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29
Chapter- III
COMPONENTS USED IN
CONTROL PANEL 3.1 INTRODUCTION [29]:
Electrical Components plays a vital role in the design process of control panel.
Normally these components are specified with different IS Codes which specifies
the characteristics of that particular component. In this chapter the most
commonly used electronics and electrical components are studied.
A power control centre of Motor control centre which consists of bus bars,
functional units and cables apart from other devices plays a vital role in the
distribution system of a plant.. Great care should be given to the selection of right
switchgear for the right application. In spite of having selected the switchgear
properly, if proper consideration is not given for the busbar, it will again be
troublesome and dangerous.. Here we will look at the important points that has to
be considered while designing a panel, excluding the discussion on mechanical
aspects.
The three main points that has to be remembered and followed throughout
panel design are
1. Safety
2. Reliability
3. Maintainability.
A panel can be divided into three distinct zones namely
1. Busbar zone
2. Unit chamber (Functional units)
3. The cabling zone.
Each of these zones individually as well as their
30
Figure 3.1: Distinct Zones of Panel
Interaction with other units has to be designed with a view to have safety,
reliability and maintainability
3.1.1 BUSBAR ZONE:
This zone comprises of the horizontal busbars, the vertical busbars and
associated supporting system.
The busbar system design should take care of the following points.
Bus bar should be able to carry
1. The rated current.
2. The current that could flow under abnormal conditions (short circuit)
Also the busbar system should be protected against.
3. Vermin, falling tools and hardware's which tend to bridge phases and
initiate arcing faults.
4. Dust and conducting deposits which accelerate tracking and supporting
surfaces and consequently cause failure of the supporting system.
Let us, now look at the parameters for proper design.
Thermal withstand capacity and short circuit withstand capacity play an
important roll for designing a bus bar.
1. Thermal Withstand Capacity:
Thermal withstand capacity depends on the following
• Busbar Material
31
• Final Operating Temperature Of Busbars
• Configuration Of Busbars
• Orientation Of Busbars
• Single Or Multiple Busbars
• Ambient Temperature
• Volume Of Enclosure And Ventilation
• Busbar Bending
• Busbar Jointing
• Bolts And Bolting Schedule
• Contact Surface And Joint Compound
• Aluminium To Copper Connection
• Contact Pressure
2. Short circuit Withstand Capacity:
Short circuit withstand depends on the following:
• System Fault Level
• Clearance And Creepage
• Insulating Support
• Insulation Of Busbars
• Material Of Insulating Supports
3.1.1.1 THERMAL WITHSTAND CAPACITY:
1. Busbar Material:
Copper and aluminum are the two common materials used for
manufacturing the bus bar. Though copper has got better conductivity when
compared copper has got better conductivity when compared to Aluminium, the
increased cost of copper and indigenous availability of Aluminium are the points
that are in favor. Though the main reason is cost and availability, Aluminium has
got some advantages over copper. They are
32
• The density of Aluminium is 2.95 gm/cm3 compared to 9.7 gm/
cm3 of copper. Because of its lower density, an aluminium busbar
will weigh only one half for same current rating.
• Because of the increased area, the Aluminium busbar will run
cooler than their copper busbar.
• The weight ratio of copper to Aluminium is in the range of 2.00 to
2.63, which once again helps to reduce the cost.
• The IS for Aluminium Busbar is IS 5082-1981. The conductivity
of Aluminium or copper busbars are referred to international standard
copper and expressed as a percentage.
2. Final Operation Temperature of Busbars:
As we had seen, the final operating temperature is limited by the withstand
capacity of bus bar itself. For Aluminium, the final operating temperature is
limited to 850 C because
• The operating temperature of a bus bar must be limited to a level
where there will be no long term deterioration of the conductor, the
joints or to the equipment connected to the busbar. The mechanical
strength is reduced at elevated temperature. Since the busbars are
required to withstand short circuit forces, mechanical strength is of
prime importance and so the final operating temperature is
restricted to safe value of 850 C. If mechanical strength is not of
importance and by giving particular attention to factors such as
joint design and thermal expansion, the busbars may be operated at
much higher temperatures. In general busbars are not operated at
levels that will cause reduction in mechanical strength.
33
Figure 3.2: Types of Busbar Arrangement
3. Configuration of Busbars:
The current carrying capacity of busbar system is affected by the
configuration in which the busbars are placed. When a busbar installation consists
of several conductors in parallel, each conductor must dissipate the heat generated
in it since there is no metallic thermal connection between the conductors. In
horizontally arranged three phase systems, the total busbar rating is limited by the
temperature rise of one conductor even when the remaining parts may be
relatively cool. The area available for heat dissipation is more for the outer side
conductors than that of the inner ones and so they will be at lower temperature
than the inner ones.
This is explained in the figure 3.2 , where the current carrying capacity are
given for various configuration of conductors of same cross-sectional area and for
the same final temperature.
4. Orientation of Busbars:
When the busbars are arranged in a panel, for a 3 phase system, they can
be arranged in two ways as shown in fig. 3.3**** The volume of space available
above the busbar decides the amount of heat dissipated and in horizontal
orientation, the busbars have more space above than the vertical orientation. The
vertical orientation helps in reducing the magnitude of short circuit forces.
34
Figure 3.3: Orientation of Busbars
5. Single Busbar or Multi Busbar:
The current distribution in a busbar (in case of AC currents) is affected by
the inductive effect due to the current in the conductor itself. This is known as
Skin Effect. Because of this, the current tries to concentrate in the outer portion
of the conductor.
If two or more conductors are there, then current distribution in one
conductor is distorted by the current in the adjacent conductors. This effect is
known as Proximity Effect.
Fig. 3.4: Number of Holes in a Busbar
Part of busbar material may be removed for holes provided. In order to ensure
temperature rise within specified limits it is necessary to keep control on busbar
material removal for holes.
35
There are two guidelines provided by standards
• The remaining material at any cross section along the length of the
busbar has at least 70% of the required ampacity.
• The remaining metal in any 150 mm length of the busbar is at least
93% of the metal of a bus having the required ampacity.
1Bar 2Bar 3Bar 4Bar
Size in
mm
D.C.
50c/s
A.C.
D.C.
50c/s
A.C.
D.C.
50c/s
A.C.
D.C.
50c/s
A.C.
25.4 x
6.35
38.1 x 6
.35
50.8 x
6.35
63.5 x
6.35
356
356
520
520
672
670
820
812
718
715
1030
1020
1315
1290
1550
1510
980
970
1380
1350
1765
1705
2100
2000
1120
1100
1585
1315
2050
1940
2430
2260
76 .2 x
6.35
101.6 x
6.35
127.0 x
6.35
152.4 x
6.35
970
958
1260
1235
1545
1505
1840
1780
1805
1740
2260
2140
2700
2510
3130
2860
2440
2310
3060
2800
3660
3240
4290
3680
2860
2620
3640
3200
4410
3700
5250
4240
50.8 x
9.53
76.2 x
9.53
840
830
1210
1180
1560
1500
2180
2050
2090
1970
2940
2660
2460
2260
3510
3030
36
101.6 x
9.53
127.0 x
9.53
152.4 x
9.53
203.2 x
9.53
1550
1495
1940
1860
2260
2120
2940
2750
2710
2480
3290
2930
3770
3340
4800
4150
3660
3150
4450
3660
5140
4080
6500
4900
4400
3560
5400
4200
6300
4680
8060
5740
76.02 x
12.7
101.6 x
12.7
127.0 x
12.7
152.4 x
12.7
203.2 x
12.7
254.0 x
12.7
1405
1355
1830
1740
2230
2080
2620
2420
3380
3060
4080
3640
2450
2240
3100
2720
3720
3120
4300
3500
5450
4450
6500
5000
3290
2830
4170
3360
5040
3900
5850
4400
7420
5300
8860
6000
4000
3240
5100
3900
6170
4550
7200
5100
9110
6150
10900
6850
Table 3.1: Busbar Ampacity
Note:
1. Ratings are for 500 C rises over 350 C ambient temperatures in still but
unconfined air.
2. For multiple-bar arrangements, the space between bars is equal to the bar
thickness.
3. A.C. ratings are based on spacing at which proximity-effect is negligible.
37
Figure 3.5: Derating factor for proximity effect
Table 3.2 gives derating factor for proximity effect.
Phase Spacing Derating Factor
3W
4W
5W
6W
> 6W
0.82
0.89
0.95
0.99
1.00
Table 3.2: Derating Factor for Proximity Effect
t = Thickness of busbar
w = width of busbar
The area available for heat dissipation for the busbar that is placed in middle is
reduced. Because of this and due to skin effect and proximity effect, the current
carrying capacity of busbar system is not increased by the multiples of no. of
busbars. Current rating of Aluminium busbar given by one of the leading
manufacturer is given in table 3.2 for reference.
38
Figure 3.6: Phase Spacing
6. Average Ambient Temperature: Referring to table No. 3.2, the foot note
says, 'Ratings are for 500 C rise over 350 C ambient temperature. The ambient
temperature, referred here, is the temperature surrounding the bus bars inside the
enclosure and not the atmospheric temperature. In Indian conditions the ambient
temperature is around 450 C to 500 C. So the ratings given in the table 3.2, are to
be suitably derated to take care of this.
The derating factor can be found out from empirical formula:
Drating Factor =
1/1.7T 2
T1
Where,
T1 = Temperature rise at the ambient
Temperature as referred by the busbar manufacturer.
T2 = Temperature rise at the ambient temperature
Where the panel will be installed.
The derating factor to be applied on the ratings given in for 500 C ambient is =
1/1.735
50
= 0.81
39
7. Volume of Enclosure and Ventilation:
When the busbars temperature increases heat is dissipated through air.
Now the amount of heat dissipated is directly related to the volume of air
available inside the busbar chamber which is decided by the volume of busbars to
the total volume of busbar chamber.
Busbar rating claimed by the manufactures is in open condition. When the
busbars are kept inside the panel, they have to be suitably derated.
The amount of heat carried away depends upon the ventilating system. Busbars
are assumed to have natural ventilation. The derating factors for volume of
enclosure / ventilation are given in table 3.3
Table 3.3: Derating Factor for volume of enclosure / ventilation
8. Busbar Bending:
When making busbar joints, bending is an important process. Aluminium
busbar can easily be bend up to 900 and the bend radius varies between 1t to 2t ('t'
= thickness of busbar).
Enclosure
Installation
Cross-Sectional Area
Of Busbar/Total
Cross-Sectional Area
Of Enclosure
Derating
Factor
Outdoors ….
< 1%
5%
10%
0.95
0.90
0.85
Indoors where the
enclosure itself in a Well-
ventilated room
< 1%
5%
10%
0.85
0.70
0.65
Indoors where the
enclosure itself is poorely
ventilated and the room
temperature is high
< 1%
5%
10%
0.65
0.60
0.50
40
If the bend radius is require is less than t or the bend angle is more than
900, this is possible to achieve by heating the bar under controlled conditions upto
2500 C.
It has to be ensured that there shall not be any crack developed at the bend
portion. Because it will reduce the cross section and mechanical strength of
busbars.
9. Jointing:
The simplest and most widely used method of jointing sections of busbar
is to overlap the busbar and bolting or clamp the sections together. Bolting and
clamping methods provide joints that have a fully satisfactory service life.
The main requirement for any busbar joint is that electrical efficiency should
remain stable under all conditions of service. To achieve this, the following
factors have to be taken into account when the joint is made. They are:
• Proper contact pressure must be applied and maintained.
• The surface of the Aluminium must be cleaned before bolting up.
• Air and Moisture must be excluded from the joint.
10. Bolts and Bolting Schedule:
Bolts for busbar joints may be of various grades of steel or aluminum
alloys, in case of Al. busbars.
Bolt size used shall depend on the width of flat normally following
guidelines are followed
Flat Width
Mm
Bolt Size
Mm
Min. No. of
Bolts / Joint
40 – 60
80 – 120
160 – 200
M12
M12
M12
2
4
8
Table 3.4: Guidelines for Bolt Sizes
Washers shall have sufficient thickness, proportionate to their size. There is little
point in increasing the size of the washer unless the thickness is also increased
sufficiently to prevent the washer from bending.
In addition to the plain washers, spring lock washers should also be used.
41
If flat width is more than 80 mm and./or flat thickness is more than 6 mm
then it is recommended to have Slitting of busbar
Figure 3.7: Slitting of Busbar
The length of slit shall be more than width of overlap of busbars.
11. Contact Surfaces and Joint Compounds:
Before bolting up a joint, oxide and dirt must be removed from the contact
surfaces. Mechanical removal is preferred when comparing to chemical cleaning.
Grease based compounds designed to provide high stability and long term
protection against the ingress of moisture, can be applied.
The requirements to establish a good joint are:
• The surface of the Busbar must be clean and free of oxide.
• Air and moisture must be excluded from the joint to prevent formation
of the oxide during service.
• Proper contact pressure must be applied and maintained.
To achieve these,
• The contact surfaces of the Busbars should be cleaned
vigorously by hand with dry coarse emery cloth or by a power
driven wire brush. This should be done immediately prior to
the assembly of the joint.
42
• A light application of a grease compound should be made to
the contact surface of each bar immediately after cleaning and
the joint should be made immediately.
The function of this compound is to exclude air and moisture from the surfaces.
The compound should be applied like a thin film. Over/liberal application of
compound can be harmful. The use of petroleum jelly is not recommended
because of its tendency to run off the joint at elevated temperatures.
