Mending A Blown Fuse – Fixing Faulty Fuses


Mending a blown fuse is a simple repair job that most of us have had to do at one time or another. What’s much more difficult is finding out what caused it to blow.


Domestic electrical fuses are deliberate weak links in the system, designed to break when a fault occurs. Electrical faults take many different forms and can occur almost anywhere in the wiring, but most have one thing in common: they cause an abnormally large amount of current to flow in the adjacent flex or cable.

Mending A Blown Fuse, Fixing Faulty FusesSimple fuses are wires combining high resistance — which means they heat up fast — with a low melting point. So when an abnormal current flows through a high-resistance fuse, the fuse melts and breaks the circuit. Current is measured in amps, which is also what fuses are rated in. Consequently a 5 amp fuse would melt if a current of 13 amps were passed through it: a 13 amp fuse would not.

In Britain, household fuse systems are designed to provide several lines of defence which protect both you and the electrical hardware. The two main weapons in this system are the fuses themselves, which halt a dangerous build-up of current —and the earthing system which disposes of it safely.

Although some appliances have their own built-in fuse, the first line of defence is generally

Types of fuse: from top, cartridges (plugs and connection units), rewireable and cartridge fuses (main circuit) and MCBs the cartridge fuse that is found in the plug.

Plug fuses are rated at 3 amps for small appliances consuming under 750 watts — toasters, hifi ind lamps for example — and 13 amps for over750 watt appliances, like fires, kettles, tele visions and videos. Confusingly, other ratings of fuse are avail able (such as 5 amp), but they are not required for home use.

Most people happily operate small appliances on a 13 amp plug fuses — happily, that is, until the appliance fails. Although it’s not really dangerous to do so, it does make sense to give your equipment as much protection as possible — especially when that protection comes so cheaply in the form of a fuse.

As well as being fitted in plugs, plug fuses are also found in fused connection units — the fixed sockets used to supply kitchen appliances, wall heaters and the like.

After that comes the circuit fuse — they’re the ones you’ll find in a row in your fuse box or consumer unit. Each lighting and socket circuit in the house has a circuit fuse and there are additional ones for the individual circuits supplying high power equipment such as the cooker and immersion heater.

Circuit fuses are rated according to the current flowing in the circuit they are protecting:

Lighting circuits are 5 amps.

Socket (power) circuits are 15 amps.

Small cooker, shower and immersion heater circuits are 20 amps.

Large cooker, shower and immersion heater circuits are 30 or 45 amps.

Conventional fuses can be of two types. Older, rewireable fuses consist of a thin strand of fuse wire running between the terminals of the fuse holder. The wire is currentrated according to the list above.

Newer fuses have a cartridge similar to a plug fuse, only larger — easier to replace than rewireable fuses, but you can’t

quickly tell if they’ve blown. Both these types have their fuse holders colour coded:

White — 5 amp.

Blue — 15 amp.

Yellow — 20 amp.

Red — 30 amp.

Green — 45 amp.

In more sophisticated modern consumer units miniature circuit breakers (MCBs) are fitted instead of fuses.


If an appliance goes dead but none of the others fed from sockets on the same circuit seem to be affected, suspect a blown plug fuse.

1. If you have not already done so switch off before opening up the plug. The fuse may look charred, or it may not — but in any case you must check the appliance, the flex and the plug. For faults before fitting a new one.

First of all check the rating of the fuse — and then the wattage of the appliance. If the former is only 3 amps and the latter is over 750 watts, you’ve found the fault.

2. Follow on by looking at the plug connections: they should all be tight and there should be no stray strands that could short from one to another.

Check the plug in the socket. If it’s a loose fit, one or the other may be badly worn and in need of replacement.

Move on to the flex. It should be tightly held in the plug’s cord grip. Along its length, there should be no evidence of fraying, splitting, kinking or twisting — any of which may cause a blown fuse.

3.Finally, check the appliance. Start with where the flex enters it — the rubber grommet should be intact and there should be no appreciable movement when you pull it. If there is, this indicates loose terminals or cord grips inside.

Often, misuse or maltreatment of an appliance can blow a fuse. For example, an excessive buildup of dust or dirt can cause over heating, and the same problem results from restricting the air flow around it — particularly on hifis and videos. Foreign objects, too, cause problems and bear in mind that something may have fallen in accidentally.

If, after checking, you replace the fuse and it blows again, lose no time in having the appliance checked by an electrician.

Damaged flexes can be replaced in part (using an extension box) or in their entirety, but make certain that you take a sample of the old one with you to the shop so that the rating of the replacement can be matched exactly.


You can save yourself a lot of trouble later by taking two simple measures as soon as you move into a property.

First, turn off the main switch at the consumer unit and remove all the fuse holders but one (on MCBs press the small ‘off’ button below the reset button).

Turn on the electricity and

note what lights, sockets or appliances are served by the fuse that you’ve left. Note the information down and stick it to the fuseholder. Then check that the fuse’s colour coding is correct.

Repeat this procedure for each fuse in turn, always remembering to shut off the main switch between times. Once you have circuit information on all the fuse holders you won’t be left in the dark if a fuse does blow.

Secondly, make sure that you have fuse wire or cartridges (as applicable), a screwdriver, pliers and torch in a handy place near the consumer unit. Rummaging around in the dark can be dangerous and awkward.


