6-earting tn
TRANSCRIPT
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GroundingGrounding
Professor Ahdab Elmorshedy
The objective of a grounding system are:1. To provide safety to personnel during normal
and fault conditions by limiting step and touch
potential.
2. To assure correct operation of electrical/
electronic devices.
3. To prevent damage to electrical/electronicapparatus.
4. To dissipate lightning strokes.
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5. To stabilize voltage during transient
conditions and to minimize the probabilityof flashover during transients.
6. To divert stray RF energy from sensitive
audio, video, control, and computer
equipment.
1. CLASSIFICATION OF ELECTRICALSYSTEMS
• An electrical system comprises a source
of energy and an electrical installation.
• According to the relationship between
the source and Earth and between theexposed conductive parts and Earth, a
system can be classified as follows:
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a. TN System
• The system has one or more points of the
source of energy directly earthed and the
exposed and extraneous conductive parts of
the installation are connected only by
means of protective conductors to the
earthed point(s) of the source.
Extraneous: not forming an essential or proper part:
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TN systems are subdivided into the following:
• TN-C systems where the neutral and protective
functions are combined in a single conductorthroughout the system.
• TN-S systems where there are separate neutraland protective conductors throughout thesystem.
• TN-C-S systems where the neutral and
protective functions are combined in a singleconductor but only in a part of a system.
b. TT System• The system has one or more points of the
source of energy directly earthed and the
exposed and extraneous conductive parts of
the installation are connected to a local earth
electrode that are electrically independent of
the source earth(s).
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c. IT System
• The system has the source either unearthed
or earthed through a high impedance and
the exposed conductive parts of the
installation are connected to an electrically
independent earth electrode
2. PURPOSE OF GROUNDING
• The purpose of grounding is to connect
together all metalwork, other than that
intended to carry current, to the earth, so
that dangerous potential differences
cannot exist, either between different
metal parts, or between metal and earth.
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• The system is connected to earth at the
secondary winding of the supply
transformer, where one conductor, which
is usually the neutral, is connected to an
earth electrode, buried in the mass of
earth.
Advantages of Grounding
• The whole system is tied to the potential of the
general mass of earth, and cannot 'float' at
another potential.
• For example, we know that the neutral of our
system is at or very close to zero volts
(reference potential) and not above or below it
when becoming charged.
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• By connecting to earth metalwork not
intended to carry current, a path is provided
for leakage current which can then be
detected, and if necessary, can be cut.
Disadvantages of Grounding
• Cost - the provision of a complete system of
protective conductors, earth electrodes, etc., is
expensive.
• Safety - the argument is made that complete
isolation from earth will prevent shock from
exposed conductive parts because there is no
complete path for the shock current.
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• This approach ignores the effect of leakage
resistance and of phase-to-earth capacitance(the insulation behaves as the dielectric
between the lines and earth).
• In many situations, the combined impedance of
leakage resistance and earth capacitive
reactance is low enough to allow significant
shock current to flow.
GROUNDING FOR TN-S SYSTEM
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a. Principle
• The principle of 'earthed equipotential bondingand automatic disconnection of the supply in a
TN-S supply system' is adopted here to explain
the typical arrangement.
• In addition, the grounding practices are based
mainly upon the IEE Wiring Regulation (BS7671)
and the Code of practice for Grounding (BS7430).
• This method is generally applicable to preventthe occurrence of a voltage of such magnitude
and duration between simultaneously accessible
conductive parts that danger could arise.
• With this method, the characteristics of the
protective devices for automatic disconnection,
the earth arrangements for the installation and
the relevant impedances of the circuits
concerned shall be coordinated.
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• A system is satisfactorily earthed if the
protective gear operates to remove
danger in the event of a fault to any
metalwork having a continuous metallic
connection to the system neutral.
b. Earth Mat
• Earth mats are installed in each substation.
• The overall earth mat design including number
and location is to meet the maximum allowable
step and touch potentials.
• Earth electrodes are jointed together and brought
out to be the principle earth conductor.
• A bolted copper link is normally provided for
connecting and disconnecting the principle earth
conductor from the grounding network to
facilitate testing.
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c. Earth Electrode
Common types of earth electrodes or means
of grounding include:
(a) Vertical plates, rods & pipes
(b) Horizontal strip or round conductor
(c) Uninsulated metallic sheaths and armour of cables
(d) Underground structural steelwork
(e) Sheet steel piling and steel reinforcement of
concrete piling
•Water and service pipes, gas or drainage, should
not be used as the means of grounding but should be
bonded to the protective conductors.
• Material for an earth electrode shall be resistant to
corrosion in the type of soil in which it will be used.
• Copper is one of the better and commonly used
material.
• The resistance to earth of an electrode depends upon
the size and shape of the conductor and the
electrical resistivity of the soil in which it is installed.
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d. Grounding Network
• From the test link, a vertical grounding conductorrising main with an grounding terminal at aconvenient location in each level is provided.
• From the grounding terminal in each levelincorporating electrical equipment rooms, acopper tape protective conductor is routedthrough all electrical equipment rooms.
• For substation incorporating more than one earth
mat, a grounding conductor of copperinterconnecting the various earth mats is provided.
e. Equipotential Bonding
• Main equipotential bonding conductors shallconnect to the earth network for the largeextraneous conductive parts not alreadyearthed by circuit protective conductor.