The recommended compounds are HPCL multipurpose grease and Indian
Oil Corporation multipurpose grease.
12. Aluminium to Copper Connections:
The same procedures as detailed above should be used in making
Aluminium to copper connections. Preferably the copper should be cleaned with a
separate wire brush or emery cloth that will not be used on Aluminium busbars,
after ords. Since in Aluminium to copper joints, two dissimilar metals are in
electrical contact, it is necessary to ensure that moisture is excluded. If no
moisture is present, there is no possibility of electrolytic or corrosion.
13. Contact Pressure:
In bolted joints the pressure exerted on the joint interface by the bolts is
concentrated almost entirely upon the area beneath the heads of the bolts.
If fish plates are used in the joint, a slit can be provided on the fish plates.
The slit improves the joint efficiency by utilizing a greater portion of the bolt
torque in applying joint pressures, rather than on overcoming distortions on the
fish plate.
14. Material of Insulating Supports:
The final operating temperature of bus bar is limited by
• Withstand temperature of supports
• Busbar itself.
As the temperature increases, insulating Material loses the insulating
property which may lead to a flashover. But with the latest insulating materials
like SMC which can safely withstand 1500 C, temperature withstand capacity of
insulating supports is not a constraint.
43
3.1.1.2 Short Circuit Withstand Capacity:
1. System Fault Level:
In case of short circuit fault busbars and insulating supports should
withstand electrodynamic forces developed during fault. While design busbars
and its supports structure it is necessary to find out maximum fault current of the
system. Refer chapter 6 for fault level calculations.
2. Clearance between Busbars:
Clearance is defined as the shortest distance between two conductors at
different potential. Clearance plays an important role in deciding the size of the
panel because more the clearance between busbars, bigger will be the size of the
panel.
Indian Standard specifies the safe minimum value of clearances, which are given
in IS but with certain assumptions. Hence it is required to give more clearance
depending upon the conditions prevailing in the area where the panel is to be
installed.
3. Creepage Distances:
The distance between two conductors at different potential, measured
along the Table 6 Minimum values of clearances and creepage distances Surface
of the insulating support is called as creepage distance. To increase the creepage
distance, it is normal practice to have the insulating supports with ribs.
Figure 3.8: Creepage Distance
44
IS specifics the safe minimum value of creepage distance and the method for
measuring the creepage distance. The values are given in table 3.5.
Rated
Insulation
Voltage
Clearances
Mm
Creepage Distance
mm
I < 63A I 63 > A I < 63A I > 63A
Up to 60
60 to 250
251 to 380
381 to 500
501 to 660
661 to 750 ac
661 to 800 dc
751 to 1000
dc
800 to 1200
dc
L-L L-A L-L L-A L-L L-A L-L L-A
2
3
4
6
6
10
10
14
14
3
5
6
8
8
14
14
20
20
3
5
6
8
8
10
10
14
14
5
6
8
10
10
14
14
20
20
2
3
4
6
8
10
10
14
14
3
4
6
10
12
14
14
20
20
3
5
6
8
10
14
14
20
20
4
8
10
12
14
20
20
28
28
Table 3.5: Minimum Values of Creepage Distance
Note 1 – When the clearance L-A is greater than the corresponding creepage
distance specified in co 'a' or col 'b', then the creepage distance from the live part
to the accidentally dangerous part must be not less than the clearance.
Note 2 – The clearance and creepage distances for control and auxiliary circuits
should be those given for I L 63 A.
"The values of clearances and creepage distances specified in table 5 are based on
the assumption that the air is clean and its relative humidity does not exceed 50
percent at temperature of 400 C".
Measurement of Creepage Distance:
In determining a creepage distance, ridges less than 2 mm high should be
neglected which is shown in above figure (3.9)
45
Figure 3.9: Measurement of Creepage Distance
Those at least 2 mm high are either measured along their contour, if they one
integral part of a component in insulating material.
Figure 3.10: Measurement of Creepage Distance Insulating Material
Figure 3.11: Recommended Creepage Distance
The application of the recommendation is illustrated in the figure, 3.10 and 3.11
where,
A = Insulating Material
C = Conducting Part
46
F = Creepage Distance
4. Insulating Supports:
The supports play a vitally important role in the overall quality of the busbar
system. The material used for the supports should have the following desirable
properties.
1. High mechanical strength
2. High dielectric strength
3. High temperature withstand
4. Non flammable properties
5. Non hygroscopic properties
6. High comparative tracking Index.
Figure 3.12: Insulating Supports
1) High mechanical strength is required because the support should be able to
withstand short circuit forces developed. The mechanical strength is expressed in
kgf / mm2
2) High dielectric strength:
High dielectric strength is also required because the dimension of
support will be affected. The unit for dielectric strength is volt/mm. If the material
is having more dielectric strength then the size of support becomes low.
47
3) High temperature withstand:
The final operating temperature of panel will be decided by the
temperature withstand capacity of busbars and the insulating support should have
non-hygroscopic property, which may lead to a flashover if it is otherwise.
4) Non Flammable properties:
Non flammable property is required because in case of any flashover in
the panel, the insulating material should not propagate the fire.
5) Non hygroscopic properties:
Hygroscopic means the tendency of the material to absorb moisture. Insulating
support should have non-hygroscopic property, which may lead to a flashover if it
is otherwise.
6) High comparative tracking Index:
The importance of having high comparative tracking Index will be best
understood, if we go through the definition on Comparative Tracking Index.
(Abbreviated as C.T.I.)
Insulation breakdown occurs either through its volume or over its surface
or in both ways. Such failure may arise from flashover or from the progressive
degradation of the insulation by small localized sparks.
Such sparks are the result of breaking if a surface film of conducting
contaminant through it. The rapid break in the leakage current produces an over
voltage at the size of discontinuity and an electric spark is produced.
These sparks may cause carbonization on organic insulation and may lead
to a carbon track between points at different potentials.
This process is known as tracking which is explained in the figure 3.13 in
steps.
One of the most common forms of contamination is atmospheric dust,
which in the presence of moisture, forms a surface film.
48
Figure 3.13: Tracking Process
This surface film is conducive to track formation on the underlying insulation.
Table 3.6 shows comparison of some Insulation Material
Property Hylam Porcelain Epoxy SMC/DMC
Mechanical
strength
Dielectric
strength
CTI (VOLTS)
Tempt. Withstand
Inflammable
Moisture
absorption
Moulding to
Different shapes
Low
High
133
120
Yes
High
NO
Brittle
High
500
High
No
Low
Difficult
High
High
800
250
No
Low
Yes
High
High
1000
200-250
No
Low
Yes
Table 3.6: Comparison of Insulating Materials
49
Comparative Tracking Index
The surface profile of the insulator / support is also extremely important.
Ribs and creepage barriers, which are integrated into the supports, break the
continuity of the conducting deposits and prevent tracking due to leakage currents
and surface break down.
Test for CTI:
The composition of dust is so varied that the tests using dust as the
contaminant are not suitable for standard purpose. So tests using arbitrary surface
contamination have been developed to classify materials in order of tracking
resistance (Ammonium Chloride)
Figure 3.14: Test for CTI
The unit of CTI is volts. The voltage which will cause failure with the application
of 50 drops of the electrolyte and used as a measure of the susceptibility of the
material to tracking.
The CTI is a measure of the susceptibility of the insulation material to
electrical tracking. It is defined as the voltage, determined under specific
conditions, which will cause failure with the application of 50 drops of an
electrolyte. The electrolyte essentially simulates the pollutant in industry.
Some typical CTI values for different insulating materials are given in table 3.7
below.
50
Table 3.7: CTI Values
6. Insulation of Busbar:
An analysis of faults which occurred in the seventies indicate that 85% of
the faults are due to vermin of falling tools or hardware shorting the phases
momentarily and initiating arcing faults.
The easiest way to reduce possibility of arcing faults is to insulate the bus
bars. Refer Chapter 13 page no.67 gives detailed information about insulation of
busbar.
7. Short Circuit Forces On Busbars:
Electro magnetic forces will be appreciably higher than all the other forces
like weight, and thermal expansion forces.
When current is passing in a conductor, a magnetic field is set up around the
conductor. Now if another current carrying conductor is placed in parallel to the
former, electro magnetic forces will be produced between the conductors. The
forces will be produced between the conductors. The forces may be either
attractive or repulsive depending upon the direction of current.
When the currents in the two conductors have the same direction, the
forces are attractive.
When the currents in the conductors are in opposite direction, the forces
are repulsive.
Material Typical value of
CTI (in V)
1. Phenolic moldings
2. Phenolic laminated
(SRBP / SRBF)
3.Glass fiber reinforced
Polyester moldings
(SMC / DMC)
130
130
1000
51
The magnitude of force depends upon the value of current (peak value of current)
and the distance between the conductors.
Figure 3.15 and 3.16: Short Circuit Forces on Busbars
The formula for finding out the Electro magnetic forces (peak) in case of three
phase short circuit between the phase conductors, is
3.2 POWER CONTACTORS:
Contactor is a mechanical switching device capable of making, carrying
and breaking currents under normal circuit conditions including operating
overload conditions.
Fm = 0.02(IP)2 ×
L
a----- kgf
Where,
Fm = Major force kgf
IP = Peak Current in kA.
L = Support Span in mm.
a = Center to center
distance between phases in mm
52
Figure 3.17: Contractor
3.2.1 ELECTROMAGNETIC CONTACTORS:
A contactor in which the force for closing the normally open main
contacts or opening the normally closed main contacts is provided by an electro
magnet.
Contactor is primarily a switching device. All the regular switching
operations are done by contactor. Due to this contactor needs to have high
mechanical electrical life.
Some of the applications require switching on and off at very high
frequency. This demands that contactor should be capable of working at very high
frequency of operations. Contactors can be remotely operated.
Figure 3.18: Contractor Features
53
Contactors are not designed to break short circuit currents. Thus, they are said to
have limited breaking capacity. Due to this reason backup short circuit switching
device like MPCB or MCCB is required in feeder circuits. The contactor must
however withstand the short circuit current till the back-up device clear the fault.
Normal Current
Overload Current
Short Circuit Current
A contactor has two separate electrical circuits
1. Main Circuit
2. Control Circuit
Figure 3.19: Contractor Circuits
1. Main Circuit:
The main circuit involves power contacts and handles the power. The main circuit
mainly consists of contacts and terminals. Double break contact system ensures
long contact life. Typical power contact is shown in Fig. 3.20.
2. Control Circuit:
The control circuit involves coil and auxiliary contacts. Auxiliary contacts are in
built and are used for interlocking signaling etc. The coils are vacuum
CONTACTOR Break
x
Contactor
Main Circuit Control Circuit
54
impregnated or resin cast. The coil, if totally enclosed by the power magnet
ensures low power consumption. Change in control circuit may necessitate
additional auxiliary contacts. Add-on auxiliary contact blocks are provided to
meet this need.
Figure 3.20: Typical Power Contact
Contact Properties:
(a) To make circuit:
• Good resistance to environmental conditions.
• High hardness to withstand impact on closing and to permit
developing the required contact pressure.
• Surface properties that allow metallic contacts to be established during
the closing operation.
(b) To carry the current:
• Good electrical and thermal conductivity and low contact surface
resistance to reduce heating.
• Relatively higher contact pressure and larger terminals improve the
capacity to carry current.
• High melting temperature to resist excessive deformation under
current flow.
• Freedom from formation of permanent weld at the contact pressures,
within the range of current carried.
55
(c) To break the current:
• Good resistance to welding and to arcing damage, to allow separation
of contacts and to conserve contact material
• High melting point to reduce material loss.
• Good thermal conductivity to reduce material loss by removing heat
from the arc spot. Relatively larger terminals help this. Strong
contactor help to break the welds formed during continuous operation.
Following are some of the factors affecting the life and performance of contacts:
1. Humidity: Saline atmosphere and humidity damage contact surface and
increase effective surface resistance.
2. Dust: Dust prevents metallic contacts from being established and reduces heat
dissipation.
Utilization Categories
A combination of specified requirements related to the conditions in which
the switching device or the fuse fulfils its purpose, selected to represent a
characteristic group of practical applications. The specified requirements may
concern for e.g. the value of making capacities (if applicable), breaking capacities
and other characteristics, the associated circuits and the relevant conditions of use
and behavior.
Table 3.8 shows utilization categories
Kind of
Current
Utilization
Categories
Typical applications
A.C.
AC-1
AC-2
AC-3
Non-inductive or slightly inductive
Loads, resistance furnaces.
Slip ring motors: starting, switching off
Squirrel-cage motors: starting
Switching off motors during running.
56
D.C.
AC-4
AC-5a
AC-5b
AC-6a
AC-6b
AC-7a
AC-7a
AC-8a
AC-8b
DC-1
Squirrel-cage motors: starting, plugging,
inching.
Switching of electrical discharge lamp
control.
Switching of incandescent lamps.
Switching of transformers.
Switching of capacitor banks.
Slightly inductive loads in household
Application and similar applications.
Motor-loads for household application.
Hermetic refrigerant compressor motor
Control with manual resetting of overload
releases.
Hermetic refrigerant compressor motor
Control with automatic resetting of over
load releases.
Non-inductive of slightly inductive loads,
resistance furnaces.
Shunt-motors: starting, plugging, inching
dynamic breaking of D.C. motors.
Series-motors: starting, plugging, inching
57
DC-3
DC-5
DC-6
dynamic breaking of D.C. motors.
Switching of incandescent lamps.
