Why do I need an RCD?

Answer:

The answer to this question requires the study of what happens to a normal installation under earth fault conditions and depends upon the type of earthing system employed.

In TT earthing systems, the neutral is generally distributed to allow single phase load supply. Exposed conductive parts are locally connected to earth. The earth fault current involves the secondary winding of the transformer, the phase conductor, the fault resistance, the earth electrode resistance and the neutral – earth resistance

In TN systems ,the neutral is directly earthed in the substation and exposed conductive parts are connected to the same earthing point as the neutral conductor . The fault loop is formed by the connection of the phase conductor and the protective conductor

The fault current flows from line and load to earth. The earth fault current is therefore limited by the total impedance of the earth loop path.
For example, if the total impedance of the earth-loop path in a 230V TT installation is in the order of 20 Ohms, the maximum leakage current would only be 11.5A. This current would be carried indefinitely by a 16A fuse or circuit breaker. As a result any metalwork bonded to the installation earth terminal would now be permanently connected to the live circuit at up to the full mains voltage constituting a major shock and fire hazard.

The 16th Edition of the IEE wiring regulations have adopted maximum allowed earth loop impedances using the worst-case conditions. The standard gives indications of the maximum disconnection times for protective devices in order to avoid pathophysiological effects for people touching live parts. Where earth leakage protection by means of fuses and standard circuit breakers cannot be achieved due to earth loop impedances above the maximum allowed, RCD devices should be used.

RCDs should also be used where a higher degree of protection is required than can be obtained by conventional overcurrent devices such as for portable or transportable equipment where there is a higher risk of loss of continuity of the earthing conductor.