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Earthing configurationsfor
LT distribution systemsby
Anjuli Chandra
Director CEA
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Earthing for LT systems – Deficient A earthing system should be installed in a manner
that it will limit the effect of ground potential
gradients to such voltage and current level that willnot endanger the safety of people or equipmentunder normal and fault conditions, as well as assurecontinuity of service.
Earthing systems adopted at Generating Stations,Substations are well designed for safety
Substations usually have ground grid system withground mat as extra protection. Grid system has the
form of horizontally buried grid conductor,supplemented by a number of vertical ground rodsconnected to the grid.
It is however observed that this area is most often
neglected especially in the LT Distribution networkand consumer premises.
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Earthing of LT networks
The earthing system for Distributionnetworks
Fixes the potential of live conductors withrespect to the earth in normal operation sothat it is consistent with the level of insulation.
Limits voltage between the frames of electrical equipment and earth should aninsulation fault occur
Enables having low zero phase sequenceimpedance to keep the higher fault currentand enable operation of relays for circuitbreaking under fault conditions
Limits rises in potential due to MV faults.
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LT earthing practices
The world has several different power Distribution
system grounding configurations.
In IEC standards, the various power systemconfigurations have designations based on theconnection of the Neutral wire to earth, and on theconnection of the Protective Conductor connection to
earth.
The IEC designation is comprised of two letters, thefirst for the Neutral conductor, and the second for theProtective Conductor.
T=Earth N = Neutral S = Separate C = Combined I = Isolated
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Types of Earthing Configurations
Three types of earthing
configurations or their combinationare generally followed:
1. TT
2. TN2.2 TN-C
2.3 TN-S
2.4 TN-C-S3. IT
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Neutral of the source connected and earthed .
Consumer or licensee must provide connection to
earth at consumer premises, i.e. by installing a earth
electrode local to the installation
The protective conductor is connected to its own
ground rod, remote from the neutral ground rod. In some
cases, the ground rod may be the steel frame of the
building. In any case, there is no direct copper connection between the consumer earth and the supply
system.
TT System of Earthing
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TN-S System
Neutral of the source of energy connected withearth at one point only, at or as near as isreasonably practicable to the source.
The protective earth conductor is connected to theneutral
The “S” in the designation means that theprotective earth conductor is a separate systemconductor.
Consumer’s earthing terminal is typicallyconnected to the metallic sheath or armour of thedistributor’s service cable into the premises or to aseparate protective conductor, for instance, in anoverhead supply
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L
L
L L
N
E
Source
Transfor
mer
TN-S System
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TN-S System
Unlike the TT and IT systems, in the TN-S systemthe equipment and man are grounded throughdifferent paths. If the current through the differentpaths is different, then a potential difference willoccur between the equipment and the man, andcurrent will pass through the man.
To minimize the potential difference due to thedifference between the equipment and the man, itis imperative to keep the equipment ground circuit
resistance as low as practicable.
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TN-S system
For a TN-S system, grounding does not provide anequipotential environment due to the finiteresistances of the equipment grounding circuit.
However, equipment grounding through itsprotective conductor does serve to limit the
voltage for low fault currents.
For higher fault currents, another scheme providesprotection against electric shock: limited durationof the current through the body by means of
automatic disconnection of the supply (operationof the circuit-breaker).
Consumers having dedicated transformers for theirinstallation and installed adjacent to or within
their premises. In such situations the usual form ofsystem earthing is TN-S.
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TN-S System
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TN-C system
The TN-C system has a single ground rod. Source star point is earthed and the function of
neutral and earth is combined throughout thesystem.
The protective earth conductor is connected to the
neutral in the equipment. There is no separate protective conductor.
The “C” in the designation means that theprotective earth conductor is combined with the
neutral conductor.
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TNC System
L
L
L L
N
Source
Transformer
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TN-C-S system earthing
A TN-C-S system has the supply neutral conductor of a
distribution main connected with earth at source and atintervals along its run. This is usually referred to as
protective Multiple earthing (PME).
With this arrangement the distributor’s neutral conductor is
also used to return earth fault currents arising in the
consumer’s installation safely to the source.
To achieve this, the distributor will provide a Consumer’s
earthing terminal which is linked to the incoming neutral
conductor.
In TN-C-S system neutral must be connected to the earth
only on supply side of the customer disconnecting switch.
So TN-C-S system is a combination of TN-C for the power
supply network and TN-S for the customer Network
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Protective Multiple Earthing (PME) systemWhere an PME earthing system is used, the following
conditions should be met:
Neutral conductor should be earthed at or near each
LV distribution centre
Neutral should be earthed at other points along the
distribution system.
Earthing should be arranged so as to ensure that
resistance of neutral to earth at any location does not
exceed prescribed limits.
