electrical earthing · grounding methods 5 • petersen coil grounding (arc suppression) •...
TRANSCRIPT
ELECTRICAL
EARTHING
SPECIAL NOTE:
•THE APPROACH HERE IS BASED ON STANDARD IEEE 80
(SAFETY IN AC SUBSTATION GROUNDING)
•THESE DISCUSSIONS ARE FOR ILLUSTRATION ONLY
•GROUNDING PRACTICES ARE SUBJECT TO LOCAL
REGULATIONS/CODES WHICH WILL TAKE PRECEDENCE
OBJECTIVES OF GROUNDING:•PROVIDES AN ELECTRICAL SUPPLY SYSTEM WITH A
REFERENCE TO THE GROUNDMASS (SYSTEM GROUNDING)
•PROTECTIVE GROUNDING OF ELECTRICAL EQUIPMENT ENCLOSURES• MAKES THEM SAFE TO PERSONS WHO MAY COME INTO CONTACT WITH THEM • ENABLES THE FLOW OF FAULT CURRENT IN THE EVENT OF A FAILURE
•PROVIDES A LOW IMPEDANCE PATH FOR ACCUMULATED STATIC CHARGES AND SURGES (LIGHTNING PROTECTION GROUNDING)
•HELPS IN MITIGATING THE GENERATION AND PROPAGATION OF NOISE (GROUNDING OF SHIELDS AND SIGNAL REFERENCE PLANES)
EARTHING SYSTEMSHALL SATISFY SAFETY, FUNCTIONAL REQUIREMENTS OF
ELECTRICAL INSTALLATION
SHALL ENSURE
• PROTECTION AGAINST INDIRECT CONTACT
• PROPER FUNCTIONING OF ELECTRICAL PROTECTIVE DEVICES
• PROTECTIVE AND FUNCTIONAL REQUIREMENTS ARE MET UNDER EXPECTED
CONDITIONS
• EARTH FAULT, EARTH LEAKAGE CURRENTS CAN BE CARRIED SAFELY
• ADEQUATE STRENGTH APPROPRIATE TO EXTERNAL INFLUENCES
• ADEQUATE VALUE OF EARTHING RESISTANCE
BENEFITS (1)
•FAULT DAMAGE NOW MINIMAL
• REDUCES FIRE HAZARD (ESPECIALLY IN MINES)
•LOWER OUTAGE TIMES
• LESS LOST PRODUCTION, LESS LOST REVENUE
•TOUCH POTENTIALS KEPT WITHIN SAFE LIMITS
• PROTECTS HUMAN LIFE
BENEFITS (2)
•LOW FAULT CURRENTS REDUCE POSSIBILITY OF IGNITING GASES•MINIMIZES EXPLOSION HAZARD
•LOWER MAGNETIC OR THERMAL STRESSES IMPOSED ON PLANT DURING FAULT
•TRANSIENT OVERVOLTAGES LIMITED• PREVENTS STRESSING OF INSULATION, BREAKER RESTRIKES
FAULT IN AN UNGROUNDED SYSTEM:
EFFECT OF NEUTRAL (SYSTEM) GROUNDING:
GROUNDING METHODS 2
SOLID GROUNDING• NEUTRAL CONNECTION ON
GENERATOR / TRANSFORMERIS CONNECTED TO EARTH BY ASOLID CONDUCTOR
• COST REDUCTIUONS DUE TOAVOIDANCE OF SENSITIVERELAYS AND GROUNDINGDEVICE, GRADING OFINSULATION TOWARDSNEUTRAL END.
• BUT CIRCULATION OF THIRDHARMONIC CURRENTSBETWEEN NEUTRALS
GROUNDING METHODS 3
•RESISTANCE GROUNDING
• NEUTRAL CONNECTION ON
GENERATOR / TRANSFORMER IS
CONNECTED TO EARTH (0V)
THROUGH A FIXED RESISTANCE
TO LIMIT THE EARTH FAULT
CURRENT
• MAINLY USED BELOW 33 KV
• FULL LINE TO LINE INSULATION
REQUIRED TOWARDS NEUTRAL
GROUNDING METHODS 4
•REACTANCE GROUNDING
• NEUTRAL CONNECTION ON
GENERATOR / TRANSFORMER
IS CONNECTED TO EARTH (0V)
THROUGH A FIXED REACTANCE
TO LIMIT THE EARTH FAULT
CURRENT
• CAN BE CHEAPER COMPARED
TO RESISTANCE
GROUNDING METHODS 5
•PETERSEN COIL GROUNDING
(ARC SUPPRESSION)
• NEUTRAL CONNECTION ON
TRANSFORMER IS CONNECTED TO
EARTH (0V) THROUGH A VARIABLE
REACTANCE TO NEUTRALISE THE
CAPACITIVE EARTH FAULT CURRENT.