Table 3.8: Utilization Categories
1. AC-3 category may be used for occasional inching (jogging) or plugging for
limited time periods such as machine set-up; during such limited time periods the
number of such operations should not exceed five per minute or more than ten in
a min. period.
2. Hermetic refrigerant compressor motor is a combination consisting of a
compressor and a motor, both of which are enclosed in the same housing, with no
external shaft of shaft seals, the motor operating in the refrigerant.
Resistive loads draw only full load current during starting. Therefore, making
current is In. Breaking current is also In. The breaking duty is simple because
current is broken at almost unity P.F.
Figure 3.21: Utilization Category AC1
Current
Time
Utilization Category AC1
Making Current In Breaking Current In
58
Figure 3.22: Utilization Category AC2
Squirrel cage induction motor draws 6 x In on DOL starting. Motor is switched
off after the starting current has come down to the load current. Breaking current
is In+6.
Figure 3.23: Utilization Category AC3
Squirrel cage induction motor with inching duty is switched off on starting
current. Breaking current is also 6 x In.
Current
Time
Utilization Category AC3
Making Current In
6 * In
Breaking Current In
Current
Time
Utilization Category AC2
Making Current In
2.5 * In
Breaking Current In
2.5 * In
In
59
Figure 3.24: Utilization Category AC4
How to read life curve of a contactor:
Both X-axis and Y-axis are in log scale. Assume the life curve is for utilization
category AC-3. We take an example of 70 A Contactor. The vertical straight line
ab presents AC-3. Life for 70 A contactor. Point b projected on Y-axis is e. The
life of contactor for 70 A current is e. No. of operating cycles.
Figure 3.25: Life Curve of Contractor
The same contactor when used for 40 A current gives life cd Point d projected on
Y-axis is f. The life 70 A contactor for 40 A current is f. no. of operating cycles.
Here, f>e
1 10 40 70 100 1000
f
e b
d
c
Current in Amperes
Current
Utilization Category AC4
Making Current In 6 * In
Breaking Current In 6 * In
Time
60
There is increase in no. of operations for a contactor when used for lower current
than rated.
Control Circuit:
The control circuit voltage has extra significance in defining (1) the circuit
voltage in which the contactor will hold positively and (2) the drop-off voltage.
Various coils for different voltages can be given for both a.c. and d.c.
Limits of Operation of contactor (a.c.Coil):
Electromagnetic contactors shall close satisfactorily at any value between
85% and 110% of their rated control supply voltage (Us), for wide band coils,
85% shall apply to the lower value and 110% to the higher.
Figure 3.26: Limits of Operation of Contractor
The limits between which contactors shall drop off out and open fully are 75% to
20% for a.c. their rated control supply voltage (Us). For wide band coils, 20%
shall apply to the higher value and 75% to the lower. These limits apply to a.c.
declared frequency. Pick-up band for d.c. coils is same as that of a.c. coils. The
limits between which contactors will d.c. coils shall drop out and open fully are
75% to 10% of rated control supply voltage. Where the range is declared 10%
shall apply to the higher value and 75% to the lower.
For a.c. coil
100
Drop off Band
Pick Up Band
0 20 75 85 110
61
Limits of operation of contactors (a.c.coil):
Figure 3.27: Operation of Contractor
Frequency:
In a.c. magnet system contactor coils are designed for 50 Hz frequency at the
rated coil voltage. For frequency other than 50 Hz, V/f ratio shall be kept
constant. Special 60 Hz, keeping V/f constant are available as a standard feature.
This is required to maintain the coil performance as specified above. Special coils
for 60Hz application are also available.
Voltage at frequency f Hz:
Uf f
-------- = -------
U 50 50
Uf = Voltage at frequency f Hz
U50 = Voltage at frequency 50 Hz
Pick up and Hold-on Vas:
Pick-up VA is product of pick-up current and rated coil voltage.
For a.c. coil
100
Drop off Band
Pick Up Band
0 10 75 85 110
62
Calculations of coil currents for AC coils:
For coil voltage = 110V.
Pick-up current = = 2100/100
= 19.09 A.
Pick-up current = = 95/100
= 0.86 A.
When the contactor is open, air gap is present between moving magnet and fixed
magnet. The air gap introduces high reluctance in the magnetic circuit.
Due to high reluctance, magnetic flux is low. Lower magnetic flux reduces
the reactance of the coil. Lower coil reactance leads to higher current during pick-
up.
Figure 3.28: Contractor Open and Close
Once the contactor has closed, air gap is zero. This given rise to a high magnetic
flux, in the magnet. This magnetic flux, linking to the coil, increases the
reactance. Increased reactance leads to reduction in the coil current during hold-
on.
Pick Up VA
Coil Vtg
Hold On VA
Coil Vtg
63
Application of a.c. coil for d.c.:
A quick adaptation of a.c. coil to d.c. is possible by the use of economy
resistance. An economy resistor helps reduce the d.c. current to the level of a.c.
current. The arrangement is as shown in the figure. Deposite its quick adaptation
it has the disadvantage of (a) power dissipation in Rg and (b) time delay in the nC
contact.
`
Figure 3.29: Application of AC coil for DC
3.3 THERMAL OVERLOAD RELAYS:
3.3.1 Motor failures:
Fig. 3.30 shows general reasons for motor failure. This pie chart also gives
percentage contribution of each reason towards failure.
From pie chart overload and single phasing put together contribute to
about 44% towards motor failure.
Bearing Fail
13%
Overloads
30%
Contaminates
19%
Misc
9%
Single Phasing
14%
Rotor Fail
5%
+
-
Rg
64
Figure 3.30: Motor Failures
3.3.2 Factors Leading to Overheating:
• Overload
• Supply variations
• Heavy starting
• High Duty
• High Starting frequency
• Single phasing
• Locked rotor
• Faulty bearing and other mechanical reasons.
• High ambient
• Anomalies in cooling
Temperature Withstand of Motors:
A large percentage of motor failure are due to our misconceptions
concerning motor ratings and motor protection, Fig 2.2 shows a typical
temperature withstand curve for a motor.
Figure 3.31 shows typical Motor Damage Curve.
This curve indicates that the motor winding may carry its full rated current for an
indefinite period of time without any damage. This condition is possible because
the heat generated in the motor winding as a result of I2R can be properly
dissipated before an abnormal temperature is reached. As long as the motor
winding is clean and dry, the air passages in the motor winding are open and a
65
supply of relatively cool air is available to dissipate the heat generated, the above
conditions will prevail.
Figure 3.31: Motor Damage Curve
The same curves indicate that if twice the normal full-load current is passed
through the motor winding, the heat generated in motor is mote than the heat
dissipation and therefore the motor temperature will rise. However there is a
specific time interval during which this may be allowed to happen. The same
holds goods if 300, 400, 500, 600 of even 1000 percent of the motor full load
current is passed through the motor windings. Heat generation will be greater than
heat dissipation, but there will be a specific time interval before any damage is
done to the motor winding. The objective of motor protection is to prevent an
excessive temperature rise which would damage the motor winding.
Device of protection:
Various types of protection devices can be broadly classified in three categories:
(a) Temperature Sensing
(b) Current Sensing
(c) Voltage Sensing
Following figure 3.32 illustrates the various devices that are available today for
protecting the motors. This covers only devices for general application and is not
exhaustive.
Current (% motor full load)
66
The user, depending on the importance of the application, severity of
environment and supply
Figure 3.32: Motor Protection Devices
Conditions and expenses that he is prepared to incur, has to make a judicious
choice.
Out of these devices those that offer closet protection are:
Temperature sensing device:
These devices, to the extent that they are able to sense that exact temperature of
winding, offer protection against all the abnormal situations listed above.
Current Sensing devices:
These devices, being an indirect sensing device cannot offer protection against:
(a) Excessive ambient temperature
(b) Over Voltage
(c) Frequency variation.
These have to be taken care by other means. Thermal bimetallic relays.
They are the most economical devices of all protective devices that are available.
Relays offer protection both against single phasing as well as overloads and thus
67
are one of the most popular devices for motor protection. The principle of
operation is as follows,
A thermal bimetallic relay works on a principle that the bimetal deflects
when heated and if one end of the bimetal is fixed, the other deflects in proportion
to the heat input.
In an overload relay intended to protect a 3 phase motor, there are three
bimetals for sensing line or phase currents.
The bimetal deflects when heated and, on reaching a predetermined condition,
trips a mechanism (open a contact). The amount or deflection required to trip is
more or less depending on the setting of the relay.
A compensating bimetal is used to ensure a consistent trip time in case of
variation in ambient temperature.
Relay characteristics:
Designers make their thermal overload relay with heat shortage
characteristics similar to those of the motor, but enough faster so that the relay
will always reach the tripping temperature before the danger temperature. The
relay characteristics should be below the motor damage curve. If it lies very much
below the motor curve, it can lead to unwanted trippings. The relay characteristics
chosen should be as close as possible to the motor damage curve.
Specification:
IS 13947 (Part 4/Sec 1) :1993/IEC 60947 (Part 4/Sec 1) lays down the
specifications that are to be satisfied by thermal bimetallic relays.
Table 3.9 shows this
Conditions to be met with at Test current as multiples of set current
Conditions Max. non-trip
currents
A
Min. Trip
Currents
B
1. All poles energized
2. When two poles are energized
3P 1.05
3P 1.2
68
Relays without protection
against single phasing
Relay with protection against
single phasing
2P 1.05
2P 1.0
1P 0.9
2P 1.32
2P 1.15
1P 0
Table 3.9: Thermal Bimetallic Relay Specifications
The relays shall comply the requirements given in table 11when tested as follows
as per IS 13947/IEC 60947 Part 4. Table 3.10 shows this
TRIP CLASS TRIPPInG TIME In SECOnDS WHEN
TESTED AT CURREnT 7.2 TIMES
10 A
10
20
30
2 < Tp < = 10
4 < Tp < = 10
6 < Tp < = 20
9 < Tp < = 30
Table 3.10: Relay Compliments
Tips for application:
Further for relays operating on double slide principle the protection
against single phasing is substantially better at maximum setting. The following
tips will therefore be useful to achieve the best protection against single phasing.
• Select a relay such that motor current lies towards maximum setting.
• This necessitates that the relay family should be such that there is a
consistent overlap between two successive ranges.
• Setting ratio of the relay range should be the maximum. Relay
available today have a setting ratio of 1.3 to 1.66
69
Max. Relay set current
Setting ratio = --------------------------------------
Min. Relay set current
Generally Relay setting is recommended as per actual current drawn by the
Motor.
Too large or too small cross section of cable terminals may result in
excessive temperature rise and nuisance tripping, or non-tripping of relay at
higher overloads.
Overload relay with manual reset should always be used where continuous
contact devices are connected (e.g. limit switches, pressure switches) to avoid
restarting automatically. Reset button to be fitted as an external feature in order to
make it accessible to all personnel. Overload with auto reset can be used only
with impulse contact devices such as push buttons, because on these the cooling
of the bimetal strip cannot lead to automatic reconnection.
Backup fuse:
Backup fuse for instantaneous trip are needed to protect not only the motor
but also the relay against the effect of short circuit. These are clearly marked on
the relay as MAX.BACKUP FUSE. However a lower rating of fuse can be
selected to minimize the damage to the contactor and other system. Refer fuse
selection for further clarification.
Overload protection of high starting time motors:
The starting time of a motor depends upon the motor and load torque and the total
inertia of the system.
• A is main Contactor
• B is by pass Contactor
• Control by timer
70
Figure 3.33: Overload Protection for Motors
The starting time can be long when the motor is started on heavy inertia loads.
This results in high current to flow through the relay for longer duration. The
normal trip time for thermal overload relays at 6 x In is in the range of 5 – 12 sec.
Hence, the relay set at normal full load current for a motor trips during heavy
starting, resulting in nuisance tripping. To avoid this problem two solutions are
available.
• Saturable core CT operated relay.
• Relay bypass during starting.
Saturable core CT operated relay:
Saturable of the core of CT is achieved by using a saturation kit in series
with the relay. The secondary current of the CT is not as per the current
transformation ratio of CT due to saturation.
To bypass the overload relay during starting:
This method is to be used when motor starting time is beyond permitted
by Saturable core CT operated relay. In this method, the relay is bypassed during
starting by the addition of a contactor connected in parallel with the relay. After a
set time delay‘t’, the contactor B is de-energized and the relay is introduced in the
power circuit. The power scheme is shown in Fig. 3.32.
Thermistor protection devices:
Thermistors are semi-conductor device whose resistance varies with
temperature. There are two types of thermistors viz.
nTC – negative temperature coefficient.
71
PTC – Positive temperature coefficient.
Resistance of nTC thermistors decreases sharply with specified increase in
temperature, resistance of PTC thermistor increase with temperature and also it
has a short cutt-off temperature, while for PTCs there is a sharp cut-off
temperature. These have to be embedded in the motor winding. Typically 3 or 6
such devices are used.
PTCs and nTCs give signal to control unit which in turn trips the motor.
Because of the sharp cut-off characteristics of PTCs it is possible to design a
standard control unit for all temperature, while nTCs requires the unit to be tailor-
made.
In PTC based unit, the increase in resistance is detected as fault. Any
break in lead, therefore, indicates a fault. PTC based systems are thus fail safe. BS
4999 prohibites the use of nTC based devices for protection of motors PTCs are
available for 1000C to 1400C in steps of 100C since it is the temperature of the
winding that is sensed. Thermistors offer the ultimate in protection.