In PME system the high voltage and low voltage
earthing systems should be kept separate.
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IT system
The distribution system has no connection toearth at all, or it has only a high impedanceconnection.
The neutral is connected through an impedanceto its ground rod in the second case
The protective conductor is connected to its ownground rod, remote from the neutral ground rod.In some cases, the ground rod may be the steelframe of the building.
In any case, there is no direct copper connection
between the enclosure and the supply system.
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One characteristic of the IT system is that the systemis tolerant of a fault to ground. That is, a fault to grounddoes not operate the circuit breaker, so the systemremains operational. (An alarm identifies the fault toground, but the system continues to operate.)
•Advantages are all voltages with respect to earth are
kept as low as possible and also allows an earth fault
to develop without causing a trip.
•Such systems must always have Earth Fault Warning
Devices, if it is imperative for a trip to occur on anearth fault, yet the advantage of reduced voltages with
respect to Earth are required.
IT system
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IT system
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Governed by Rule33
•The supplier shall provide and maintain on the
consumer’s premises for the consumer’s use, a suitableearthed terminal in an accessible position at or near the
point of commencement of supply as defined under rule
58.
•
Provided that in the case of medium, high or extra highvoltage installation the consumer shall, in addition to
the aforementioned earthing arrangement, provide his
own earthing system with an independent electrode.
•Provided further that the supplier may not provide any
earthed terminal in the case of installations already
connected to his system or before the 30th June, 1966
if he is satisfied that the consumer’s earthing
arrangement is efficient.
Earthing at consumer premises
I E RULES REGARDING SAFETY AND EARTHING
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I.E.RULES REGARDING SAFETY AND EARTHING
29: Rating sufficient for power, insulation andestimated fault current to construct in such a manner to
ensure safety of human beings, animals and property.
To follow relevant code of practice
30:The supplier shall ensure that all electric supply
lines, wires ,fittings and apparatus belonging to him or under his control, which are on a consumer premises
are in safe conditions and in all respects fit for supplying
energy and the supplier shall take due precautions to
avoid danger arising on such premises from such supply
lines, wires, fittings and apparatus.
The consumer shall also ensure that the installation
under his control is maintained in a safe condition.
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I.E.RULES REGARDING SAFETY AND EARTHING
46: Periodical inspection and testing of installation
(1) Existing installations to be checked once in five years by
the inspector or any other appointed to assist the inspector
47: Testing of consumer installation
(1)The supplier shall inspect and test the applicant’s
installation for a new or additional supply of energy
48: Precautions against leakage before connection
The supplier to check for leakage before connection and
the insulation resistanceshall be at least one Mega Ohm or
as specified in relevant Indian standard.
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I.E.RULES REGARDING SAFETY AND EARTHING
61 Connection with the earth
Neutral conductor shall be earthed by not less than two
separate and distinct connections with a minimum of
two different earth electrodes at sub-stations or such
large number as may be necessary to bring the earth
resistance to a satisfactory value both at generating
stations and sub stations.
The earth electrodes so provided, may also be
interconnected to reduce earth resistance.
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I.E.RULES REGARDING SAFETY AND EARTHING
61 Connection with the earth
it may also be earthed at one or more points along the
distribution system or service line in addition to any
connection with earth which may be at the consumer’s
premises.
Limit earth resistance sufficiently low to permit
adequate fault current for the operation of protective
device in time and to reduce neutral shifting.
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I.E.RULES REGARDING SAFETY AND EARTHING
61 A : Earth Leakage Protective device
The supply of energy to every electrical installations
other than low voltage installation below 5 KW and those
low voltage installations, which do not attract
provisions of section 30 0f the Indian Electricity act shall
be controlled by an earth leakage protective device so
as to disconnect the supply instantly on the occurrence
of earth fault or leakage of current.
Provided that the above shall not apply to overhead
supply lines having protective devices which are
effectively bonded to the neutral of supply transformers
and conforming to rule 91 of I.E. Rules.
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Draft safety regulations: ELPD:
The supply of electricity to every electrical installationother than voltage not exceeding 250 V below 5kW
and those installations of voltage not exceeding 250V
which do not attract provisions of Section 54 of the
Electricity Act, 2003,shall be controlled by an earth
leakage protective device so as to disconnect thesupply instantly on the occurrence of earth fault or
leakage of current:
Conclusion
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Under present I.E.Rules It is responsibility of DISCOM to
ensure safety
The three earthing systems of TT, IT and TN for LTnetworks have their own advantages anddisadvantages.
The choice depends upon the Distribution Company,type of installations, type of consumers to be served,
operation philosophies.
TT system are difficult to maintain in India
TN-S or TN-C-S system are better as it provide low
impedance path (except in situations like petrol pumps
etc).
TN-S system offers best properties for EMC
Conclusion
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