Fuse
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
Fuse Ratings
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
Mains Plug Fuses
Mains plug fuses are available in ratings of
- 1A
- 2A
- 3A
- 5A
- 7A
- 10A
- 13A
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.
Equipment Fuses
Glass Fuses
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.
Example markings:
- F1A - 1A quick blow fuse, uses a plain fuse wire.
- T5A - time delay 5A, uses a spring & solder joint
- T2A - slow blow 2A, uses fuse wire with a few small nodules
The spring and wire with nodules types of fuse are not equivalent, spring types blow a good deal slower, and this time delay is needed where relatively high internal surge currents are involved, as is routinely the case inside appliance power supplies and with motors.
Microwave Ovens
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.
Others
Other fuse types are also sometimes used in appliances.
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
Fuses designed for vehicles should never be used at mains voltage, as they are likely to be unable to stop the flow of current.
Mains fuses can safely be used at low voltage. The differing voltage does not affect their current rating.
Rewirable Fuses
- 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 have become more popular than rewirable fuses due to pushbutton ease of resetting, and the popular belief that they offer of extra safety. For details see Fuse vs MCB
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.
Incomer Fuses
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.
Fuse Safety
Fuses are safety devices, serving to prevent many cases of fire, shock and equipment damage.
Abuse
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 should be kept on the fusebox to eliminate most such temptation.
Rating
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. This is why there is a difference in rating between 30A fuses and 32A MCBs.
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 Fuse vs MCB