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Mains electricity and electrical safety
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Mains electricity is alternating current. Insulation, earthing, fuses and circuit breakers protect people and appliances.
a.c. and d.c.
Direct current (d.c.) flows in one direction only, as from a cell or battery.
Alternating current (a.c.) keeps changing direction. Mains electricity is a.c. (in the UK about 230 V, 50 Hz: it goes through 50 complete cycles every second).
Heating effect: when a current flows through a resistor, electrons collide with the ions in the metal, transferring energy. The resistor's temperature rises. This is useful in heaters and kettles, but wastes energy in cables.
Alternating current (a.c.) keeps changing direction. Mains electricity is a.c. (in the UK about 230 V, 50 Hz: it goes through 50 complete cycles every second).
Heating effect: when a current flows through a resistor, electrons collide with the ions in the metal, transferring energy. The resistor's temperature rises. This is useful in heaters and kettles, but wastes energy in cables.
Insulation and earthing
Insulation: plastic covers wires so nobody touches live metal.
Double insulation: the appliance has a plastic case, and no live part can touch an outer metal part, so it needs no earth wire.
Earthing: a metal case is joined to the earth wire. If the live wire touches the case, a large current flows to earth, which blows the fuse and stops the case staying live.
Double insulation: the appliance has a plastic case, and no live part can touch an outer metal part, so it needs no earth wire.
Earthing: a metal case is joined to the earth wire. If the live wire touches the case, a large current flows to earth, which blows the fuse and stops the case staying live.
Fuses and circuit breakers
A fuse is a thin wire in the live wire. If the current is too large, it melts and breaks the circuit.
Choose a fuse rating a little above the normal current. A 2300 W kettle on 230 V takes I = P ÷ V = 10 A, so use a 13 A fuse (not 3 A or 5 A).
A circuit breaker is an automatic switch that trips when the current is too large. It works faster than a fuse and can be reset.
Choose a fuse rating a little above the normal current. A 2300 W kettle on 230 V takes I = P ÷ V = 10 A, so use a 13 A fuse (not 3 A or 5 A).
A circuit breaker is an automatic switch that trips when the current is too large. It works faster than a fuse and can be reset.
A 690 W toaster runs on 230 V. Which fuse should it have: 3 A, 5 A or 13 A?
- I = P ÷ V = 690 ÷ 230 = 3 A
- The fuse must be above the normal current.
- 3 A would blow in normal use, so choose the next one up.
Answer: 5 A
Mains is a.c.; cells give d.c. A current in a resistor transfers energy and heats it. Insulation and double insulation stop contact with live parts. Earthing plus a fuse makes a faulty metal case safe. Fuses melt and circuit breakers trip when the current is too large.
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