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Fuses and circuit breakers protect electrical circuits and appliances.

Fuses

The fuse breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs:

• if the device works at 3 A, use a 5 A fuse
• if the device works at 10 A, use a 13 A fuse

Calculating the value of the fuse required

The equation P = I × V can be rearranged to find the current if the power and voltage are known.

I = P ÷ V

For example, what current flows through a 1.4 kW electric fire at a potential difference of 230 V? Remember that 1.4 kW is equal to 1,400 W.

Current = 1,400 ÷ 230 = 6 A

The best fuse to use in this example would be the 13 A fuse. The 3 A and 5 A fuses would melt and break the circuit even when the fire was working normally.

Circuit breakers

These are automatically operated electrical switches that protect electrical circuits from overloading or short circuiting. They detect faults and then stop the flow of electricity. Small circuit breakers protect individual household appliances, whereas larger ones can protect high voltage circuits supplying electricity to entire cities.

Miniature circuit breakers

An MCB (miniature circuit breaker) can be reset and uses an electromagnet to open a switch if the current goes above a certain value. MCBs switch off the current faster than a fuse.

Residual current circuit breakers

An RCCB (residual current circuit breaker) switches off the circuit when there is a difference between the currents in the live and neutral wires of the appliance. RCCBs are more sensitive than MCBs.