The purpose of fuses
A fuse should be the weakest link in an electrical circuit, so that it burns out before more expensive equipment, or acts as a safety device, to ensure the circuit is not overloaded, causing overheating of wiring, etc.
It is rated in amps. (A), which is the current at which it melts and cuts off the electric current. This is the current flowing from the live terminal. This current usually goes to the neutral terminal, but it could also be leaking to earth. It does not prevent electrocution; it is there to protect the wiring or appliance. Electrocution is only prevented using RCCD circuit breakers, either at the consumer unit (main fuse box) or with an RCCD adaptor or plug at the socket where the appliance is plugged in. 'Retro-fit' MCBs plugged into a consumer unit, in place of fuses, do not prevent electrocution; they merely replace fuses, so that they can be easily reset without needing the expense or trouble of replacing a fuse.
Consumer Unit (Mains Fuse Box) Fuses
Consumer units (in order of age) can have the following types of fuses, or equivalents, fitted:
1. Rewirable fuse holders. These have a fuse wire going through the fuse holder. This comes on a card in 5, 15 and 30A fuse wire, and is often colour coded in white, blue and red respectively.
2. Cartridge fuses These usually are 5A (white) for lighting, 15A (blue) for boilers and similar equipment, 20A (yellow) for boilers, immersion heaters, etc. 30A (red) for power (sockets), and 45A (green) for cookers and showers below 11kW. Apart from the 15 and 20A, they are all different sizes and cannot be interchanged. This is to prevent people fitting a higher fuse that the existing wiring warrants.
3. Retro-fit MCBs These act the same as 2, are colour coded the same, although in some cases the current rating may be a little higher. They are fitted for convenience only, since if they trip, a switch or button has to be reset, instead of the fuse being replaced.
4. RCCD circuit breakers These have to be fitted to a consumer unit designed to take them (they will not fit into the old type of unit). They trip if either the current from live to neutral exceeds their rating, or if there is a leakage from live to earth, usually greater than 30mA, in a short enough time to prevent electrocution. They have a greater range of values than 1,2 and 3, and all new installations should have these fitted.
Fuses or breakers blow if the circuit is overloaded. This can commonly happen if two many high current devices are in use at once, there is a fault with the wiring, or sometimes if an incandescent bulb blows.
Plug fuses
Fuses fitted in plugs are designed to protect the appliance only. The same fuses are also fitted in spurs. They commonly come in 3 current ratings (although others are available). It is important to fit the correct size of fuse in the plug. The 3 common ones are:
3A (red) for all appliances up to 700W. This includes most lights.
5A (black) for appliances up to 1200W.
13A (brown) for appliances above 1200W (to 3250W). This includes most heaters.
If a plug fuse blows it is usually because of a fault in the appliance. In the case of a light, this could merely because the bulb has blown.
Appliance Fuses
These are usually glass, although they can be opaque. They can be quick blow, slow blow or anti-surge. If in doubt, fit a quick blow fuse, as this gives more protection. They come in a vast range of current ratings. This can usually be found on one of the end caps - typically it will say T5A 250V, indicating a 5A fuse. They are usually there to protect a specific part of the appliance; e.g. a fuse at the speaker output of an amplifier will blow if the terminals to the speaker are accidentally short circuited. If this happened without the protection of the fuse, expensive damage to the amplifier could occur. They are also commonly in 2 lengths - 20mm and 32mm. Some fuses are designed to work at higher temperatures. These are usually slow blow, and must be used in microwaves. There is nothing to stop you using these in other appliances too, even though the high temperature rating is not needed.
Barrie Newton B.Sc.
Owner of Rhyl Lightworks Co
http://www.thelightworks.net
or for common fuses see: http://www.thelightworks.net/index.php?main_page=index&cPath=98
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