Protection by fuses and other devices

Here the general requirement is similar to that for discrimination between two fuselinks, in that only the downstream device is required to operate. It is this latter device which has to be chosen first, because its time current characteristics must provide the necessary protection for its associated circuit. Thereafter the upstream device must have a characteristic which will ensure discrimination.

In practice, two alternative arrangements are encountered. One in which the upstream device may be a fuse whilst the downstream devices may be small or miniature circuit breakers incorporating overcurrent protective features, and the other in which an upstream circuit breaker and downstream fuses are used.

With the first arrangement there is always an actual or potential upper limit to the fault current at which discrimination can be obtained. This is because the circuit breaker or other downstream device always has a definite minimum operating time resulting from the delays in the overcurrent detection equipment and the circuit breaker itself, plus its own arcing time, of which the latter is not likely to be less than the duration of one half cycle. The operating time of the upstream fuse, on the other hand, decreases continually with increase in current giving an upper current limit at which discrimination can be achieved.

With the second arrangement, there is usually little difficulty in choosing characteristics which enable full discrimination to be obtained.