Dealing with old Blown Circuit Fuses

If a fuse blows or an M.C.B. trips, cutting off the lights or power in part of the house, turn off the mains switch at the consumer unit and trace the device concerned. This is easy if you have M.C.B.s: the switch will simply have flipped into the “off” position and can be restored to the “on” position as soon as the fault has been rectified. In the case of fuses, there should be a list on or near the consumer unit showing which device controls which circuit.

If there is no list, then you will need to remove and examine in turn each fuse with the same rating as that of the affected circuit. For example, if a lighting circuit cuts out, then you should check the 5 amp fuses. Where a power circuit cuts out, check the 30 amp fuses – 20 amp if you have radial wiring. Individual appliances such as electric cookers, immersion heaters and night storage heaters will have separate circuits, each with a fuse at the consumer unit. Bear in mind that a cartridge fuse, unlike a rewirable fuse, usually shows no visible signs of damage: it must be checked with a continuity tester.

Before repairing or replacing a fuse, you should try to find out why it blew. Usually, the cause will be obvious. If a fuse blows immediately you switch on a light, then there is probably something wrong with either the bulb or the flex. If a fuse blows after you switch on the light, this could be a sign that the fuse wire has deteriorated with age and is no longer capable of carrying its rated current. However, if the fuse is of the cartridge type, which does not deteriorate, there is probably a fault in the wiring. A ring final circuit fuse will blow only through serious overloading or, again, because of a fault in the wiring. If there is a fault in an appliance, this will usually blow the plug fuse, leaving the circuit intact. Where you suspect that the wiring is defective, call in an electrician. Never try to solve a wiring or an overload problem by upgrating the fuse: the circuit could become dangerously overheated.

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