Fuses and plugs are the two parts of a home’s electrical system that most closely concern the householder and it’s important to know what they do and how to use them.
A fuse is a safety device inserted into an electrical circuit to protect the mains wiring or any appliance from damage by overloading. This can be caused by anything from too many appliances overloading a particular circuit to a short circuit within an appliance, its plug or in some other part of the system. Basically the fuse is a thin wire that melts (’blows’) and breaks the circuit if too much current flows through it. Circuit fuses are located in the house’s main fuse box or consumer unit. Fuses are also fitted in modern plugs.
Earth leakage circuit breaker (ELCB)
A current-operated ELCB may be fitted in conjunction with the house’s earthing system, and usually protects all circuits in the house. It cuts off the power supply if it detects an electric current flowing to earth – this is what happens when someone receives an electric shock, or insulation fails on a mains cable. An ELCB is activated by far less current than is necessary to blow a fuse or trip an MCB, and operates within a fraction of a second.
Circuit fuses protect the fixed wiring and apparatus, and if they ‘blow’ no equipment will operate on the circuit affected.
Every circuit fuse has a current rating in amps (A) appropriate to the rating of the circuit, and is colour-coded: white is for a 5A fuse (for lighting circuits), blue for a 15A, yellow for a 20A, red for a 30A (ring mains) and green for a 45A (cooker circuit).
There are two types of circuit fuse – rewirable and cartridge. A rewirable fuse is the least sensitive method of protection, but it can be mended easily by connecting fuse wire of the correct rating between the terminals of the fuse carrier.
In a cartridge fuse, the fuse wire is enclosed in a clip-in ceramic cartridge filled with quartz powder (sand) and if it blows you simply replace the whole cartridge. Unlike the rewirable fuse where it’s possible to fit the wrongly rated fuse wire, it’s impossible to fit a larger amperage cartridge as the size of fuse holder usually depends on the rating of the circuit to which it is fitted. The only exception to this is with 15A and 20A fuses which are the same size, but as the cartridges are colour-coded, a mistake is avoidable.
Miniature circuit breaker (MCB)
MCBs can be fitted instead of circuit fuses in modern consumer units. An MCB is a switching device that is activated by the surge of current caused by overloading or a short circuit. It has the advantage that it can be reset at the push of a button or the flick of a switch – unless the fault that caused it to switch off in the first place is still present.
MCBs are more sensitive and faster reacting than circuit fuses. Some kinds have the same colour-coding as conventional circuit fuses; others are labelled with the circuit rating.
Circuit fuses and faults
If a circuit fuse blows immediately after it has been mended, the cause could be a fault in an appliance, in the circuit wiring or in a socket outlet. A fixed appliance such as a shower unit or immersion heater (which has no plug or intermediate fuse) could also be to blame. Such faults should be investigated by a qualified electrician.
Continuous blowing of a lighting circuit fuse is probably caused by a short circuit in the flex or lampholder of a pendant light.
Modern 13A flat-pin plugs contain fuses that protect the individual appliance and its flex in case a fault occurs. When they blow they only isolate the appliance concerned, so other appliances plugged into the main circuit will still function.
Plug fuses are of the cartridge type which fit neatly into a carrier in the plug, but are a different size to circuit cartridge fuses. The two standard ones have current ratings of 3A and 13A and are colour-coded red and brown respectively. Fit a 3A fuse for appliances rated at less than 700 watts, a 13A fuse otherwise (and always for colour TV sets). Plug fuses can also be fitted into connection units used to supply fixed appliances such as a night storage heater or a freezer.
When a plug fuse blows
If an appliance doesn’t work, the most likely cause is a blown plug fuse. Replace it with a new one of I the correct rating for the appliance 3 , then put the plug g> back in the socket and switch on. O If nothing happens, check that the socket is live by plugging in an appliance that you know is working. Should this not work, check the circuit fuse.
The function of a plug is to connect a portable appliance (eg a lamp or power tool) to the fixed wiring via a socket outlet anywhere around the house.
The modern standard plug is a 13A fused three-pin plug, which has flat pins. There is also a moulded-on unfused two-pin plug used exclusively for electric shavers in conjunction with a special socket outlet. In older installations, instead of 13A plugs, round-pin plugs with current ratings of 2A, 5A and 15A are used. These are not fused.
A recent development is a 13A fused plug that is moulded onto the PVC sheathing of the flex and can’t be dismantled. However, it’s possible to change the fuse by lifting up a flap between the pins of the plug. It’s also possible to change the fuse in some conventional plugs without removing the plug top. If you ever need to replace a moulded-on plug you have to cut off the flex as close to the plug as possible and replace it with a conventional 13Aplug.