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.
