3.7.5 -  Back-up protection

There are times when the overload protection has insufficient breaking capacity safely to interrupt the prospective short circuit current at the point of the installation where it is situated. An instance would be where a large number of low-rated miniature circuit breakers each with a breaking capacity of 3 kA are fed by a large cable. In the event of a short circuit which gives a current of 8 kA, there is a good chance that the miniature circuit breaker concerned will be unable to break the fault.

Perhaps the fault current may continue to flow in the form of an arc across the opened circuit breaker contacts, causing a very high temperature and the danger of fire. Of course, if this happened the circuit breaker would be destroyed.

Fig 3.24 Back-up protection

The normal method of protection is to 'back up' the circuit with a protective device which has the necessary breaking capacity. For example, the group of miniature circuit breakers mentioned above could be backed up by an HBC fuse as shown in (Fig 3.24}.

It will be appreciated that when a protective device operates it does not do so instantaneously, and fault current will flow through it to the circuit it seeks to protect. The time for which such a current flows is a critical factor in the damage that may be done to the system before the fault clears. Damage applies to cables, switchgear, protective devices, etc. if the fault is not cleared quickly enough. This damage will be the result of the release of the energy of the fault current, and the system designer will aim to minimise it by calculation and by consulting manufacturer's data of energy let-through.