Electronic Single Phasing Preventors:
The text Single Phasing Preventor is a misnomer. The devices do not prevent
any single phasing but protect the motors from getting damaged irreparably due to
single phasing. Protection against overload conditions should be taken care by a
relay. The devices operating on two different principles are available today.
•••• Current sensing: These work on the principle of negative phase sequence
currents. When motor gets only two phases – ve phase sequence currents
are produced and these are sensed by relay. It isolates the motor from
supply irrespective of the load on the motor. To avoid nuisance tripping
due to unbalance currents in winding these devices tolerate imbalance up
to 10%. This limit can be lowered if better supply conditions can be
assured. Typical operating time is 10-20 seconds on On LOAD and 5-15
seconds on FULL LOAD. These are to be connected in series with the
motor.
•••• Voltage Sensing: These devices sense the absence of voltage, and are to
be connected across the line. Refer Fig 3.19
72
They suffer following drawbacks-
- Downstream faults are not protected.
- Current unbalance due to inter-turn short is not detected.
Figure 3.34: Single Phasing Preventors.
3.4 STARTERS FOR SQUIRREL CAGE INDUCTION
MOTOR:
Three phase squirrel cage induction motor constitute major portion of
industrial drives. Smooth starting of motor is important for load and source as
well. This chapter deals with purpose and operation of DOL and STAR- DELTA
starters for three phase squirrel cage induction motors.
Generally, percentage of squirrel cage induction motor load goes as high as
90% of total industrial load as illustrated in Fig 3.34. Thus we would be
concentrating the discussion mainly on starting of Squirrel cage motors.
73
Figure 3.35: Squirel Cage Motor % Use
Fig. 3.36 shows location of starter in a motor feeder circuit.
3-phase supply:
Figure 3.36: Location of Starter
The need of starter for S.C.I.M.
1. Switch On/OFF
2. Remote On/OFF
3. Limit starting current/torque.
Fuse Switch Unit
STARTER
MOTOR
74
Types of squirrel cage Induction Motor Starter:
Figure 3.37: Types of SCIM
DOL Starter:
Motor Starter windings are permanently connected in Delta. Full 3-phase line
voltage is applied at starting. The starting current drawn by the motor is about 6-8
times the rated current of the motor.
Starting torque developed by motor with DOL starter.
Figure 3.38: DOL Starters
Starter
Full Voltage Reduced Voltage
Star - Delta Auto Transformer
DOL Direct On Line
75
For Induction Motors
V2
T a --------- Where
S Tr = Full load torque
I r = Full load current
Sr = Slip at full load
Tst ( Ist )2 Tst= Torque at starting
---- = ------ x Sr Ist= Starting current
Tr ( Sr)
Advantage of DOL starter:
Following are advantages of DOL starters
• High starting torque.
• Rapid acceleration of motor.
• Low cost of control gear required.
Limitations of DOL starter:
• Heavy inrush starting current can cause a dip in the system voltage.
• Starting of large motors on DOL would demand increase in capacity of
sources.
• kVA maximum demand would increase.
Star – Delta Starter:
First motor stator windings are connected in star.
Figure 3.39: Star Starters
76
Motor windings get phase voltage although full 3-phase line voltage is switched
on. Thus, it is reduced voltage starting. Motor is allowed to run in STAR till such
a time it has reached 80% if the rated speed. After this, motor is reconnected in
DELTA
Figure 3.40: Star Delta Starters
Power circuit of Star-Delta starter:
Sequence of contactor operations:
1. Star contactor pick-up
2. Hold-on contactor pick-up. Motor starts in star.
3. Star contactor opens.
4. Delta contactor pick-up. Motor switched from star to Delta.
Figure 3.41: Power Circuit of Star Delta Starter
77
Types of Star Delta Starters:
Figure 3.42: Types of Star Delta Starters
Changeover scheme for hand-operated starter:
Figure 3.43: Changeover for Handheld Starters
Changeover scheme for semiautomatic Star-Delta starter:
The motor would continue to run in star till the star push button is pressed. When
start push button is released the motor would be switched to delta.
Star Delta Starter
Fully Automatic Semi-Automatic Hand Operated
Thermal Timer Electronic Timer
78
Figure 3.44: Changeover for Semiautomatic Starters
Change over scheme for fully automatic starter with thermal timer:
Figure 3.45: Changeover for Fully Automatic Thermal Starters
Change over scheme for fully automatic starter with electronic timer:
Pause time: During the change over from star to delta, star contactor drops.
Arcing occur between the contacts for a short period. During the same time if
79
delta contactor picks-up before arching in star, which results in leading to welding
of delta contacts and blowing of fuse.
Figure 3.46: Changeover for Fully Automatic Electronic Starters
Torque developed during star-delta starting:
The voltage applied at starting is phase voltage. Thus, the starting current drawn
by the motor is ILR/3 where ILR is locked rotor current of the motor.
For Induction Motor
V2 Where,
T a --------- T = Torque
S V = Applied Voltage
S = Slip
__________________________________
We know that,
I2 (V/ 3)2
T f a ---------- and Tst a -------------
Sf St
----------------------------------------------------------------------------
Therefore
T 1 ( I )2
80
-------st- = ------ -----LR-- x Sf
T f 3 (If)
Typical curves of currents during star-delta start.
Figure 3.46: Torque Characteristic for Star Delta Starters
Typical curves of torque during star-delta start
CMD = Motor torque
CMY= Load torque
Figure 3.48: Typical Curve of Star Delta
81
Timers for fully automatic star-delta starter:
Thermal timer:
Figure 3.49: Timers for Fully Automatic Starters
The time delay in the timer is achieved through displacement of a bimetallic
element which operates on current drawn from the secondary of a transformer.
After each timing cycle, the operating coil must be de-energized so that bimetal
cools and resets to its initial position.
Electronic Timer:
Figure 3.50: Electronic Timer Circuit
On energization of the timer, the output star relay energizes instantly. After
completion of preset delay time, output delta relay energizes after fixed pause
time this pause time is 60 ms. This provides shortest possible ‘current off’ pause
and simultaneously ensures smooth changeover.
82
Figure 3.51: Electronic Timer Connection
Auto-transformer starter:
A starter for an induction motor which uses for starting one or more reduced
voltages derived from an auto-transformer.
Transition types with ATS:
Figure 3.52: Transition types with ATS
Open transition: A circuit arrangement such that the supply of the motor is
interrupted and reconnected with changing over from one step to another.
Closed transition: A circuit arrangement such that the supply of the motor is not
interrupted when changing over from one step to another.
ATS
Open transition Closed transition
83
Typical curves of currents during ATS start:
IR = Motor current at rated voltage
LT= Motor current at reduced voltage
I L= Line current at reduced voltage
Figure 3.53: Current Characteristics during ATS Start
Typical curves of torques during ATS start:
CR = Load torque
CM = Motor torque
CMR= at rated voltage
CMT= at reduced voltage
Figure 3.54: Torque Characteristic during ATS Start
84
3.5 SWITCH, SWITCH DISCONNECTOR AND FUSE
COMBINATION UNIT:
Disconnector:
A mechanically switching device which, in an open position,
complies with the requirements specified for the isolating function.
A disconnector is capable of opening and closing a circuit when
either a negligible current is broken or made, or when no significant change in the
voltage across the terminals of each of the poles of the disconnector occurs. It is
also capable of carrying current under normal circuit conditions and carrying for a
specified time currents under abnormal conditions such as those of short circuits.
Switch:
A mechanical switching device capable of making, carrying and
breaking current under normal circuit conditions which may includes specified
operating overload conditions and also carrying for a specified time currents
under specified abnormal circuit conditions such as those of short-circuit.
A switch may be capable of making, but not breaking, short-circuit
currents.
Switch disconnector:
A switch which, in the open position, satisfies the isolating
requirements specified for a disconnector,
Switch-disconnector:
A switch which, in the open position, satisfies the isolating
requirements specified for a disconnector.
Disconnector-fuse:
A disconnector in which one or more poles have a fuse in a
composite unit.
Switch disconnector fuse:
A switch –disconnector in which one or more poles have a fuse in
series in a composite unit.
Fuse-disconnector:
85
A disconnector in which a fuse-link or fuse-carrier with fuse-link
forms the moving contact.
Fuse-switch:
A switch in which a fuse-link or fuse a fuse-carrier with fuse-link
forms the moving contact.
Fuse-switch-disconnector:
A switch-disconnector in which a fuse-link or a fuse-carrier with
fuse-link forms the moving contact.
Distinction between AC-23 and AC-3 rating of switches:
AC-23 duty is defined in IS 13947, (Part3) as switching of motor
loads or other highly inductive loads.
NAC-23n utilization category referred in IS 13947 (part3) does not
apply to equipments normally used to start or stop individual motors. AC-3
utilization category is squirrel cage motors: starting and switching off of motors
during running.
Hence, AC-23 application necessarily means that two switching
devices are connected in series. One is to be used for switching of motor (starter,
for e.g.) and other (switch) as backup. Therefore a starter is to be used for normal
on / off operation of the motor and the switch is used only as a back up devices
required operating sparingly.
Figure 3.55: Distinction of AC-23 and AC3 Switches
86
Under normal condition, i.e. when contactor is functional, the switch is normally
expected to make / break rated current at rated voltage. However, if the contactor
is welded, & the rotor of the motor is locked, them the switch may have to make /
break 10 or 8 times the rated current at the rated voltage. This is because the
switch is used as a backup device.
AC-3 rating apply only is the switch directly switches on the motor
as shown above. If the switch is of backup, AC-3 rating has no significance.
Table 3.11 shows utilization categories for AC-3 switches
Utilization Category Making Make-break
I/Ie U/Ue Cos Ø Ic/Ie UrUe Cos Ø
AC – 20 A/B
AC – 21 A/B
AC – 22 A/B
AC – 23 A/B
---
1.5
3
10
---
1.05
1.05
1.05
---
0.95
0.65
0.45
---
1.5
3
8
---
1.05
1.05
1.05
---
0.95
0.65
0.45
Table 3.11: Utilization Categories for AC-3 Switches
Utilization
Category
Typical Application
AC – 20 A/
AC – 20 B
AC – 21 A/
AC – 21 B
AC – 22 A/
Connecting and disconnecting under
no-load conditions
Switching of resistive loads including
Moderate overloads.
Switching of mixed resistive and
87
AC – 22 B
AC – 23 A/
AC – 23 B
Inductive loads, including moderate overloads.
Switching of motor loads or other3
Highly inductive loads.
DC – 20 A/
DC – 20 B
DC – 21 A/
DC – 21 B
DC – 22 A/
DC – 22 B
DC – 23 A/
Connecting and disconnecting under
No-load conditions.
Switching of resistive loads including
Moderate overloads.
Switching of mixed resistive and
Inductive loads, including moderate overloads.
Switching of highly inductive loads.
Table 3.12: Utilization Categories for Different Switches
3.6 HRC FUSE LINKS:
Fuse:
A switching device that, by the fusion of one or more of it’s
specially designed and proportioned components, opens the current in which it is
inserted and breaks the current when this exceeds a given value for a sufficient
time. The fuse comprises all the parts that forms the complete switching device.
Fuses have four things to do in a circuit:
• The fuse must sense faults.
• The fuse must open quickly and clear itself when a short circuit occurs.
88
• The fuse must also sense a normal or harmless overload before it becomes
excessive or prolonged.
• The fuse must not change or later the characteristic of the circuit during
normal operation.
Construction:
Figure 3.56: HRC Switch Construction
Fuse link parts Material
1. Fuse body
2. Quartz powder
3. Knife
4. Fuse element
5. Globute
Steatite.
Quartz
Copper with silver plating.
Copper with silver plating.
Solder / Tin.
Table 3.13: Fuse Link Parts and Material
Definition:
Refer Fig. 3.42
P – Prospective peak current of a circuit
C – Cut-off current
t1 – Pre-arcing time.
t2 – Arcing time.
89
T – Total operating time.
Pre-arcing time (Melting time t1):
The time between the commencement of a current large enough to
cause the fuse element(s) to melt and the instance when an arc is initiated.
Figure 3.57: Characteristic of Pre-arcing Time
Arcing-time (t 2):
The interval of time between the instance of the initiation and the
instant of final are extension.
Operating time (T):
The sum of the pre-arcing time and the arcing-time.
Prospective current (P):
The current that would flow in a circuit if a fuse situated therein was replaced by a
link of negligible impedance without any other change in the circuit or of the
supply.
Cut-off current(C):
The maximum instantaneous value reached by the current during
the breaking operation of a fuse when the fuse operates in such a manner as to
prevent the current from reaching the otherwise attainable maximum.
Fuse characteristics:
• Time / current characteristics
90
• I2t characteristics.
• Cut-off and let-through current characteristics.
1. Time/Current characteristic:
A curve giving the pre-arcing time as a function of the prospective
breaking current under stated conditions of operations.
2. I2t (joule integral):
The integral of the square of the current for a given time interval is
t1
I2t = I2dt
to
I 2t characteristics:
A curve giving I2t values (pre-arcing I2t and / or operating I2t) as a
function of prospective current under stated conditions of operation. HRC fuse-
links offer one of the best forms of protection for motor starting applications in
view of very low short circuit stresses generated in case of a fault as compared to
other protective devices. However, the best possible rating of a HRC fuse-link
selected / recommended for such application depends on following basic
requirements:
(a) Ability to withstand motor starting current:
The fuse-link selected should withstand repeated starting currents without any
deterioration. The rating of fuse-link is decided by:
1. The motor starting current and its starting time. This will depend on two
factors:
- Type of starter (e.g. DOL or Star-Delta)
- Motor characteristics.
