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Answer Posted / ganesan.n
EARTHING SYSTEMS
These have been designated in the IEE Regulations using the
letters: T, N, C and S. These letters stand
for:
T - terre (French for earth) and meaning a direct
connection to earth.
N - neutral
C - combined
S - separate.
When these letters are grouped, they form the
classification of a type of system.
The first letter denotes how the supply source is
earthed.
The second denotes how the metalwork of an installation is
earthed. The third and fourth indicate the functions of
neutral and protective conductors.
TT SYSTEM
A TT system has a direct connection to the supply source to
earth and a direct connection of the installation metalwork
to earth. An example is an overhead line supply with earth
electrodes, and the mass of earth as a return path as shown
below.
Note that only single-phase systems have been shown for
simplicity.
TN-S SYSTEM
A TN-S system has the supply source directly connected to
earth, the installation metalwork connected to the neutral
of the supply source via the lead sheath of the supply
cable, and the neutral and protective conductors throughout
the whole system performing separate functions.
The resistance around the loop P-B-N-E should be no more
than 0.8 ohms.
TN-C-S SYSTEM
A TN-C-S system is as the TN-S but the supply cable sheath
is also the neutral, i.e. it forms a combined earth/neutral
conductor known as a PEN (protective earthed neutral)
conductor.
The installation earth and neutral are separate conductors.
This system is also known as PME (protective multiple
earthing).
The resistance around the P-B-N-N loop should be less than
0.35 ohms.
SUMMARY OF EARTHING SYSTEMS
The TT method is used mostly in country areas with overhead
transmission lines. In contrast to the TN-S system there is
no metallic path from the consumer's terminals back to the
sub-station transformer secondary windings. Because the
earth path may be of high resistance, a residual current
circuit-breaker (R.C.C.B.) is often fitted so that if a
fault current flows in the earth path then a trip
disconnects the phase supply.
For protection against indirect contact in domestic
premises, every socket outlet requires an RCCB with a
maximum rated current of 30mA.
The TN-S system of wiring uses the incoming cable sheath as
the earth return path and the phase and neutral have
separate conductors. The neutral is then connected to
earth back at the transformer sub-station.
Remember in TN-S, the T stands for earth (terre), N for
neutral and S denotes that the protective (earth) and
neutral conductors are separate.
The TN-C-S system has only two conductors in the incoming
cable, one phase and the other neutral. The earth is
linked to the neutral at the consumer unit. The neutral
therefore is really a combined earth/neutral conductor
hence the name PME.
In order to avoid the risk of serious electric shock, it is
important to provide a path for earth leakage currents to
operate the circuit protection, and to endeavour to
maintain all metalwork at the same potential. This is
achieved by bonding together all metalwork of electrical
and non-electrical systems to earth.
The path for leakage currents would then be via the earth
itself in TT systems or by a metallic return path in TN-S
or TN-C-S systems.
NOTES
Older houses in towns use TNS (solid) i.e. separate earth
say cable sheath.
Around Towns new houses use (PME) TNCS i.e. neutral and
earth shared.
Single House in country with own transformer uses TT i.e.
own buried earth electrode.
Petrol stations, Swimming pools, Changing rooms etc. are
not allowed to be PME.
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