Fuses & Circuit Breakers

As may be necessary to avoid danger, all systems and installations must have efficient and suitably located means for the protection from excess of current.

 

A major source of electrical fires is overheating caused by conductors carrying more current than they were designed to safely carry. The resulting insulation breakdown leads to further overheating and possible short circuits. Given these dangers, provision of suitable over-current protection is a fundamental safety requirement in all electrical systems and installations and usually takes one of two forms, fuses or miniature circuit breakers.

 

Fuses: are the most commonly used form of over-current protection. The fuse should be the weakest link in the circuit and should be designed to fail in a safe manner. Replacement fuses must be of the correct rating and type. Fuses should normally be placed in the phase (line) conductor such that should the fuse blow, the equipment being supplied, and as much of the circuit as possible, is disconnected from the source of power. Fuses placed in both the phase and neutral conductors (double pole fusing) are not permitted in ac circuits because should the neutral fuse be the one to blow, a greater part of the circuit would still be connected to the phase side of the supply and thus remain live. However, fuses in both conductors is permissible if the supply has been floated (e.g. through use of centre-tapped transformers) so that the conductors can no longer be designated phase and neutral.

 

In many electrical systems it is normal practice to use a number of fuses to protect various parts of the system; this is called discrimination. Two things need to be remembered:

The maximum current which the fuse can safely and continuously interrupt at the rated voltage is called its breaking capacity. Should it be subjected to a greater current, there is a danger that an arc may be struck between the two ends of the fuse and the fuse could explode. When determining the correct fuse for a system, it is important to ensure that it has a sufficiently high breaking capacity to safely interrupt any prospective short circuit current.

 

Miniature circuit breakers (MCB): an MCB is a device which will break the circuit under over-current conditions but which can subsequently be re-set. Many of the things mentioned in the section on fuses, such as where fuses should be placed in systems, replacement types and the desirability of discrimination, are equally applicable to MCBs.

 

Note: the prime purpose of over-current protection devices is to protect equipment from overheating. By itself, over-current protection does not protect against electric shock. It is only able to protect people if it is installed in conjunction with a properly specified earthing system; and even then, it is only effective under fault conditions. Over-current protection devices do not protect people if they become connected across the line and neutral of a supply circuit.