2. Time current characteristics of the fuse link.
(b) SCPD co-ordination:
As per the standard IS 13947, part4 section1/IEC 60947 part 4 sec. 1, for the short
circuit protective device co-ordination, ntwo types of co-ordinations are
91
permissible ‘Type 1 or 2’. Thus the fuse-link selection will also depend on its
short-circuit characteristics i.e. the cut-off current and I2t characteristics.
(c) Discrimination:
There should exist discrimination between the overload relay and the SCPD i.e.
the fuse link This means that for overload currents less than Ic (Ic=the current
corresponding to the intersection point of overload relay and the fuselink
characteristics.) the relay should operate and protect the device and not the fuse-
link. Also there should be no damage to the starter. The contactor breaking
capacity should be greater than Ic. For overload currents more than Ic, the
fuselink should operate before the relay protect the circuit. Here the fuse link is
selected based on its time current characteristics and the time-current
characteristics of the overload relay.
(d) Low power loss:
One of the most important factors while selecting a HRC fuse-link is its power-
loss. The rated power-loss of the fuse-link is the power-loss value as stated by the
manufacturer when the fuse-link is carrying its current under specified conditions.
The fuse-link selected should preferably have low power-loss.
The advantages of having such fuse-link with low power loss are:
• Saving in power / energy.
• Lower temperature rise at fuse-switch terminals and fuse-knives.
Type 2 co-ordination:
IS/IEC standard actually defines different levels of protection for system’s motor
starters after a fault or over current has occurred. These levels are defined as
nType 1n and nType 2n co-ordination.
For Type 1 co-ordination, the standard states that under fault conditions
the contactor or starter shall cause no danger to persons or installation and may
not be suitable for further service without repair or replacement of parts.
For Type 2 co-ordination, a higher level of protection, the IS/IEC standard
states that under fault conditions the contactor or starter shall cause no danger to
persons or installation and shall be suitable for further use. The risk of light
contact welding is noted and each manufacturer should provide direction to
92
maintain their equipment. Type 2 protections reduces fire and safely hazards,
minimizes disruption in production and system downtime, minimizes replacement
costs, provides solutions for no damage protection requests and provides simple
starter selection.
Obtaining type 2 protection: (p,q, r current tests):
Each combination of contactor, overload relay, and short circuit protective
device must be evaluated and tested. The tests are as follows.
Discrimination test: (‘p’ test)
Discrimination testing verifies that the overload relay will protect from
over current conditions and that the fuse will protect under fault conditions. This
test must be performed to verify overload protection. The currents for the tests
shall be:
1. 0.75 Ic + 0%, -5% and (Relay must trip)
2. 1.25 Ic + 5%, -0% (Fuse must blow)
Ic being the current corresponding to the crossover point of the mean curves
representing the time current characteristics of the overload relay and the SCPD
respectively.
Low level fault test:
A discrete low level fault test is performed at rated voltage, related to
starter’s operating current. This is the more severe test on the starters and the test
that will most likely cause contact welding because of the slower operation of the
short circuit protective device.
Performance under short circuit: (‘r’ test)
Test at protective current nrn.
The test is performed at appropriate value of prospective test current nrn. The
value very with operational current of starter and are given in following table as
per IS 13947 (part 4/Sec 1): 1993/ IEC 60947 Part4, Sec.1
The circuit shall be adjusted to the prospective test current. The contactor
or starter and the associated SCPD or the combination or the protected starter,
shall then be connected in the circuit. The following sequence of operations shall
be performed:
93
• One breaking operation of the SCPD with all the switching devices closed
prior to the test.
• One breaking operation of SCPD by closing contactor.
3.7 FAULT LEVEL CALCULATIONS:
All important criteria for the proper selection of a circuit breaker or other
interrupting device for use at any point in an electrical network is the knowledge
of fault current likely to be available at that point.
Two types of fault currents need to be calculations.
• Min. fault current – This decides settings for short circuit protection.
• Max. fault current – This decides equipments withstand capacity. Proper
selection of breaking capacity for the circuit opening device in any
network result in more safe design of the system.
Type of faults:
1. Symmetrical fault:
- 3 phase fault
R
Y
B
2. Unsymmetrical faults:
- Line to line fault
- Double line to earth fault
- Line to earth fault
R
Y
B
E
94
R
Y
B
E
R
Y
B
E
Figure 3.58: Types of Fault
Typical oscillogram of a short circuit current. The short circuit current consists of
following two components. AC component & DC component. AC part of the fault
current is fed by the supply. DC part comes from sorted energy in inductances and
capacitance of network components. If fault occurs at zero on the sinusoidal wave
from, magnitude of DC components is maximum. AC components are
symmetrical and sinusoidal where as DC component is exponentially decaying.
The decay depends on the time constant of the system which can be given by ratio
X / R. AC components is superimposed on DC component. Thus short circuit
wave is initially disposed asymmetrically about the zero axis. DC components die
out in 3 to 4 cycles. This gives AC sinusoidal current as sustained fault current.
Figure 3.59: Current Characteristic
95
= Initial symmetrical rms S/C current
= Steady state rms S/C current
= Peak S/C current
= Initial Value Of decay in DC component
Sources of short circuit currents mainly are
1. Source of supply
- Transformers
- D.G. Sets (Information on DG set will be shared in next article)
2. Loads
- Induction Motor
- Synchronous Motors
Shunt Capacitors
3.8 AIR CIRCUIT BREAKERS:
A circuit breaker is an electrical device that opens and closes a set of electrical
contacts of an electrical circuit.
Figure 3.60: Circuit Breaker
Fig. 3.61 shows circuit breaker contacts in closed position.
Figure 3.61: Closed Circuit Breaker
RRRR YYYY
BBBB
RRRR YYYY
BBBB
96
Fig. 3.62 shows circuit breaker contacts in open position.
Figure 3.62: Open Circuit Breaker
Fig 3.63 shows a four pole circuit breaker.
The neutral is switched ON and OFF along with three poles in four pole circuit
breakers. This figure shows circuit breaker contacts in open position. Circuit
breaker is a mechanical switching device, capable of making, carrying and
breaking current under normal circuit conditions and also making, carrying for a
specified time and breaking currents under specified abnormal circuit conditions
such at those of short circuit. In air circuit breakers the contacts open and close
in air, at atmospheric pressure.
Fig 3.64 shows a simplified diagram of an air circuit breaker. The wires represent
the breaker’s connection to the main power circuit. The major parts of the circuit
breaker are a stationery contact, a movable contact, a spring and a latching
mechanism. The spring is compressed or charged and contacts are closed. The
latching mechanism holds the cotacts closed.
Figure 3.63: Four Pole Circuit Breaker
RRRR YYYY BBBB
NNNN
97
In the Fig. 3.65 the latching mechanism shown is operated manually by
pressing a button. After the button has been pressed, the latch has moved, and the
spring has discharged to force the contacts open. Any action that causes the
contacts to open is known as tripping the breaker.
Figure 3.64: Latching Mechanism for Circuit Breaker
Arcing:
When circuit breaker contacts are opened under load, an arc is drawn between
them. Arcing is undesirable because:
• It causes pitting of the contacts, leading to high resistance, and
• Current flow in the circuit continues as long as the arc remains.
Circuit breakers are designed to contain and rapidly extinguish arcing.
Effects of arcing:
Even if arcing is quenched quickly, the circuit breaker contacts will still become
pitted over a period of time. Pitting reduces the area of smooth contact surface,
resulting in increased resistance across the contacts. Current flowing through the
contacts can cause overheating, leading to further damage.
Circuit breaker contacts:
To prevent the effects of arcing from interfering with the operation of the circuit
breaker, two sets of contacts are used:
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• The arcing contacts close first and open last, making and breaking the
circuit.
• The main contacts carry the full load current.
Figure 3.65: Circuit Breaker Contacts
The main contacts are the breaker’s main current carrying conductors when a
circuit operates at normal load. The arcing contacts are connected in parallel with
the main contacts. They are designed to handle arcs that occur as the breaker
opens. The contacts associated with any one phase of a breaker are often referred
to as poles.
Fig.3.66 shows the position of the movable main and arc contacts as a
breaker trips. The main contacts open first. No arc occurs, because the primary
power circuit is still complete through the arcing contacts. They are thus
controlled by confirming its occurrence to the gap between the movable arcing
contacts and the stationary arcing contact. The arcing contacts are more rugged
and are made to withstand the heat of the arc. As the movable arc contacts pulls
away from the stationery contacts, the arc forms. At this point, the arc can be
extinguished.
Formation of Arc:
Circuit breakers operate by putting a set of contacts apart. As the contacts
separate, a gap forms between them. The current flowing through the circuit tries
99
to maintain the circuit by jumping the gap between the contacts. This results in an
arc, as shown in Fig. 3.67
Figure 3.66: Formation of Arcs
At normal room temperature, air is a very poor conductor of electricity. However,
when air becomes hot enough, it becomes a very good conductor. If the arc is not
extinguished, it will eventually vaporize the contacts and other circuit components
in its path. Even an arc that lasts only a fraction of a second may be able to cause
significant circuit and plant damage.
Factors in Extinguishing Arcs:
To extinguish arcs, breakers utilize three physical factors: Speed, distance
and cooling. In terms of speed, the rate at which the movable arc contacts separate
from the stationery arc contacts has a direct bearing on weather an arc is
extinguished. The faster the arc formation, because the air has less time to get hot
enough to maintain current flow between the contacts.
As the distance increases when the movable arc contacts separate from the
stationary contacts, an arc stretches, or elongates. Fig 3.68 is a simplified drawing
of an arc forming between a breaker’s movable and stationery arc contacts that
are closed together. Fig. 7.8 B shows the same attempt to sustain current flow
between the two contacts. Elongating the arc increases the chance of
extinguishing it, because the greater the distance an arc must travel, the greater
the voltage needed to sustain it.
100
Figure 3.67: Effect of Distance on an arc.
Arc Chutes: In circuit breakers, arc chutes are installed above the movable and
stationery arc contacts. The arc chutes provide a good conducting material to
direct the path of the arc. The arc chutes are surrounded by a material that
insulates the fins from one another. An arc is electrical current: so, as in any
current-carrying conductor, a magnetic field is produced around an arc as it forms.
The magnetic field will induce a voltage in conductors such as de-ion plate. This
induced voltage is opposite to the voltage that is forcing the arc to jump the gap
between the movable and stationary arc contacts. The voltage that is induced in a
single de-ion plate is not sufficient to overcome the arc’s voltage. The sum of the
induced chutes voltages opposes enough of the arc’s voltage to reduce current
flow as the arc passes through the rack of de-ion plates.
Figure 3.68: Arc Cuts
101
Since cool air is a good insular, mixing cool air with the heated air that occurs
when an arc forms will help cool and extinguish the arc. Cooler air is directed at
the arc’s path when an arc is confined in arc chute. An arc chute is a device that is
similar in operation to a simple chimney. The chute confines and directs the arc
and the air that surrounds it. Hot air rises within the chute, and cooler air is drawn
in at the base of the chute and directed t the arc. The cooler air will help cool and
extinguish the arc.
Type of Mechanism:
Breaker can be operated manually or electrically. In manually operated
breaker the closing is made independent of the force offered by the operator by
means of closing spring. An operator has to go to the breaker for nONn operation.
In electrically operated breaker the charging of the spring is by a motor.
The spring stores energy for closing and a closing command enables the breaker
to switch nONn. Spring charging motors are available to suit different control
voltages.
Figure 3.69: Different Breaker Mechanisms
1. MF
- Manually operated Fixed type
ACB
Manually Operated
Electrically Operated
Fixed Fixed
Drawout Drawout
102
2. MDO
- Manually operated Draw out type
3. EF
- Electrically operated fixed type
4. EDO
- Electrically operated Draw out type
Accessories:
1. Shunt release:
The latching mechanism shown in Fig. 3.70 is operated electrically by means of
device called a shunt trip. In this example, the shunt trip consists of a coil and a
movable metal plunger. Wires connect the coil to a separate control source. Often,
this control is actuated from a remote location.
Figure 3.70: Shunt Release of Breaker
Figure 3.71: Remote Control of Breaker
103
In Fig. 3.72, the breaker contacts are closed. In Fig. 3.72 the control circuit has
been activated. When the coil is energized, the shunt trip is activated. Magnetic
attraction draws the plunger up and extends the rod that moves the latch
mechanism tripping the breaker.
2. Under voltage release:
This release could be instantaneous or fixed time delay with different voltages,
Releases with time delay are used where transient voltage fluctuations are more.
Under voltage coils are continuously rated.
3. Earth fault releases:
These releases are generally required for giving E.F. protection. It will be
necessary to specify the percentage of earth fault setting and also to specify a 3
phase 3 wire systems or a 3 phase 4 wire systems.
4. Micro switches:
These can be incorporated in the ACB for given various trip and position in
dictations, indicating circuit load should not exceed the rated current value and
burden of the micro switches.
5. Auxiliary contacts:
Number of auxiliary contacts and rating are important from the point of view of
the maximum load and number of control contacts that can be wired through a set
of contacts.
6. Safety shutters:
These are required only in case of draw-out breakers. These shutters provide
protection against accidental contacts with the power circuit where the breaker is
isolated.
