Discrimination and
co-ordination

Overcurrent discrimination is defined as: "co-ordination of the relevant characteristics of two or more overcurrent protective devices such that, on the occurrence of overcurrents within stated limits, the device intended to operate within these limits does so, while the other(s) do(es) not".

Most circuits contain several protective devices and some of these are effectively in series. They must all be co-ordinated so that correct discrimination is achieved under all fault conditions and only the minimum of interruption should occur to clear any fault condition.

The co-ordination of the circuit protective devices PD’s, is affected by their operating characteristics and there are several possibilities which may arise; for example, a network may contain a considerable number of fuses which must be chosen to discriminate or alternatively a fuse may have to operate in series with a circuit breaker which is tripped by a protective relay. These situations are considered separately using the simple network shown below.