Question

With reference to the protection of electrical power circuit

Explain the term discrimination and describe how it is achieved

This is the ability of a protection system to disconnect only faulted circuits and to maintain the electrical supplies to health circuits.

Discrimination is achieved by ‘co-ordinating’ the current ratings and time settings of the fuses and over current relays used between the generator and load: the devices nearest the load having the lowest current rating and shortest operating time; those nearest the generator having the highest current rating and longest operating time.

If a short-circuit fault occurs in the lamp holder below, the fault current will be large enough to operate all protection devices from the generators to the fault. However, fuse ‘A’ having the lowest current rating and shortest operating time will ‘blow’ first to clear the fault and leave all other healthy circuits still connected.