Indications:
If the breaker is made to communicate its status through certain
indications the pressure on the maintenance personnel can be eased. It will be
necessary to look into what indications the breaker can offer like:
1. ON / OFF
2. TRIP INDICATION
- OVER LOAD TRIP
104
- SHORT CIRCUIT TRIP
- UNDER VOLTAGE TRIP
- SHUNT TRIP
- EARTH FAULT TRIP
3. POSITION INDICATIONS
- SERVICE
- TEST
- ISOLATED
4. SPRING CHARGED / DISCHARGED
Locks and interlocks:
Consideration must be given to see the type of lock and interlocks that can
be provided.
Locks –
• Lockable trip push button.
• Service / Test / Isolated position.
Interlocks –
• Door interlock
• Between 2 or 3 circuit breakers.
Overload protection:
Figure 3.72: Overload Protection Latching Mechanism
If a circuit problem occurs so that current flow exceeds a pre determined value,
the bimetallic strip heats and bends. The direction of bending and the amount of
105
the bend are determined by the construction of the bimetallic strip. As the strip
bends, the latching mechanism is tripped and the contacts open.
Figure 3.73: Overload Protection
Short circuit protection:
Fig. 3.75 illustrates typical electro magnetic trip device connected to a
breaker. The trip device consists of a coil, connected to secondary side of the CTs,
and a movable metal core. The core is inside the coil. The core is held in place by
the spring, which acts a restraining mechanism. The latching mechanism consists
of two parts, an insulated latch and a metal trip bar.
Figure 3.74: Electromagnetic Trip Device
If an excessive amount of current flows through the circuit, the magnetic field
intensifies. The increased magnetic attraction draws the core into the coil,
overcoming the resistance of the spring. The metal trip bar is attached to the core
and it moves, forcing the latch to move, as shown in Fig. 3.75. This action trips
the breaker.
106
Figure 3.75: Short Circuit Protection
Selection of ACB:
1. Current rating
Current Rating is decided based on application
1) For source Protection [transformer & D.G. Set]
Transformer Rating in KVA
KVA x 1000
I Full Load = ---------------------
3 x VL
Most Transformers normally can be overloaded for 10% of its capacity.
2) For load Protection [P.C.C. feeder MCC feeder & loads]. Full load current
& its overloading capacity.
The nominal rating of ACB should be equal to more than source rating or
continuous current of load. Rating specified by the manufacturer as per IS are at
400C. Correction factors may have to be applied for higher ambient.
Considering above two factors the rating of ACB (A) is greater than or equal to
1.1 x I Full Load.
2. Rated Ultimate Short-Circuit Breaking Capacity: (Icu)
It is the value of ultimate short-circuits breaking capacity assigned to that
circuit Breaker by the manufacturer for the corresponding rated operational
voltage under specified condition. The prescribed condition according to the
specified test sequence does not include the capability of the circuit breaking to
carry its rated current continuously. It is expressed as the value of the prospective
breaking current in kA (RMS).
107
Rated service breaking capacity: (Ics)
It is the value of service short circuit breaker by the manufacturer for the
corresponding rated operational voltage under prescribed condition according to
the specified test sequence include the capacity of the circuit breaker to carry its
rated current continuously. It is expressed as the value of prospective breaking
current in kA (RMS), corresponding to one of the specified percentage of the
rated ultimate short circuit breaking capacity. Ics & Icu.rating should be greater
than or equal to IF Fault current calculated as per fault level calculation.
3. Making capacity:
Important condition arises when the ACB is closed on fault. When the
breaker is closed on such a fault the current rises from zero to fault level i.e. a
switching source takes place, putting the ACB to a more severe duty.
Making capacity depends on:
• Power factor
• Instant of switching
The multiplying factor ‘n’ for finding the making capacity can be found from the
following table 3.13
Breaking Capacity Std.P.F. Min. Making Capacity
I cn (Amps) CosØ (n x 1cn)
Icn < 1500
1500 < Icn < 3000
3000 < Icn < 4500
4500 < Icn < 6000
6000 < Icn < 104
104 < Icn < 2 x 104
2 x 104 < Icn < 5 x 104
5 x 104 < Icn
0.95
0.90
0.80
0.70
0.50
0.30
0.25
0.20
1.41
1.42
1.47
1.53
1.70
2.00
2.10
2.20
Table 3.14: Making Capacity
108
4. Type of protective teleases:
Depends on type of releases
1. Thermo-magnetic 2. Solid state
3. Microprocessor based 4. Numeric
In order to get more accurate protection and wider range of setting, the
Electronic releases were developed. These releases are capable of offering not
only, overload, short circuit protection but also earth fault protection without any
other coil or accessories. Like electromechanical release, they can also be
designed to give direct mechanical trip command to circuit breaker for cleaning
the fault. Besides,
They can give all protection with wider range of pick up setting and adjustable I-T
curve, i.e. with time delay. To protect the system against repeated overload fault,
release can have thermal memory also due to which breaker trips faster on such
faults.
For short circuit protection, these releases offer wider range of pick up
setting and also selectable variable time delay in milliseconds. Hence only one
variety of release is required in irrespective of location of breaker in the system
close or away from transformer and thus reduction in inventory carrying cost.
For earth fault protection also release offers wider range of pick up setting
and selectable variable time delay in milliseconds. Due to provision of time delay
system can be discriminated (time base discrimination)easily, and nuisance
tripping due to system disturbance can be avoided.
In order to understand on which fault circuit breaker has tripped
annunciator module is used. For achieving intelligent protection & co-ordination,
micro-processor is used in the release. The advantages of microprocessor are
more – accurate protection, fine range of settings, flags for displaying the type of
fault, communication between two releases and self diagnostic of circuitry.
These releases also offer overload and short circuit protection with time delay,
instantaneous and earth fault protection. The salient features of various
protections with microprocessor-based releases are:
109
• The overload protection with fine range of pick up setting and to
protect the system against repeated overload faults, release has
selectable thermal memory i.e., this memory can be blocked also when
not required.
• The short circuit protection is also with fine range of pick up setting
and also wide adjustable time delay in milliseconds. This type of
release can give blocking signal to upstream release of same make,
thus helps in achieving better time base co-ordination between circuit
breakers.
• An additional, instantaneous protection (short circuit) with wider range
of pick up current having no intentional time delay trips circuit breaker
on very high magnitude fault.
• The earth fault protection with fine range of pick up setting and also
adjustable time delay in milliseconds. Due to provision of time delay
and communication, it helps in achieving better time base co-
ordination between circuit breaker.
In modern protection system the expectations form protection release is not only
to protect system against various faults but also to record the fault data,
measurement of various parameters, control of circuit breakers and computer
communication. The numerical (digital) release performs above mentioned
activities by using micro-processor along with memory for data storage. The basic
advantage of digital technology is inputs are converted in to digital values, then
compared with set values (which are digital) and finally appropriate digital output
signals are produced. This release offers following protections:
• Overload protection (as low set) with all features of microprocessor
based release.
• Short circuit protection (as High set) with additional facility of
Automatic Doubling of High set value during switching on the feeder
circuit breaker to avoid nuisance tripping of circuit breaker. These can
also give blocking signal to upstream any make numerical (digital)
110
release, thus help in achieving better time base co-ordination between
circuit breaker.
• Earth fault protection with all features of microprocessor based release
and with additional features like third harmonics filter to avoid
unwanted tripping of circuit breaker. Due to provision of time delay
and blocking function, it helps in achieving better time base co-
ordination between circuit breakers.
The setting of these releases is done by push buttons, which are on front panel of
the release. In addition to protection, the release display current value of all three
phases continuously in scrolling mode and the starting current of the feeder. It
also records on which fault circuit breaker has tripped, magnitude of fault current,
faulty phase and how many times faults has occurred in past. They have the
capability of communication with personnel computer or laptop through a RS 485
to RS 232 converter. These facilities to view all trip data on personnel computer,
one can do programming or setting from control room and the circuit breaker
control through personnel computer.
5. Rated Short time withstand Current (Icw):
It is the value of the current that the circuit breaker can carry for short
period (1 sec or 3 sec.) without damaging Circuit Breaker, under short circuit fault
condition.
6. Type of mechanism MF, MDO, EF, and EDO
7. Accessories
8. Indications
9. Locks and Interlocks
10. Ease of maintenance
11. Available of spares
111
Figure 3.76: ACB Layout
Typical layout of system (fig) Fig. shows typical layout of system. In this system
there are three breakers in series (A, B, C) when overload fault of 3200A occurs
at location F1, only Circuit breaker C trips. The CBs A, B, & C are coordinated
for overload protection because (current rating of A>B>C.)
Figure 3.77: CB Layout
When short circuit torque of 15 KA occurs at location F1. It is expected that C
circuit breaker should trip. But all three CBs trips. Thus the continuity of supply
to healthy feeders is affected; hence to overcome this situation two methods are
available.
• Documents base discrimination
• Time based discrimination
1) Current based discrimination:
For achieving current base discrimination short circuit pick up setting of
protection release of immediate upstream ACB should be greater than fault level
at the location of downstream ACB i.e. [circuit breaker A’s short circuit setting,
greater than circuit breaker B greater than circuit breaker C]
112
Figure 3.78: Current Based Discrimination
2) Time based discrimination:
For achieving time base discrimination short circuit setting of protection
release of all Circuit Breaker may be lesser than or equal to the fault current at
given location.
The minimum impulsion time (MIT) of protect ion release of immediate
upstream ACB should be greater than total operation time of downstream ACB.
e.g. if CBC’s MIT is 25ms & its total operation time is 90 ms than Circuit Breaker
B’s MIT should be greater than 90ms.
3.9 MCCB AND MCB:
Moulded case circuit breaker is a totally insulated air circuit breaker. It
can be used in distribution scheme in place of ACB. It can also replace a fuse
switch unit in a motor feeder.
(a) Protections in MCCB:
1. Overload protection
2. Short Circuit protection
Overload protection is given by bimetal strips Short Circuit protection is achieved
by magnetic actuation.
(b) Accessories for MCCB:
1. Under voltage release
2. Shunt release
3. Auxiliary contact block
4. Rotary operating handle
5. Remote operator
113
6. Earth fault relay
There are two types of MCCBs
1. Conventional MCCB
2. Current Limiting MCCB
1. Conventional MCCB :
In event of short circuit, the conventional MDDB, the fault is cleared in
about 15-20 msec. During this half cycle the entire system would under go
tremendous stress, at times resulting in damage to the downstream equipment.
2. Current Limiting MCCB :
Figure 3.79: Current Limiting MCCB
As per IS 13947/IEC 60947 (Part2): 1993 current limiting breaker should have
break time short enough to prevent the short circuit current reaching its
prospective peak value. To meet this requirement, the current limiting MCCB
must respond quickly in case of fault. To achieve high speed contact separation,
closely spaced contact fingers carrying current in opposite direction create a
strong magnetic repulsion between the conductors. High speed contact separation
is actually produced by electromagnetic repulsion forces generated by the fault
current itself. The higher the current, the greater is the force pushing the contact
apart. Although rapid contact opening is important, just opening the contact
quickly is not enough. The next concern is to control the arc voltage across the
contacts to ensure proper arc extinction. This is accomplished by forcing the arc
Upper Contact
Repulsive Force
Incoming Terminal Outgoing Terminal
Lower Contact
114
into the chute and is cooled and broken into segments in the arc chute until it is
de-ionized and ceases to conduct current, thus being extinguished
MCCB for motor protection:
Figure 3.80: Fuse and MCCB as SCPD
Fuse as SCPD:
Fuses are traditionally used as SCPDs because of the low initial capital
cost and there capability to interrupt faults faster. Fuses have excellent current
limiting characteristics and can be applied on systems having high fault levels
using fuses for short circuit protection is found to be the easiest and most effective
way of achieving Type 2 co-ordination,
Comparison of fuses with MCCBs:
M M
Relay
Contractor
Fuse as
SCPD
MCCB as
SCPD
115
One constraint against MCCB is the initial cost when compared to the cost
of SFU / FSU. However with MCCB as SCPD following are the factors which are
superior to fuse links.
• Ease of maintenance
• Reduced down time
• Simplicity of operation
• No recurring costs.
Current limiting MCCBs do have a number of practical advantages over fuses if
one considers these to be worth the extra cost. These advantages will have to be
weighed against the advantages offered by fuses like low initial cost, very high
breaking capacity, very low peak cut-off current and let-through energy, etc.
A fuse is better than a current limiting MCCB in providing protection
against the effects of short circuits at very high fault levels. But in most of the
cases, the short circuit currents are limited by the contactor, relay, cable and
source impedance, generally upto 30 times the rated current of the motor. In such
events, the MCCB acts faster than any other type of protective device available.
MCCB selection criteria:
In selecting MCCBs for motor protection,the following points are to be taken care
of
• The thermal rating of MCCB should be greater than or equal to the
motor full load current.
• The breaking capacity of the MCCB should be greater than or equal to
the prospective fault current at its installation point.
• The magnetic threshold of the MCCB should be selected In such a way
so as to avoid nuisance tripping during starting of the motor.
• The contactor should be able to break any current up to the magnetic
threshold of the breaker.
• The starter should be able to withstand the let-through energy of the
breaker.
• The selection of MCCB can be done on the basis of application. There
are certain applications where we need high breaking capacity medium
116
breaking capacity and low breaking capacity MCCBs. In motor feeder
application we need only short circuit protection from MCCB.
• For protection sources there are MCCBs suitable for transformer
protection and stand by DG set protection. The special application like
monitoring of HRC fuses.
Following are the functions a MCB can do:
Figure 3.81: MCCB Functions
Advantage of MCB:
• It can be used by skilled/unskilled workmen.
