EARTHING SYSTEMS

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.