The main switch of the electrical system in your home switches off - and on - the supply of electricity to the entire house. Before any electrical inspection or work is carried out - such as checking and replacing a blown fuse - it is vital that the main switch is turned off. The main switch is located on the fuse box or the consumer unit. A problem with older-style systems, which have wall mounted re-wireable fuses with unscrewable safety covers, is that it is possible to get at a fuse without actually switching off the supply. The design of modern consumer units, on the other hand, means that the fuse covers can only be removed once the mains power has been turned off.

In some older houses, it is possible to find a mixture of 'fuse boxes' attached to the fuse board alongside the meter. These were often the result of additional installed circuits. This 'sprouting' of sub-boxes can often mean that the wiring is poorly labelled, so you won't know for certain whether a circuit is isolated safely or if a particular fuse is correctly rated unless you know what type of circuit it is protecting.

Furthermore, old cabling was usually covered in rubber and cotton, which perishes after about 25 years. Such wires are dangerous because they may become exposed in a metal conduit, which may be earthed to pipes, and serious, if not fatal, electric shocks are very possible.

Your Old Fuse Box

If you have, or suspect you have, an old-style fuse box, get it inspected and tested by a fully qualified electrician. This must be done before you carry out work on any part of the electrical system in your home. A qualified electrician will be able to advise you whether to replace the old fuse box with a modern consumer unit. If your fuse box is found to be in good working order, ask your electrician to label the various circuits for you.

Modern Consumer Unit

Examine your consumer unit and become familiar with it. Remember that even when the unit is switched off, the cable that connects the main switch to your meter is still live. On some units, the main switch is in the form of an RCCB - a residual current circuit breaker. This can trip automatically, switching off the main supply of power to the entire house in case of a serious fault, or can be operated manually.

Fuses

Switch off the main switch and pull out an individual fuse carrier. The carrier contains a fuse that will be one of two types: either a single or double-bladed re-wireable contact, or a cartridge fuse. The re-wireable carrier will have a thin wire running from one contact to the other and will be secured at each end with a screw.

Fuse wire comes in a variety of different thicknesses, which have been calculated to melt or 'blow' at a particular temperature. If you over-load a circuit, the temperature rises, and the fuse blows and 'shuts down' the circuit. Cartridge fuses look a little like the 13 amp fuses you find in plugs, only bigger. Like fuse wire, cartridges vary in size depending on their 'rating'.

In an emergency, and before undertaking any inspection of fuses at the consumer unit, always switch off the supply of electricity to the entire house by operating the main switch on the consumer unit.You can then remove the individual circuit fuse or MCB for the part of the circuit you need to work on.