• The overload and short circuit settings can not be tempered since they
are non adjustable.
• Mechanism of MCB is trip free.
• It can be used as a functional switch.
• MCB can be used as an isolator.
• It’s a fully enclosed unit and hence no ageing problems.
• MCB is a cost effective device.
Motor Protection Breaker:
Conventional motor feeder: Conventional Motor feeder consist of SDF &
fuse link switching isolation & short circuit protection purpose OLR for overload
protection & contactor for remote operation & no volt protection as well as
frequent ON/OFF operation though it is possible to achieve co-ordination between
the equipments it is very difficult to maintain even a small change in the current
rating may effect complete co-ordination & to be specific (type-2) co-coordinator.
117
Thus the very purpose of providing “Type-2” co-ordination gets defeated cause of
simple but unavoidable reasons the using wrong letting of use cause unavailability
of correct single rating of fuse in the stock/ store or replacing relay / contractor
rating.
To achieve and maintain a temper proof ‘type-2’ co-ordination motor
protection circuit breaker is the solution. Fig. 3.83 and 3.84 shows the ‘type-2’ co-
ordination graph for conventional motor feeder and for MPCB motor feeder.
Figure 3.82: Motor Protective Breaker
Protections offered by MPCB:
• Overload protection.
• Short Circuit protection.
• Phase loss sensitivity.
• Under voltage protection – if a contractor is used with MPCB.
118
Figure 3.83: MPCB and Contactor
The motor protection circuit breaker can be connected in 2 ways, as stand alone
application and in association with contactor as shown below.
Figure 3.84: Fuse and Relays Characteristic
To achieve type-2 co-ordination with MNPCB is much simpler as compare to fuse
relay combination as in case of conventional motor feeder. Fig. 7.8 shows the
type-2 characteristics for conventional motor feeder and fig. 7.9 shows
characteristics with MPCB. It is clear that in case of MPCB since both overload
and short circuit protection are offered by single unit, to achieve and maintain
type-2 co-ordination is simpler.
119
Figure 3.85: Coordination Characteristic
Applications of MPCB:
• Stand alone motor protection
• DC switching
• Fuse Monitoring
A special MPCB is used for monitoring fuses in the system. The current rating of
this MPCB is very low and magnetic threshold is also set at low value of 1A or
lower. This is required to be connected as shown in fig. 3.87.
Fuse monitoring MPCB operates whenever any of the fuse in line blows as
the current in that line gets diverted thru MPCB. The auxiliary contact associated
with MPCB can be used for audio/visual alarm, thus announcing single phasing
condition. In case of healthy conditions the current passes thru fuses as it offers
least resistance path.
3.10 CURRENT TRANSFORMERS:
OBJECTIVE:
In this chapter we would learn:
Selection of current transformer for metering & protection application.
A current transformer (CT) is used to transform primary current quality in terms
of its magnitude and phase to a secondary value such that in normal condition a
secondary value is substantially proportional to the primary value for measuring
and protective application.
120
Figure 3.86: Current Transformer
The advantages of using CTs are
1. Moderate size instruments are used for metering
2. Low power consumption
3. Meter & relay circuit is electrically isolated
4. Instruments & meter can be standardized so that over all saving in cost.
Depending upon the construction the CTs can be classified as:
1. Wound type ACT having a primary winding of more than one full turn wound
on core
2. Bar type ACT in which primary winding consists of a bar of adequate size &
material forming an integral part of the current transformer
3. Ring Type ACT as an opening of circular safe to accommodate primary
conductor thru it
Definitions:
1. Actual transformation ratio: The ratio of the actual primary current to the
actual secondary current.
121
2. Ratio Error: The error which transformer introduces into the measurement of a
current and which arise form the fact that the actual transformation ratio.
(Kn Is – Ip)
% Ratio Error =-------------------- x 100
Ip
Kn = Rated Transformation ratio.
Ip = Actual Primary Current.
Is = Actual Secondary Current.
3. Rated Transformation Ratio: The ratio of the rated primary current to the rated
secondary current.
4. Phase displacement: The difference in phase between the primary and
secondary current vectors, the direction of the vectors being so chosen that the
angle is zero for a perfect transformer. The phase displacement is said to be
positive when the secondary current vectors leads the primary current vectors.
It is usually expressed in minutes.
5. Accuracy Class: A designation assigned to a current transformer. It states that
the error remain within specified limits.
6. Rated Burden: The burden on which accuracy requirements are based is called
as rated burden. The burden is usually expressed as the apparent power in VA
absorbed at a specific p.f. and it the rated secondary current.
7. Rated Secondary current: It is the value of the secondary current on which the
performance of the C.T. is based on secondary current can be 1 Amp. Or 5
Amp. The 1 Amp secondary current of C.T. is used for large length of
secondary cable where as 5Amp C.T. is used for shorter length of secondary
cables. CTs are broadly classified in three types
• Measuring CT
• Protection CT
• Class PS CT
122
1. Measuring CT:
CT intended to supply indicating instruments, integrating meters and similar
apparatus is called measuring current transformer. Measuring CTs need to
perform very accurately but only over the normal range of load up to, say, 120%
full load current. Measuring CTs are specified in terms of:
• Ratio
• Rated VA secondary burden
• Accuracy class
Accuracy classes recognized by IEC 185 are 0.1, 0.2, 0.5, and 1, Accuracy classes
3 and 5 are available from manufacturers. For each class. The ratio and phase
angle error must be within specified limits at 5, 20, 100, and 120% of rated
Primary current.
A class 0.2 measuring CT mean that at 100 – 120 % of the rated current
the percentage ratio error will be+ 0.2% i.e. for a class 0.2 CT with a rated
secondary current the of 5A the actual secondary current would be 5A + 0.01A.
Phase displacement error is also specified in the IEC standard. For special
applications an extended current range up to 200% may be specified. Above these
ranges accuracy is considered to be unimportant since these conditions will only
occur under abnormal fault conditions. There is an advantage in the CT being
designed to saturate under fault conditions so that the connected metering
equipment will have a lower short – time thermal withstand requirement.
Accuracy
Class
+ Percentage Current Error AT % of
Rated Current
5 20 100
120
+ Phase
Displacement in
Minute At 100%
Rated Current
0.1
0.2
0.5
0.4 0.2 0.1
0.1
0.75 0.35 0.2
0.2
5
10
30
123
1.0 1.5 0.75 0.5
0.5
3.0 1.5 1.0
1.0
60
Table 3.15: Accuracy Class and Rated Current
The CT is to be marked by rated output followed by the accuracy class e.g. 15 VA
class 0.5 or 15/0.5
2. Protective Current Transformers:
A current transformer intended to supply protective devices (relays, trip
coils) is called as protective current transformer.
Protection CTs, unlike measuring CTs may be required to operate at many
times full load current. Line rarity under these conditions is not of great
importance. The essential point is that saturation must be high enough to drive the
magnetizing current and the secondary current under fault conditions. Various
Terms are used in connection with protection CTs and these are described below.
1. Composite Error : The rms value of the difference (Kn is – ip) integrated over
the cycle under steady state condition gives the composite error
100 T
Composite Error = ---------- --------- (Kn Is - Ip )2 dt
Ip T
Ip = Rms value of primary Current
T = Duration of one Cycle in Sec.
Kn = Rated Transformation ratio
Is = Instantaneous secondary crrent
Ip = Instantaneous primary current
The composite error is generally expressed as a % of the rms value of the primary
current.
124
2. Rated accuracy limit primary current: It is the value of the highest primary
current up to which CT will comply with the specified limits of composite
error.
3. Accuracy Class: The accuracy class shall be designed to the highest
permissible percentage Composition error at the rated accuracy limit primary
current prescribed for the accuracy class concerned followed by the letter “P”
(meaning protection). The standard accuracy class for CT shall be 5P, 10P,
15P
4. Accuracy limit factor: To designate a protective CT standard accuracy class is
allowed by accuracy limit factor.
Rated limit primary current
Accuracy limit factor = ----------------------------------------
Rated primary current
The standard accuracy limit factor shall be 5, 10, 15, 20 and 30.
Hence the protective CT designation becomes
Figure 3.87: Accuracy Limit Factor
Accuracy Class 5P 10P 15P
Current rated At Rated Ip (in %) +1 +3 +5
Phase Displacement at Rated Ip
(In Minutes)
+60 --- ---
Composite Error at Accuracy
Limit Primary Current
5 10 15
Table 3.16: Limits of Error for Standard Accuracy
30 / 5P 10P
VA Burden
Accuracy Class
Accuracy Limit Factor
125
Limits of error for standard accuracy class are as shown in table 3.15.
3. Class PS CTs:
When the operation of the relays is more precisely dependent upon the
magnitude and phase relationship of voltage and current. The CTs used are called
class PS CTs. e.g. restricted earth fault and differential protection.
For such applications CT characteristics are normally specified in terms of
1. Knee point voltage
2. Exciting current at knee point voltage
3. Resistance of secondary winding.
Knee point voltage:
It is that voltage appearing at the secondary terminals of the CT, with all
other windings being open circuited, which when increased by 10%, causes the
exciting current to be increased by 50%. A typical CT magnetizing characteristics
is shown in above figure.
Figure 3.88: Knee Point Voltage
3.11.POWER CABLES:
Selection procedure of power cables:
Electricity is the most versatile form of energy. It can be easily converted
into other form and is easier when it comes to transmission. Electrical wires and
cables are the vehicle in transmission of electricity.
Cable is a general name given to an insulated conductor or group of conductors
and is extensively used in indoor and outdoor distribution systems.
Fig. 3.90 show the cut section view of the cable
126
The first covering over the core (Conductor) is called insulation. Commonly used
cables in L T systems are as follows.
• PVC insulated cables
• Paper Insulated cables
• Cross linked polyethyl e n e cables (XLPE)
• Flame retardant low smoke cables (FRLS)
1) PVC INSULATED CABLES:
PVC cables consist of polyvinyl-chloride synthetic material with vinyl
chloride as a principal material. The main advantages of PVC cables are-High
dielectric & mechanical strength over a wide range of temperature Moisture,
Acids, Alkalies do not affect PVC insulated cables – cables can be bent to smaller
radius,- flame retardant cables, does not support combustion.
In PVC cables, serious damages can occur, when cables are subjected to
appreciably higher temperature, even for short period than those permissible for
continuous operation. Therefore the current rating of PVC cables are determined
not only by maximum conductor temperature admissible for continuous running,
but also by the temperature likely to be attained under conditions of excess
current, PVC cables are suitable for maximum conductor terperature up to 700C.
2) PAPER INSULATED CABLES:
The paper insulation consist of a number of layers of impregnated paper tape of
0.1 to 0.15 mm thickness. These cables are suitable for maximum conductor
temperature of 800C.
3) CROSS LINKED POLYETHYLENE CABLES (XLPE):
XLPE cables are being used extensively in power stations and in industrial plants.
These cables are ideally suited for chemical and fertilizer industries where cable
insulation is exposed to chemical corrosion or in heavy industries where severe
load fluctuations occur. The excellent bending properties permit the cable to be
used even under most difficult cable routing conditions and also is cramped.
XLPE cables permit maximum continuous conductor operating temperature
of 900C and short ckt. Temperature of 2500C. XLPE cables are having greater
short circuit withstand capacity than PVC or paper cables.
127
Other advantages of XLPE cables are
• Low dielectric loss and hence saving in running cost.
• Low weight and small bending radius required so laying and
installation of cable is very easy.
• High safely against mechanical damage and vibrations.
4) FLAME RETARDANT LOW SMOKE CABLES (FRLS):
FRLS cables are specially designed for building and installations where a high
degree of safety of personnel and equipment is desired. These cables are
especially recommended for use in the hospitals, theatre, underground trains,
industrial complex, and schools etc.
FRLS cables should fulfill the following four criteria.
1) Flame retardance
- Ability to restrict flame propagation
2) Low smoke emission
- Smoke emitted should not obstruct visibility
3) Low acid gas emission
- Gases which when combined with water produce high corrosive
acids which damages plant and equipment.
4) Low toxic gas emission
- Gases less injurious to health.
FRLS cables are available with PVC and XLPE based insulation. Continuous
current ratings and short circuit ratings of these cables are corresponding to those
for PVC and XLPE insulated cables.
Types of Cables:
Type Designation:
Codes Abbreviation
A
Y
W
Aluminium conductors
PVC insulation
Steel round wire armour
128
F
WW
FF
Y
AW
sm
nm
Re
Steel trip armour
Steel double round wire armour
Steel double strip armour
PVC outer sheath
Aluminium wire armour
Sector shaped conductor
Multi stranded circular conduct
Single stranded conductor
Table 3.17: Type Designation of Cables
Selection of Cable:
While selecting cable following parameters needs to be considered
1) Type of insulation: - Paper PVC, XLPE … etc.
Table 3.17 shows comparison of properties of insulating material used for low
tension cables.
2) Type of conductor: - Copper or Aluminium
Aluminium is the most common material used as the current carrying part of
power cables. Aluminium is cheap and lighter in weight in comparison with
copper, Handling of aluminium cable is more easy as compared to copper cables,
Generally in practice copper is used as a conductor in control cables.
3) Type of cable: - Armoured or unarmoured Single core or Multi core.
Type of cable to be used depends upon conditions of installation. For examples, it
there is no likely possibility of mechanical damage after laying of cable, cheaper
unarmoured cables can-be used instead armoured cables. Sometimes especially in
power stations, it would be economical to use single core cable instead of multi
core power cable on account of their low bending raddi.
