Fuses and Circuit Breakers
From the electricity meter, the current passes to a consumer unit with an integral mains switch, or, in older installations, to a fuse board with separate switches and fuse boxes. The power is then transferred to the wiring circuits either by means of individual fuses or by circuit breakers. Either type of device serves an essential safety function. If a fault or an overload results in more current flowing through a circuit than it is designed to carry, the fuse will melt or the circuit breaker will open, thereby cutting off the circuit from the mains supply.
In this way, the excess current is prevented from generating the kind of heat that usually causes a fire. Miniature circuit breakers (M.C.B.s) are more convenient than fuses, which have to be repaired or replaced after a circuit interruption. Once the cause of the interruption has been corrected, a circuit breaker is simply reset by pushing the switch or button to the “on” position. M.C.B.s are a fairly recent development, however, and most electrical installations are still protected by fuses. These are of two types—rewirable and cartridge—and they are housed i n porcelain or plastic fuse holders which plug into the fuseways of the consumer unit or fuse box. Plastic holders are usually colour coded according to their current rating: 5 amp is coloured white, 15 amp blue, 20 amp yellow, 30 amp red and 45 amp green. The most widely used of the two fuses is the rewirable type, in which the fuse element – a piece of thin wire—is held in position by a screw terminal on either side of the holder.
If a fuse blows, melting the wire between the terminals, first the remnants of the old wire are removed and then a new piece of the same current rating is fitted in its place. In the case of the cartridge fuse, the element is enclosed in a small, metalcapped tube which slots into two spring contacts on the holder. The disadvantage of a cartridge fuse is that it cannot be repaired; once it has blown, it has to be replaced . It is also more expensive to replace than the rewirable type.
On the other hand, cartridge fuses come in different sizes according to their current rating, so that it is impossible to fit, say, a 20 amp fuse into a 5 amp holder. Like plastic fuse holders, cartridge fuses are colour coded so that they can be easily identified. The most important advantage of all is that a cartridge fuse provides greater protection for the circuit than a rewirable fuse. This is because i t takes less excess current to blow the cartridge fuse. For example, i t would need only 45 amps to blow a 30 amp cartridge fuse compared to about 60 amps for a 30 amp rewirable fuse.
In addition to protecting the circuits in your home, fuses and M.C.B.s perform another and all-important safety function. Because fuses can be removed and M.C.B.s switched off, they enable you to ” kill” a circuit so that you can work on it without the risk of receiving a shock.