A Fuse is a thin piece of wire which melts when excess current is passed, thus breaking the circuit.
Fuses are used:
- in mains plugs
- inside appliances
- in the CU or fusebox
- MCBs and RCBOs may be used instead in these
- at the supply company's mains incomer
A fuse's rated current is the current it will pass indefinitely, not the current at which it fuses, which is significantly higher.
- Very slow fusing occurs at currents moderately above rated current
- Very fast fusing occurs at currents several times rated current
- To illustrate, a 13A mains plug fuse will pass 25A for about 30 minutes before it blows.
Mains Plug Fuses
Mains plug fuses are available in ratings of
With 3A & 13A being by far the most common. 1A, 7A & 10A are not common today.
Mains plug fuses use sand filled ceramic cartridges for extra breaking capacity. Do not use glass fuses in mains plugs, as these do not have the breaking capacity sometimes required.
20mm & the older 1.25" glass fuses are widely use in appliances. There are 3 common types of these fuses:
- Quick blow
- Slow blow
- Time delay
The type is indicated by the construction inside the glass and the marking on one metal endcap.
- F1A - 1A quick blow fuse, uses a plain fuse wire.
- T2A - slow blow 2A, uses fuse wire with a few small nodules on it
- T5A - time delay 5A, uses a spring & solder joint
The slow blow 'nodules' and time delay 'spring' types of fuses are not equivalent, time delays blow a good deal slower than slow blow, and that extra time delay is needed in some applications, such as where relatively high internal switch on currents are involved, as is routinely the case inside appliance power supplies and with motors.
Ceramic microwave oven fuses should not be replaced with glass fuses. The fuse inside a microwave is a safety critical component, much more so than other electrical fuses. It is what prevents the user being cooked if the interlock system fails.
Wire Link Fuses
Wire link fuses are occasionally used in low cost appliances, such as wall warts. These are simply a link of very thin wire. When repairing such goods, if you find a very thin filament of wire in use, it may be a bad idea to replace it with ordinary thicker wire.
Other fuse types are also sometimes used in appliances. These include small round encapsulated fuses soldered to a PCB, sometimes surface mount fuses, safety resistors, and less often car type fuses. There are also self resetting thermal devices sometimes used to replace fuses; these only need be left to cool down to reset.
Interpreting Fuse Failure
It is possible to glean some information about the nature of the fault that caused a fuse to blow:
- Wire ends melted, surroundings clean: moderate overload, very slow blow.
- Wire ends melted, surroundings clean, but signs of overheating of the fuseholder: long standing moderate overload, eventual blow.
- Wire ends melted, a little blackening around fuse wire: fairly large overload current, quick blow.
- Just a blackened mess: Very high overload current, very fast blow.
- Surroundings clean, no blob of molten fusewire on either broken end: fusewire fractured due to mechanical causes, not an overload fault. Typically seen with very low current fuses (2A and below) and glass fuses with a loose end.
Low Voltage Fuses
All fuses have a voltage rating above which they may be unable to break the circuit.
Fuses designed for vehicles are usually 32v rated, should never be used at mains voltage, as they are likely to be unable to stop the flow of current.
20mm glass fuses should not be used to replace better specified fuses, as these have very low current breaking ratings inadequate for many mains voltage applications.
Mains fuses can safely be used at lower voltage. The lower voltage does not affect their current rating.
- Widely used in fuseboards and CUs
- Low cost
- Rarely nuisance trip when a light bulb fails.
- Rarely abused by householders or tenants putting thick wire in their place.
MCBs are recommended for let properties, where tenants might abuse rewirable fuses.
Fuses are recommended for lighting circuits, where MCBs sometimes produce nuisance trips on bulb failure, creating a safety issue and inconvenience.
BS 88 & BS 1361 Type II fuses are used in electrical mains incomers. This is where the mains supply enters the property, before the meter.
Pulling the Incomer Fuse
When replacing a CU or fusebox it is necessary to de-energise the supply. Working with live tails is not a good plan.
The supply company may de-energise the supply for you, and may fit an isolating switch. Sometimes they won't do it, or will but at a price.
Pulling the fuse yourself is not permitted, but for reasons I'll explain later supply companies would rather you do that than work live, so have not been known to object or take any action in practice. Pulling live fuses is common practice with professional domestic electricians.
Fuses are safety devices, serving to prevent many cases of fire, shock and equipment damage.
The one significant issue with fuses is the tendency of some users to remove the fuse and replace it with something ineffective, such as thicker wire or a bolt.
A card of fusewire kept on the fusebox eliminates most such temptation.
Fuses permit some amount of sustained overload, so cables protected by fuses which might be run at above the fuse rating, in particular domestic ring circuits, need to be able to safely carry more than the fuse's rated capacity. The degree of difference in this overcurrent between fuse and MCB is why there is a difference in rating between 30A fuses and 32A MCBs used on ring circuits.
A Common Fusebox Design Issue
Some older types of fusebox are designed in such a way that pulling out a fuse carrier with the main switch on is liable to result in electric shock. This is a matter of historic fusebox design rather than safety of the fuse itself. There are many of these boxes still in service, so a fuse should not be pulled with power switched on.
Low Voltage Fuses
Low voltage fuses should not be used on high voltage circuits.
For more on fuses and safety, see MCB#Comparison with Fuses