Free SAS Urban Survival Handbook by John Wiseman

that switching on a light or a particular piece of equipment has blown a fuse, switch it off before attempting to replace the fuse. If a circuit fuse has blown, test something else in the socket to make sure that the fault is not in the circuit. Check for possible obvious causes before retrying the equipment or you will just keep blowing fuses.
    An expansion fuse or ‘circuit breaker’ is enclosed and usually all you have to do to re-establish the circuit is to press the reset control button or flip a switch (E). You will often find circuit breakers on sink disposal units and other equipment which is designed for short spells of usage and can get overheated if run for too long (or are prone to jamming).
    Cartridge fuses (D), which you will find inside plugs and special sockets as well as in fuse boxes, are sealed capsules containing a fuse wire. A few are transparent and enable you to see the fuse, but most give no visible sign that they have blown. You must simply replace the fuse and if the appliance works you will know that the fuse was the problem. You can always test an appliance fuse by fitting it into the plug of equipment which you know is still operational.
    Old style fuses in a fuse box each consist of a short piece of wire between two screws or terminals, passing over or through an insulated pad. You will sometimes see that the middle of the wire has melted. If it looks complete, try lifting it with your screwdriver. Replace by fitting another piece of fuse wire of correct value between screws or terminals (A, B and C).

WARNING
     
    Always use the correct value of fuse in a plug for an appliance, or at the main fuse box. Don’t simply copy the fuse you found there. It may not have been the correct fuse.
     

Fuse ratings for appliances
     
    Using the correct fuse is VITAL. The lower-rated fuses, such as 3-amp, are used in the plugs of appliances with low power consumption. They are intended to ‘blow’ more easily, protecting delicate equipment. Using a fuse of a higher rating may allow a fault to cause overheating, possibly starting a fire.
    On a 240 volt supply, 3-amp fuses are used for appliances up to 750 watts—this includes computers, televisions, clock radios and fridges.
    A 5-amp fuse is suitable for appliances which consume 760-1000 watts. A 10-amp fuse suits appliances 1000-2000 watts. Above this and up to 3000 watts maximum, a 13-amp fuse is safe.

RCDs/RCBs/ELCBs
     
    Residual current devices (also called residual current breakers and earth leakage circuit breakers) come in various forms. Some plug into a socket, and the appliance is then plugged into them; some may be fitted into an extension cable ; some replace sockets altogether; or they may be part of the consumer unit. If anything goes wrong with the current, such as a sudden leak to earth (which might be you getting a severe shock!), they instantly cut the power and need to be reset. They are very responsive and are designed to give you protection, in addition to that given by fuses and earthing. ELCBs were so called because they respond to even small leakage of current to earth—a leakage which might not blow a fuse.
    RCDs are VITAL life insurance. Users of power tools, electrical gardening equipment or kitchen gadgets where liquid is involved should ALWAYS use RCDs. If upgrading your domestic wiring, consider installing these safety devices on the whole system.
     
    Any electrical dealer or electrician will be able to advise you on the correct fuse ratings. Often the appliance is labelled, or the instruction manual will indicate the amount of power consumed by the appliance. (For a general guide only, see Power consumption. )

Main circuit fuses
     
    On a 240-volt supply, and especially on special circuits for cookers, showers and storage heaters, the correct fuses MUST be used. In general, these are:
    Immersion water heater: 15-20 amp
Lighting circuits: 5 amp
Ring power (socket) circuits: 30 amp
Radial power (socket) circuits: 20-30