SAS Urban Survival Handbook (12 page)

WARNING

When a fault develops and a fuse ‘blows’, a dangerous amount of heat may have been generated in an appliance, a plug or a socket. Check the circuit for signs of overheating.

When a fuse ‘blows’

First check the fuse in the plug or socket. The easiest way to check them is to replace them, since a ‘blown’ fuse may not look any different from a good one.

If these are working, go to the fuse box/consumer unit and check the fuse there or press the reset button (see
Replacing a fuse
).

It’s usually more obvious when a fuse has blown at the fuse box (if you know which outlets are on a circuit) because a whole circuit will be rendered inoperative. If a light on one floor’s lighting circuit blows a fuse, then all the other lights on that circuit will go out.

Replacing a fuse

If you know 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 amp
Showers:
30 amp
Oven (up to 12 kW):
30 amp
Oven (over 12 kW):
45 amp
Storage heaters:
20 amp

REMEMBER

Always keep a flashlight near your electricity meter. If it is in a dark cupboard, this will help you see what you’re doing. If you need to switch off the power at night, you will need to avoid fumbling around. DON’T use a candle if the fuse box/consumer unit is near your gas meter!

CABLES/FLEXES

Live, neutral and earth wires are colour-coded for identification to make sure that the correct electrical connections are made. Unfortunately, colour coding is not standard around the world, but it is beginning to be. This can never take account of strange ‘bodged’ wiring jobs people may have created.

Main circuit wiring (cable)

Old rubber sheathing is still common and should be treated with suspicion. Rubber degrades with age and crumbles. But even modern wiring (on supply cables) will have a red live, a black neutral and a green-and-yellow earth. In practice, the earth wire is often unsheathed in the centre of the cable. Special green-and-yellow sheathing is sold for you to cut to length, to sleeve sections of the earth which are exposed when making connections. DON’T use spare pieces of black or red sheathing. DON’T use electrical insulating tape.

Appliance wiring (flex)

The live is always brown, the neutral is always blue, the earth is always green-and-yellow (when there is an earth wire).

Old appliances—which should really be rewired and updated by an expert (or discarded)—will probably have a red live, a black neutral and a green earth. If the wires are old, the colours of the sheathing may have faded or discoloured. DO NOT GUESS! Old rubber insulation is, by now, not safe.

WARNING

When connecting wiring with screw fittings, DON’T over tighten the screws. To do so may sever individual strands of multi-stranded flex. These may become dislodged and form an unwanted and dangerous connection. With cable or singlestranded flex, over tightening the screw may flatten (or even sever) the wire. Heavily-flattened wire may act like a fuse, with a reduced capacity for carrying current. It can overheat, may burn out and may start a fire.

Types of domestic cable

On a 240-volt supply, the lighting circuits are generally formed by 1.0 mm
²
twin-and-earth (a plastic casing housing three single-strand wires—one sheathed red, one black and one unsheathed) Each wire is about 1 mm in diameter.

The power cable for the main power (socket) circuits is very similar, but of a heavier weight.

On special circuits for ovens, electrical heating installations or shower heaters, even thicker cables may be used.

WARNING

No mains wiring alterations or installations should be attempted by anyone who is not qualified to do so. Because of the dangers, in some countries such as Australia, severe restrictions are imposed on the type of electrical work that you can do yourself.

Types of flex

Flexes may contain two sheathed wires—brown (live) and blue (neutral)—in an outer casing
OR
three wires, if a yellow-andgreen earth wire is included. Sometimes a thin two-core flex may have no outer casing—but this is only really suitable for wiring battery-operated doorbells.

Most flexes, inside the internal coloured sheathing, are made up of several thin strands of wire, which gives the flex FLEXibility. When stripping the sheathing from the ends, for wiring a plug, take care not to sever individual strands. Losing one or two may not matter, but if you lose too many the current-bearing capacity of the wire is reduced at that point. This could cause overheating—or even fire.

When you buy flex
, ask the dealer which is the correct one for a particular appliance. There is a rising scale of thickness to cope with different power consumptions. The flex is usually marked with its recommended maximum load.

When using longer flexes
—perhaps an extension lead to a garden tool—you will need a thicker cable to cope with the voltage drop over the distance.

If the flex needs to withstand a lot of movement
and handling (an iron, a power tool, a hairdryer), a heavier flex should be used than is suggested by the power consumption.

For pendant light fittings
, the flex needs to be heavy enough to support the lampshade, even if only a 25-watt lamp is used. NEVER expect a heavy pendant fitting to be carried by the flex alone. A chain, or similar, must be anchored into a joist above the ceiling. The flex is for carrying power—not weight.