RESULTS IN ARC EXTINCTION
GROUNDING METHODS 6
• NEC GROUNDING (WITH AND WITHOUT RESISTANCE)
• IN HV DELTA SYSTEMS NO EARTH CONNECTION IS AVAILABLE. A 3 PHASE NEUTRAL GROUNDING COMPENSATOR IS CONNECTED TO ALLOW EARTH FAULT CURRENTS TO FLOW - ALLOWING DETECTION OF THESE FAULTS
PROTECTIVE GROUNDING
•PROTECTS PERSONNEL AGAINST SHOCKS
•PERSONNEL DO NOT EXPERIENCE DANGEROUS HIGH VOLTAGES
WHEN CONTACTING ENCLOSURE ACCIDENTALLY CONNECTED
TO LIVE PARTS
•PROVIDES A LOW IMPEDANCE PATH FOR ACCUMULATED
STATIC CHARGES AND SURGES (LIGHTNING PROTECTION
GROUNDING)
•HELPS IN MITIGATING THE GENERATION AND PROPAGATION OF
NOISE (GROUNDING OF SHIELDS AND SIGNAL REFERENCE
PLANES)
IMPORTANCE FOR EARTHING
An Electrical equipment is considered dead when
• At or about zero potential
• Disconnected/ Isolated from live system
• Disconnection alone not adequate
• Can retain stored charge
• Can acquire a static charge
• Can accidentally be made alive
• Nearby live conductors may induce voltage
IMPORTANCE OF EARTHING! Ensure earthing before working on electrical equipment
Earthing
• Connect apparatus electrically to general mass of earth in such a
manner as will ensure at all times an immediate safe discharge of
electrical energy
• Connect to earthed metal earth bar or spike with good metallic
conductor
Earthing by
• Closing of earthing links
• Attaching of fixed earthing devices
• Affixing of portable earthing straps
IMPORTANCE OF EARTHING
Ensure before applying earth
• Earthing connection is mechanically, electrically in
good condition
• No broken strands
• Clamps should be rigid and without defect
• Applied properly in intimate contact with conductors
and earth-bar/ spike
• Earthing cable tails as short as possible
• Connect to earth first when installing earthing,
disconnect earth last while removing earthing
HAZARDS OF IMPROPER EARTHING
• Electrocution
• Burns from arcing
• Electric shock leading to falls
BONDING
•CONNECTING OF VARIOUS GROUNDING SYSTEMS AND
NON CURRENT CARRYING PARTS
•TO ACHIEVE POTENTIAL EQUALIZATION BETWEEN
DIFFERENT ACCESSIBLE CONDUCTING SURFACES
•POTENTIAL DIFFERENCE BETWEEN DIFFERENT
ACCESSIBLE CONDUCTING SURFACES, DIFFERENT
GROUNDING SYSTEMS HAZARDOUS
TYPICAL EARTHING SYSTEM
TOUCH, STEP, TRANSFERRED VOLTAGES
An Earth Leakage Circuit Breaker (ELCB) is a
device used to directly detect currents
leaking to earth from an installation with high
earth impedance to prevent shock.
Typical RCB circuit(Three
Phase):
This relay causes tripping of the associated circuit breaker to disconnect the power supply to the
equipment under protection
CONDUCTORS
-
--
-
High Resistance
-
-
-
-
-
-
- -
--
-
-
-
Low Resistance
A conductor has many free electrons so is good at transferring electrical
current
Good Conductor Bad Conductor
Conductance is the opposite of resistance
It is measured in ‘Mho’s (ohm backwards) ℧
DIFFERENT TYPES OF CONDUCTORS
Material Used
Copper
High conductivity
Easily soldered
Heavier & more expensive
than aluminum
Copper used in house
wiring
1mm2, 1.5mm2
4mm2 , 6mm2
Aluminum
60%
conductivity of
copper
Cheap &
lighter than
copper
Lowest
conductivity
Heavier than
aluminum
Galvanized Iron (GI)
Used in
overhead lines
DIFFERENT TYPES OF CONDUCTORS
Respective of their property
Good Conductors Bad Conductors
Medium
resistance
Used for
converting
electrical
energy into
heat, light &
sound
PVC, glass
High
resistance
Non Conductors
Carry current
Low resistance
Copper & Aluminum
Tungsten & Nichrome
Insulators
Wires & cables
use conductors
& non
conductors to
their advantage
DIFFERENT TYPES OF CONDUCTORS
Physical Appearance
Solid Conductor
Used in cables.
