How to Fit and Mend Fuses Safely

Fuses the fuses of various current rating have different physical dimensions (except the 15A and 20A which are of identical size) which prevents, say, a 5A cartridge being replaced by one of higher current rating, on 03 bath

There are two types of circuit fuses, rewirable fuses containing ordinary fuse wire, and cartridge fuses.

The cartridge fuse is &small ceramic tube with a metal cap at each end. It contains a silver fuse element connected by a wire to each end cap. The cartridge is tightly packed with quartz granules (sand). These fuses are most commonly used in plugs.

Most consumer units are fitted with rewirable fuses, but the cartridge fuse is the superior of the two. Its advantages are that

and a cartridge fuse requires less current to melt the fuse element.

A rewirable fuse on the other hand can be fitted with a heavier fuse wire than its rated capacity and endanger the circuit cable and apparatus which may start a fire.

A rewirable fuse requires a current of twice its rating to blow whereas a cartridge fuse requires a current of only 1½ its rating to blow.


A miniature circuit breaker is superior to circuit fuses and requires a current of only 11/4 its rating to operate. Also, when it trips the current the circuit is restored simply by switching on the MCB once any fault has been rectified.

Earth leakage circuit breaker (ELCB)

An ELCB is a safety device which trips if, through a fault in wiring or in an appliance, there is a slight leakage of electricity to earth, and isolates the circuit or circuits connected through it. It is required in modern houses that one socket is protected

by an ELCB, to supply power tools used outdoors. An ELCB may also be present in other houses where earthing facilities are inadequate. There are various types and models of ELCB, and you should seek professional advice as to which suits your particular requirements. For individual appliances they can be incorporated in plugs, socket outlets or plug-in units; to protect whole circuits an ELCB may replace the main isolator switch in the consumer unit or be fitted as a separate unit next to the consumer unit.

Mending fuses

To mend a rewirable fuse, replace the blown fuse wire with a new piece of the correct current rating. When a cartridge fuse blows remove the blown cartridge and fit a new cartridge. Should a fuse blow again immediately after it is renewed or an MCB trips and the fault is not obvious, call in an electrician.

Fitting a switchfuse unit

A main switch and fuse unit (switchfuse

unit) is really a one-way consumer unit comprising a double-pole isolating switch and a single-pole fuse which can be of a current rating of from 5A to 45A, of the rewirable or the cartridge type or it can have a miniature circuit breaker of the switch or push-button type.

The unit is installed to supply any circuit from a new lighting circuit to a cooker circuit or for an electricity supply to an outbuilding. Where more than one additional circuit is likely to be installed it is wise to fit a multi-way consumer unit instead of a switchfuse unit.

Where a number of night storage heaters are to be installed, each needs a separate circuit making a multi-way consumer unit necessary. Such a consumer unit is time controlled by a time switch usually fitted by the electricity board to limit the supply to the overnight off-peak hours when electricity is cheaper.

To install the switchfuse unit, fix the switchfuse unit (or multi-way consumer unit where there is to be more than one additional circuit) near the existing consumer unit and the electricity board’s meter. To the mains terminal of the unit connect a pair of single-core PVC insulated and sheathed cables, one red, the other black. The red cable is connected to the L terminal, the black to the N terminal. A 6mm2 green/yellow PVC insulated cable is connected to the E terminal of the unit.

The cables are left for the electricity board to connect to the mains. The size of cables depends on the load and on the board’s requirements.