Always try to be aware of the circumstances immediately prior to a fuse blowing. For example, if it blows the moment you switch on a rather suspect old appliance, the chances are this is the culprit. On the other hand, if the

lights go out seemingly for no reason at all, you have cause to suspect a general wiring defect.

Whatever happens though, your first step should always be to switch off the power — at the socket if it’s a blown plug fuse, otherwise at the mains.

Whenever possible try to discover what caused the fuse to blow before you mend it. On no account attempt a temporary repair by substituting a higher rated cartridge or fusewire.


With a bit of luck, your observations at the time the fuse blows should give you more than an inkling as to where the trouble lies more often than not it happens when you switch something on.

For this reason, and hopefully to save you having to conduct a systematic survey of your house’s electrical system, the checks below are grouped according to the most common individual trouble spots.

Overloading: If a socket is overloaded, you’ll almost certainly recognise the fault as soon as it happens. Threeway adaptors (which do have their own plugtype fuse) are a particularly common source of trouble and their use should be avoided.

If you must have a threeway adaptor, make sure you never run highpower appliances —such as a fan heater or kettle —from it.

Circuit overloading is harder to recognise, but it can easily happen if you have all your highpower appliances — washing machine, spin drier, TV, fan heater, kettle — running at the same time. If the problem recurs frequently, have the circuit checked by a qualified electrician.

Appliance failure: Serious faults within an appliance can cause a circuit fuse to blow as well as the plug or connection unit fuse.

Light switches: In this case of a suspect switch, turn off the electricity at the mains.

Remove the switch faceplate and inspect the connections behind.

The red and black wires should be held tight in their terminals — with no stray strands to cause short circuits —and the insulation should be complete, without nicks.

Now look at the earth wire. This should be attached to a terminal on the switch box and covered with a length of green and yellow plastic sheathing. A loose earth terminal means the switch may not be properly earthed: a bare earth wire can cause a short circuit.

Sockets: These suffer from more or less the same problems as light switches — loose connections or shorting via an un insulated earth wire (in this case all three wires — live, neutral, earth — are held in terminals in the back of the socket faceplate).

Having made sure the electricity is turned off, unscrew the faceplate and make a thorough inspection.

Incorrect fuse rating: This may seem obvious, but it’s a common fault. Even if the colour code on the fuse holder is correct, you should also check the cartridge. And if the fuses are rewireable don’t assume that the blown wire was of the correct rating. Check the figure stamped on the fuse with the ratings on the fuse wire packet to make sure you use the right one.

Among the other faults to look out for are: cracking or worn terminals. In which case the socket must be replaced: kinking of the cable where it enters the backing box (possibly due to a perished grommet), in which case the cable must be pulled through and trimmed: and damp. Which must be put right before the socket is replaced.

Light fittings: Turn off the electricity and remove the rose or the body of the fitting. As before. Be on the lookout for loose wires, stray strands and perished or broken insulation.

Outside factors: If a fuse blows as you drill through a wall or drive in a floorboard nail. Think yourself lucky — it could have been far worse: you could have got a severe shock.

In all cases where you suspect an outside factor, turn off the electricity at the mains and do some very careful exploratory work to find out exactly what has happened before you mend the fuse.

Wiring defects: These are the hardest to spot, but also the most dangerous to ignore. Any of the checks above will show up obvious ones, but the list below is of more basic defects which you should put right as soon as possible:

Incorrect cable — turn off the electricity and open up sockets.

Light switches and ceiling roses all over the house. Make sure that the cable used in each case is definitely of the correct type and rating.

If you find any old, rubber sheathed cable. Your house must be rewired immediately.

Sold sockets — some houses still have old round pin sockets working off an obsolete radial circuit. These are dangerous —the circuits must be disconnected at the fuse board.

Overlong circuits — a problem

in houses wired on the radial system, the symptoms are usually that of circuit overloading. Suspect it if the circuit seems to extend round the whole house: treat the circuit with respect until it can be rewired.

Faulty junction boxes — found on light circuits using this method of wiring. Lift upstairs floorboards and look in the roof space for signs of deterioration or poor connections at boxes.


The first rule of mending a fuse is turn off the electricity — at the socket if it’s a plug fuse, at the main switch if it’s a circuit fuse.

The second rule is find out what caused the fuse to blow and put it right before you go any further.

The third rule is never use fuses or fuse wire of an incorrect rating — no matter for how short a time. Where electricity is concerned there are no short cuts. Plug fuse: Simply open up the plug, flick out the old fuse with a screwdriver and slot the new one in. In the case of fused connection units, the fuseholder either unscrews or is prised out with a screwdriver. Slot in the new fuse and replace.

Circuit fuse (cartridge): Remove the fuseholder of the circuit that has failed. Dig out the old fuse and slot in its replacement.

Circuit fuse (rewirable): These may be one of several patterns, notably open or enclosed, but the principles are the same.

Remove the relevant fuseholder and loosen the screw terminals at either end to release any burnt wire.

Cut off sufficient new wire of the correct rating to stretch the length of the holder and wrap around the terminals.

Wrap one end clockwise round one of the terminals and tighten.

Feed the wire across or through the holder as necessary.

Wrap the other end round other terminal — again in a clockwise direction — and tighten. Replace the fuseholder. MCBs and ELCBs: Simply press the reset button once the fault has been put right.

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