e.g. copper,
aluminum, steel
Stranded Conductor
Flexible
1.13 to 3.73 mm diameter
1, 7, 19, 37 stands
Multi stranded Conductor
0.2 or 0.3 mm diameter
14, 22, 24,84 strands
Flexible Conductor
14, 23, 40 strands
<0.2 mm diameter
WIRES & CABLES
Wires
Domestic & small industry wiring
In appliances
Cables
Small & big industries
Distribution Lines
Transmission lines
The size & type of wire/cable must suit the power rating required for their
use. The higher the power the thicker the wire/cable
Wires & Cables are purpose built conductors
TYPES OF WIRES
Vulcanized India Rubber (VIR)
suitable for: low &
medium voltage supply
only
tinned copper/ aluminumCotton tape & cotton
braiding
Bitumen
Vulcanized India Rubber (VIR)
To protect against corrosion
from the VIR
Old type: not
readily available
to purchase
TYPES OF WIRES
Cabe Tyre Sheath wire (CTS)tinned copper
Rubber/plastic
Thicker
Rubber/plastic
Don’t absorb moisture
Available in 250/440V only
Old type: not
readily available
to purchase
TYPES OF WIRE
PVC Wire
copper/ aluminum
Polyvinyl chloride (PVC)
Widely used
Long life
Durable against
water, heat, oil, UV
light
Available in 600,
660, 1100 Voltage
WIRING APPLIANCES
Earth
Takes current to ground if
appliance has fault
Live
Provides current to
appliance
Neutral
Returns current to
power source
What wire is missing and
why?
What do each of these
wires do?
STANDARD WIRE GAUGE & CURRENT CARRYING CAPACITY
Higher current will heat the
cable and damage the
insulation and may result in
short circuit
Each number
represents a size of
wire
Maximum safe
current that can flow
through a cable
CABLESLarger sized conductors
Type of insulation
Types of cable are sorted by:
Cotton covered
Silk coated
Asbestos covered
Rubber coated
PVC coated
Type of conducting material
Their shape
Unarmored
Armored
Voltage Grade
Low
High
Copper
Aluminum
Mechanical protection
Flat
Round
2. UNIT PRICE ESTIMATE
UNIT PRICES ARE OBTAINED FROM DATA ON
PROJECTS ALREADY PERFORMED.
COST OF LABOR, MATERIAL, AND EQUIPMENT FOR ALL
UNITS OF WORK ARE ADDED TOGETHER AND DIVIDED BY
THE NUMBER OF UNITS INVOLVED.
PROF AWAD S. HANNA
Elemental Estimate Analysis
Gross floor Area = 250,000 ft2
4. DETAILED (DEFINITIVE )ESTIMATE
PREPARED AFTER DRAWINGS AND SPECIFICATION
ARE COMPLETED.
REQUIRES A COMPLETE QUANTITY TAKEOFF BASED
ON DRAWING AND THE COMPLETE SET OF
CONTRACT DOCUMENTS
NEED INFORMATION ON LABOR RATE
"PRODUCTIVITY", MATERIAL COST, COST OF
RENTING OR PURCHASING EQUIPMENT
VARIATION FACTORS IN ESTIMATING1. TIME
WE BASE OUR ESTIMATE ON THE COST OF EXISTING
PROJECTS THAT WERE BUILT IN THE PAST
PRICE-LEVEL CHANGES OVER TIME
WE NEED TO PROJECT COSTS OF FUTURE PROJECTS
MANY ORGANIZATIONS PUBLISH CONSTRUCTION COST DATA
ON REGULAR BASIS:
US DEPARTMENT OF COMMERCE
US DEPARTMENT OF LABOR
ENR
TURNER CONSTRUCTION COMPANY
HANDY-WHITMAN UTILITIES
1. TIME (CONT.)
COST INDICES
PUBLISHED BY R.S. MEANS AND ENR
USED TO UPDATE OLD COST INFORMATION
USES
1. TO UPDATE KNOWN HISTORICAL COSTS FOR NEW ESTIMATES
2. TO ESTIMATE REPLACEMENT COST FOR SPECIFIC ASSETS
3. TO PROVIDE FOR CONTRACT ESCALATION
LIMITATIONS
1. THEY REPRESENT COMPOSITE DATA, AVERAGE OF MANY PROJECTS.
2. THEY FAIL TO RECOGNIZE TECHNOLOGICAL CHANGES.
3. THERE IS A REPORTING TIME LOG.
P
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A
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d
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.