• They include, but do not limited to, thefollowing:
(a) all external metallic service pipes.
(b) fire main pipes.
(c) ventilation, air conditioning and chilled waterductwork.
(d) steel floor plates.
(e) cable tray.
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f. Grounding Conductor
The grounding conductor of the earth mat should be(a) sufficiently sized and supported to carry without
danger the greatest earth fault currents
(b) sufficiently robust to withstand mechanical
damage and corrosion in the ground, and
(c) compatibility with the material of the earth
electrode
g. Residual Current Device (RCD)
• RCDs are to be installed when earth fault
current in a circuit is insufficient to cause
operation of the overcurrent protective
devices within the time required.
• RCD should be installed for every socket outlet
circuit.
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• The RCD should have the product of the ratedoperating current (in A) and the earth fault loop
impedance (in ) not exceeding 50 V and be
capable of disconnecting all the phase
conductors of the circuit.
• RCDs for socket outlet circuit, in addition to
requirements above, should have a rated
residual operating current not exceeding 30 mA.
• The RCD range offers both 2 and 4 pole devicesin 30mA - 500mA trip sensitivities, with main
current ratings of 25A, 40A, 63A and 100A
• The RCD protects against residual currents
(earth leaks), where a small current may leak
from the circuit due to bad insulation.
• Critically the residual current device will react to
leakage currents as low as 30mA.
• This is an essential protection requirement.
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Residual Current Device (RCD)
h. System Grounding
High Voltage Power Distribution
(1) The secondary star point of the distribution
transformers is solidly earthed by direct
connection to the substation earth network with
copper conductors of sufficient size.
(2) The sheath and armour of all high voltagedistribution cables is bonded to the earth bar or
terminal of the switchgear panel or transformer
at each termination.
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Low Voltage Power Supply
(1) The star point of the service transformers isearthed by connection to the local earth networkwith copper tapes of appropriate size.
(2) The phase and neutral conductors of thedistribution circuits are provided with a circuitprotective conductor.
The protective conductors are bonded to thegrounding terminal or earth bar of the
switchboard, control panel or to which they areconnected.
Equipment Grounding
• The objective of electrical equipment
grounding is to ensure effective operation of
the protective gear in the event of earth fault
currents that might otherwise cause damage
to property, and to protect against danger to
life through shock due to installation
metalwork being maintained at a dangerouspotential relative to earth.
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HV & LV Switchgear
(1) All switchgear should be provided with a copperearth bar of sufficient size, running the full length
of the switchboard.
(2) All metal parts, other than those forming part of
an electrical circuit should be connected to the
earth bar.
The protective conductors of incoming and
outgoing cables should be bonded to the earth bar.
(3) Busbar and circuit grounding devices are
provided for all HV switchgear.
4) The earth bars of HV and LV main switchboards
are bonded to the substation earth network via
separately routed copper grounding conductor
connected at each end of the earth bar.
(5) Fuseboards and MCB boards are equipped with a
single earth terminal for connection of all
conducting parts, which do not form part of an
electric circuit.
The terminal is bonded to the earth network by
copper conductor.
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5. SPECIAL APPLICATION AND
SITUATION OF GROUNDINGa. Lightning Protection
• Protection of electrical equipment in a building
against induced voltages and possible side flash
can be provided by bonding the grounding
conductor of the lightning system to the main
grounding terminal of the electrical installation.
b. Clean Grounding
• If required, it should be directly connected to the
principle earth conductor isolating link using
insulated copper conductor and separated from the
grounding network and power cables to minimize
interference.
• The distribution of the insulated clean earthconductor should be in the form of star topology
or alike in order to avoid electrical noise loop back
phenomenon.
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c. Corrosion Due to Interconnection With
Another Metallic Item
• The possibility of damage to underground
services in the vicinity of earth electrodes, to
which the grounding system is to be bonded,
due to electrolytic action between dissimilar
metals should be considered.
• The rate of corrosion depends on the metals
involved and to some extent on their relative
surface areas.
• Galvanized steel is strongly electronegative to
both copper and steel in concrete so that an
earth electrode of bare galvanized steel
should not be bonded to either of them.
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d. Grounding Cathodically Protected Structures
• Cathodic protection is normally applied to
wholly or partly buried ferrous structures in
order to counteract electrolytic corrosion.
• Such a protection system relies on the
metalwork being maintained at a slightly
more negative potential with respect to theground than it would exhibit if it were
unprotected.
• If the cathodically protected structure has to
be earthed for any reason, earth electrodes of
bare copper should not be connected directly
to the structure.
• The bare copper is strongly cathodic to
ferrous materials and may require a quite
unacceptable current drain if the protection
is to be maintained.
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• In the event of failure of the source of protective
current, a copper electrode will accelerate the
rate of corrosion of the structure.
• If copper electrodes are used, the connection to
the structure should be made through a
polarization cell.
• This will drain only a small current from the
cathodic protection source, but will pass
alternating current with a low voltage drop.
TESTING AND MONITORING
Tests should be carried out on the earthing system to
determine its effectiveness at the completion of
installation and periodically after installation at a preset
interval.
The tests include the following items:
(a) Earth resistance of each earth mat.
(b) Earth fault loop impedance measurement.
(c) Touch voltage and step voltage measurement.(d) Interference voltage measurement.
(e) Inspection on the integrity of earthing conductorsand all associated connections.