Electricity Generation: Fuses and the race to protect


Fuses and circuit breakers

Fuses are the "last gasp" and sometimes the only protection found in most domestic electric appliances as well as electronic circuits. A fuse is primarily used in a mains plug-top to oversee the mains cable (power cord) and will melt if the cable is severed or damaged, when an excessive current could flow. The fuse is also intended to disconnect the mains supply should an insulation fault, such as a short to earth, arise within the apparatus. Simple electric appliances often have no other form of protection.

^ (UK) Mains plug internals with three-core wire and a 13A ceramic cartridge fuse that interrupts the Live wire.

Fuses are the most basic type of over-current protection and rely on a wire melting to interrupt the supply. They offer little protection to human beings in preventing electric shock, other than to disconnect the supply if an overload condition (e.g. a short to earth) occurs.

Old open-type fuses found in some fuseboards contain bare wire which will melt if the rated current is exceeded. However fuse wire can eventually start to oxidise which may cause premature failure. An HRC (High Rupture Capacity) fuse uses a sand-filled ceramic cartridge to prevent oxidation and also to extinguish the arc. These are typically found in UK mains plugs but larger versions are used in industry. Continental fuseboards use such ceramic cartridge fuses as well.

^ 1" ceramic 3 Amp fuse used in British plugs. (13A ones are brown. 5A are black).

^ 1.25" glass anti surge fuse - notice the coiled fusewire inside.

Photos © Alan Winstanley from my Electronic Component Photos CDROM V1 available exclusively from Wimborne Publishing Ltd.

Electronic equipment often utilises a variety of glass-bodied cartridge fuses: 20 x 5mm for up to 10A current, whilst larger 1 1/4" types are produced with ratings exceeding 25A. Some types are anti-surge, meaning that they will not "nuisance trip" when equipment surges during first powered up. It is always very important that fuses are replaced with the same type, size and rating. Failure to do so may cause both a fire and an electrocution hazard.

The fuse rating should match the appliance: using the simple formula of P = I x V then Power (in Watts) = Current (Amps) X Voltage (Volts). Anything up to 500 -750W or so (UK) should use a 3 Amp fuse in the plug. Larger appliances such as 3,000W electric heaters or washing machines need a 13A fuse.

It can be highly dangerous to use too high a fuse rating. Failures in insulation etc. may cause an electrical fire and electric shock risk.

Circuit Breakers and RCDs (GFCIs)

^ UK domestic fuseboard using four (dark grey) Miniature Circuit Breakers (MCBs) and a (light grey) Residual Current Device (RCD) to protect the whole system. The blue button is the RCD's test switch. The red switch is the main isolator.

The miniature circuit breaker (or MCB) is a resettable "trip switch" which will quickly open the circuit when an excessive current is drawn by the load. (E.g. every time a ceiling light blows in my household, the MCB trips.) The most sensitive form of protection is the Residual Current Device or RCD, also known as an earth leakage circuit breaker (ELCB) or Ground Fault Circuit Interrupter (GFCI) in the USA.

An RCD detects the imbalance between current flowing in the live and the neutral return. When any difference arises, any losses must be due to current leaking to earth, so an RCD will typically trip within 40ms or less of detecting an earth leakage current of 30mA. This is often sufficient to protect against severe shock. An RCD trip on the fuseboard shown above, hinted of insulation failure in the central heating's electrics, and prevented a possible fire risk.

A Residual Current Device offer the best personal protection against electrocution and is a very wise live-saving investment, especially when using outdoor power tools. Most RCDs now offer "double pole" protection which disconnects both the live and the neutral wires to ensure total isolation, and feature a test button. Fuseboards fitted with RCDs are now quite common.

RCDs save lives - possibly yours!

More resources:

| Introduction | Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 |