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3. SIZE
Proposed Size
Comparison Size
E - 1
PROF AWAD S. HANNA
TABLE FORUNIT AND TOTAL COST MULTIPLIERS
Buildings
Complex
Project s
Project
Size
Fact or Buildings
Complex
Project s
1.175 1.904 0.2 0.235 0.381
1.128 1.619 0.3 0.338 0.486
1.096 1.443 0.4 0.438 0.577
1.072 1.320 0.5 0.536 0.660
1.052 1.227 0.6 0.631 0.736
1.036 1.153 0.7 0.725 0.807
1.023 1.093 0.8 0.881 0.875
1.011 1.043 0.9 0.910 0.939
1.000 1.000 1.0 1.000 1.000
0.991 0.963 1.1 1.090 1.059
0.982 0.930 1.2 1.178 1.116
0.974 0.900 1.3 1.266 1.170
0.967 0.874 1.4 1.354 1.224
0.960 0.850 1.5 1.440 1.275
0.954 0.829 1.6 1.527 1.326
0.948 0.809 1.7 1.612 1.375
0.943 0.780 1.8 1.697 1.423
0.938 0.774 1.9 1.782 1.470
0.933 0.758 2.0 1.866 1.516
0.928 0.743 2.1 1.950 1.561
0.924 0.730 2.2 2.033 1.605
0.920 0.717 2.3 2.116 1.648
0.916 0.705 2.4 2.199 1.691
0.912 0.693 2.5 2.281 1.733
0.909 0.682 2.6 2.363 1.774
0.905 0.872 2.7 2.445 1.815
0.902 0.662 2.8 2.526 1.855
0.899 0.653 2.9 2.607 1.894
0.896 0.644 3.0 2.688 1.933
0.893 0.636 3.1 2.768 1.972
0.890 0.628 3.2 2.849 2.010
0.887 0.620 3.3 2.929 2.047
0.885 0.613 3.4 3.008 2.084
0.882 0.606 3.5 3.088 2.121
0.880 0.599 3.6 3.167 2.157
0.877 0.593 3.7 3.246 2.192
0.875 0.586 3.8 3.325 2.228
0.873 0.580 3.9 3.404 2.263
0.871 0.574 4.0 3.482 2.297
Use This Side for
Unit Cost Mult ipliers
(UCM Met hod)
Use This Side for
Tot al Cost Mult ipliers
(TCM Met hod)
PROF AWAD S. HANNA
CHART TO CONVERT TOTAL PROJECT COST
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
0.1 1.0 10.0
Size Factor
To
tal
Pro
ject
Co
st
Mu
ltip
lier
(TC
M)
Buildings Complex Projects
MINIATURE CIRCUIT BREAKERS (MCB)MULTI 9
• PROTECT CABLES AGAINST
OVERLOADS AND SHORT
CIRCUITS
• LET THE CURRENT FLOW
WITHOUT OVERHEATING ON
CABLES
• OPEN AND CLOSE A CIRCUIT
UNDER RATED CURRENT
• PROTECT AGAINST
INSULATION
FAULTS (SEE EARTH LEAKAGE
PROTECTION MODULE)
DEFINITIONMain functions
Miniature circuit = switch + trip unit breaker (MCB) 4
Symbol
• A MECHANICAL SYSTEM TO OPEN AND
CLOSE THE CIRCUIT
• A BIMETAL STRIP TO ACT AGAINST
OVERLOADS
• A MAGNETIC TRIP TO ACT AGAINST SHORT
CIRCUITS
DEFINITIONA Multi 9 circuit breaker has:
Miniature circuit = switch + trip unit breaker (MCB) 4
THE TECHNOLOGYActive parts
C60N NC100 / NC125
terminal
fixed contact
moving
contact
strip braid
magnetic
trip
load side
terminal
terminal
bimetral strip
strip braid
moving
contact
magnetic trip
load side
terminal
strip braidbimetal strip
4
OVERLOAD PROTECTION
bimetal
tripping barbimetal
tripping bar
screw
calibration
strip
braid
NC45 NC100 / NC125
• A BIMETAL STRIP IS USED:
O TO PROTECT THE CABLE AGAINST OVERLOADS
O SYMBOL:
• PRINCIPLE OF THE BIMETAL: IN A MULTI 9 CIRCUIT
BREAKER4
• THERMAL TRIPPING
CURVE
• THE "THERMAL" PART
OF THE
TRIPPING CURVE MUST
BE IN
ACCORDANCE WITH
THE STANDARD
OVERLOAD PROTECTION
Legend:
In = rated current
Ind = no tripping current
Id = tripping current 4
tripping
no
tripping
destruction
of bimetalaccuracy
t (s)
I/InIn Ind Id