129
4) Continuous current carrying capacity of the conductor by considering various
derating factors:
IS 3961-1967 Part I, II, III, IV, and V gives recommended current ratings for
Paper, PVC (Heavy Duty), Rubber, Polythene and PVC (Light duty) cables
respectively.
Tables 3.17 gives current rating for 1.1 KV grade PVC cables. Above current
ratings are given based on following assumptions.
a) Maximum conductor temperature 700C
b) Ambient air temperature 400C
c) Ground temperature 300C
d) Thermal resistivity of soil 1500C cm/Watts
e) Depth of laying 750mm
f) Thermal resistivity of PVC 6500C cm/Watt
If any of the parameter is different than above, the current rating of the
cable is to be corrected by applying respective rating factor.
Sr.No. Property XLPE PAPER PVC
01.
02.
03.
04.
05.
06.
Operating Temperature 0C
Emergency Overload
Temp 0C
Short Circuit Temp 0C
Dielectric Constant
Thermal Resistivity 0C
Cm/watt
Moisture Sebsitivity
90
130
250
2.35
350
Excellent
65/80
100
160
3.4
500
Poor
70
120
160
6.8
600
Good
130
07. Current carrying capacity
For 3 phase, 3 core
6.35/11kV cables in
ground
(a) 35sq. mm2 Al.
(b) 120 mm2 Al.
(c) 300mm2 Al.
120
240
385
100
205
335
105
200
330
Table 3.18: Comparison of Properties of Insulating Material
5) Voltage rating and permissible maximum voltage drop across the cable:
Cables are graded into various groups such as LT, HT, EHT cable. Select
the cable matching to required voltage level.
It is a good practice to design the cable from Transformer LT to main
distribution board in actual practiced voltage drop of 2 to 3%, keeping in mind
that another 2 to 3%, drop will occur from main distribution board to terminal
point.
In case of direct LT connection from electricity board, total voltage drop
should be limited to 3%.
Table 3.11 shows voltage drop in volts / km / Amp with respect to cross
sectional area of conductor.
6) Overload protection of cables:
Cables insulated with some thermoplastic material like PVC may sustain serious
damage when subjected, even for relatively short periods, to temperature which
are appreciably higher than those permissible for continuous operation.
Therefore current rating of PVC cables are determined not only by the
maximum conductor temperature admissible for continuous operation, but also by
the temperature likely to be attained under condition of excess current.
131
PVC insulated cables can safely withstand overload current 1.45 times its
continuous rating for the period of 4 hours.
ex motor rating = 75 H.P
Full load current of motor (ib) = 100 amp
Overload relay setting (In) = 100 amp
Minimum operating current of relay (12) = 100 x 1.2 = 120
Cable current rating (Iz) amps = 100 amps
As per IEE regulations 1.45 Iz > 12 i.r. 1.45 x 100 > 120
7) Short circuit rating of the conductor:
The cable selected shall be able to withstand the short circuit current till the time
protective devices like fuses, ACB’s MCCBs clear the fault. This time can be
maximum 1 sec. Table 12 gives permissible maximum short circuit ratings for
PVC cables. These values are based on full load conductor temperature prior to
short ckg. Is 700C.
For any other duration of short circuit the values given in table for 1 sec
should be divided by Vt, where t is the duration of short circuit. E.g. For 150
sq.mm Aluminium conductor PVC cable short circuit. Rating for 1 sec is 10.7 KA
So for 2 sec. it is 10.7 / 2 = 7.56 KA.
3.12. EQUIPMENT EARTHING:
Selection of cross sectional area of protective conductor
Selection of ELCBs
EQUIPMENT EARTHING:
In order to ensure safety in electrical installations it is essential that all
metal casings containing conductors (ex. Conduits, the frames of the Motors
and other applications) must be connected to general mass of earth. This is
known is equipment earthing or Grounding. To understand the utility of
earthing consider a non earthed Water Heater.
If fault develops causing contact between the conductor and the body, the
body becomes live with respect to earth by acquiring a potential equal to that
of the phase wire.
132
When a person touches the body of the applications, the fault current flows
through the persons body to earth. In this condition person gets shock.
Figure 3.89: Effect of Unearthed Equipment
If body is well earthed, its potential cannot rise appreciably and immediately upon
occurrence of a severe type of fault, a large current flows to the earth and earth
fault relay operates to isolate the faulty circuit.
In this case even if person touches the faulty enclosure, he does not get severe
shock, because majority of current flows through earth wire as well as persons
body resistance is considerably high as compared to resistance of earth
connection.
Figure 3.90: Current Flow for Properly Earthed Equipment.
133
Many portable electric tool such as drilling m.c. grinding m/c. … etc. also
have to be grounded.
Tools that have metal casings should be equipped with three pin plugs. Tge
third pin connects the casing to a earthing point. If an electrical tool is double
insulated, it does not have to have ground connection.
Use of battery operated portable tools is a very safe and convent practice.
Calculation for cross sectional area of protective conductors:-
Definitions:-
1. Exposed conductive part: A conductive part of electrical equipment which
can be touched and which is not normally live but which may become live
under fault conditions.
2. Protective Conductor (PE): A conductor required by some measures for
protection against electric shock for electrically connecting any of the
following parts:
- Exposed conductive parts:
- Extraneous conductive parts:
- Main earthing terminal:
- Earth electrode:
- Earthed point of the source or artificial neutral.
3. Earth Fault Current: A fault current which flows through and return to
source.
4. Protection Against Direct Contact: Prevention of dangerous contact of
person with live parts.
5. Protection Against Indirect Contact: Prevention of dangerous contact of
person with exposed conductive parts.
Calculations
(With regard to thermal stresses due to current of short duration.)
The following formula may be used to calculate cross section of protective
conductors necessary to withstand the thermal stresses.
134
I t
Sp = -------------
K
Where,
Sp = cross sectional area, in square mm
I = RMS value of a.c. fault current for a fault of negligible impedance which can
flow through the protective device in Amp.
t = Operating time of the disconnecting device in second
k = Factor dependent on the material of the protective conductor, the insulation
and other parts and the initial and the final temperature.
Selection of K value
Initial Temp. – 300C
Final Temperature - 2500C
Material of insulation – Bare conductor (busbars)
Material of conductor
Copper
K
176
Aluminium
Steel
110
64
Table 3.19: K Factor for Materials
Earth Leakage Circuit Breakers:
In our day to day life all of us would have come across the following events -
Housewives complaining about a shock while switching on the kitchen mixie of
washing machine.
- Carpenter while working with drilling machine gets a shock.
- Children getting shock while opening the bathroom taps.
All the above “ill effects” are mainly due to ground faults or current leaking to
earth through human body.
135
“ill effects” means beginning with an unpleasant sensation, it could end with
fatality .
Limiting Value Of Leakage Current On Human Body:-
On the basis of experiments carried out in India, it was decided that 30mA (peak)
should be the limit for leakage current. This is used on what is called as
unpleasant sensation.
Use of earth-leakage circuit breakers (ELCB) are an extremely effective way of
protecting human life and property. Leakage current of 300mA and above cause
insulation failure leading to electrical sparks which can spread major fires. Such
leakage can be quickly detected by ELCB and human life and property can be
saved.
Old and substandard wiring also lead to leakages. ELCB detects such leakage
currents and help us to rectify the fault and thus save the installation.
To ensure safety of life and property and minimize to the maximize possible
extent the wastage of power Indian Electricity Rules made it mandatory to use the
ELCBs.
I
Figure 3.91 – IE Rule about ELCBs
ELCB working Principle:-
Figure 3.94 shows the circuit of ELCB under normal condition. The current
flowing in the two wire is equal in magnitude and opposite in direction.
IE Rules - 1956 61-A Earth Leakage Protective Device:- The supply of energy to every electrical installation other than low voltage installation below 5KW shall be controlled by an earth leakage protective device so as to disconnect the supply instantly on occurrence of earth fault or earth leakage of current.
136
i.e. Ires = I1 + I2 = 0
Since the magnitude of fluxes will cancel each other, no voltage will be induced
on the secondary side of the Toroid.
Current flowing through toroid in healthy circuit
Ires = I1 + I2 = 0
Current flowing through toroid in circuit with earth fault 13
Ires = I3 + I1 = 12
Figure 3.92: ELCB Working Principal
However if an earth leakage fault is produced in the circuit, there will be
difference between the current flowing in the two lines and this differential will
produce in output in the secondary side of the toroid proportional to the
differential current.
i.e. Ires = I3 = I1 - 12
This output will cause the ELCB to trip. The tripping device will operate and
interrupt the circuit if the leakage current in the circuit/apparatus exceeds its and
rated sensitivity. It is most important that the line and neutral conductors are
passed through the ELCB A common cause of nuisance operation is the failure to
connect the neutral through the device.
137
ELCBs work just as well on three phase and neutral circuits, but when the
neutral is distributed it must pass through it. ELCB are also called as Residual
Current Circuit Breaker (RCCB)
RCCBs are not suitable for use on DC systems and unearthed networks.
RCCBs For domestic installation:
RCCBs can be installed in two ways
• Whole house protection.
• Selective house protection.
Whole house protection is provided typically by a distribution board where the
RCCB device serves as the main switch. Although very popular this suffers from
a disadvantage of all circuits are disconnected in the event of fault.
Selective protection can be provided by associating the RCCB with
identified high risk circuits.
The 30 mA sensitivity RCCB when installed protects a human being to the widest
extent. This RCCB trips in less than 30 ms. The risk associated with indirect as
well as direct contact with live parts is totally eliminated.
RCCB Selection:
Following parameters are generally to be considered for selection of right type of
RCCB
1) Sensitivity of Current
Sensitivity Application
30 mA
100 mA
300 mA
For protection against direct contact
For protection against indirect contact
For less sensitive protection suitable for large
installations having high level of leakage current.
Protection against electrical fire.
Table 3.20: RCCB Sensitivity and Applications
138
2) Current rating
Select the current rating of ELCB based on load current as 10, 16, 25, 32, 40, 63,
or 100 Amp.
3) No of Poles:
2 Poles – For Single phase circuits
4 Poles – For three phase circuits
In both the cases it may prove fatal unless proper precautions are taken. Proper
earthing is not the only answer.
3.13 INGRESS PROTECTION:
Selection of enclosure for electrical equipment
Ingress Protection
Right selection of enclosure for electrical equipment is essential because an
enclosure with inadequate protection shall result in
1. Reduction in the life of the equipment
2. Dangerous to human life.
The Ingress Protection (IP) for all low voltage enclosure up to 1000 V a.c. and
1500 V d.c. is defined in the identical fashion by the standards BSEN 60529 –
IEC 529 it comprises the letters IP followed by two character numerals:
The first character numerals:
Indicates the degree of protection provide by the enclosure with respect to
persons, also to the equipment inside the enclosure.
The second character numerals:
Indicates the degree of protection provide by the enclosure with respect
with respect to harmful ingress of water; a third character may be used to indicate
mechanical strength.
The First Character Numeral
Protection against Solid Substance
139
IP Test Short Description Definition
0 Non Protection No Special Protection
1
Protection against solid
objects greater than
50mm shall not be able
to touch the live parts
inside the enclosure
A large surface of the
body, such as a hand
(but no protection
against deliberate
access) solid object
exceeding 50 mm in
diameter
2
Protection against solid
objects greater than 12.5
mm shall not be able to
touch live parts
Fingers or similar
objects not exceeding
80 mm in length;
solid objects greater
than 12.5 mm
3
Protection against solid
objects greater than 2.5
mm shall not be able to
touch
Tools, wire, etc….
Of diameter of
thickness greater than
2.5mm. solid objects
exceeding 2.5 mm in
a diameter
4
Protection against solid
objects greater than 1.0
mm shall not be able to
touch
Wires or strip of
thickness greater than
1.0 mm; solid objects
exceeding than 1.00
in a diameter
140
5
Dust-Protected
Ingress of dust is not
totally prevented but
dust does not enter in
sufficient quantity to
interface with
satisfactory operation
of the equipment
The First Character Numeral
Protection Against solid Substance
IP Test Short
Description
Definition
0 Non Protected No Special Protection
6
Dust - tight No ingress of Dust
The Second Character Numeral
Protection Against Liquid Substance
IP Test Short
Description
Definition
0 Non Protected No Special Protection
141
1
Protected
against
dripping water
Dripping water
(vertically falling
drops) shall have no
harmful effect.
2
Protect against
dripping water
when titled up
Vertically dripping
water shall have no
harmful effect When
the 150C enclosure is
titled at any angle up
to 150 from its normal
position.
3
Protect against
spraying water
Water falling as spray
at an angle up to 600
from the vertical shall
have no harmful
effect
4
Protected
against
splashing water
Water splashed
against the enclosure
from any direction
shall have no harmful
effect.
142
5
Projected
against water
jets
Water protected by a
nozzle against the
enclosure from any
direction shall have
no harmful effect
The Second Character Numeral
Protection Against Liquid Substance
IP Test Short
Description
Definition
0 Non Protected No Special
Protection
6
Protected
against heavy
seas
Water from heavy
seas or water
projected in powerful
jets shall not enter the
enclosure in the
harmful quantities.
7
Protect against
the effect of
immersion
Ingress of water in a
harmful quantity shall
not be possible when
the enclosure is
immersed in water
under defined
condition of pressure
and time.
143
8
Protected
against
submersion
The equipment is
suitable for
continuous
submersion in water
under conditions
which shall be
specified by the
manufacturer.
Table 3.21: Ingress Protection