10.33–10.36Direct and alternating potential difference
Mains electricity is an ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.
You should be able to explain the difference between direct and alternating potential difference.
Those two numbers are worth learning exactly: 50 Hz and about 230 V. A frequency of 50 Hz means the direction of the current reverses and returns fifty times each second.
Common mistake
The current in an ac supply is not simply "moving back and forth without going anywhere useful". Energy is still transferred to the components — it is the direction of charge flow that reverses, not the direction of energy transfer.
- Alternating current
- ac: a current whose direction repeatedly reverses; the UK mains supply is ac at 50 Hz and about 230 V
- Direct current
- dc: a current that flows in one direction only, as supplied by a cell or battery
10.32, 10.37, 10.40Mains electricity
Most electrical appliances are connected to the mains using three-core cable. The insulation covering each wire is colour coded for easy identification:
• live wire — brown • neutral wire — blue • earth wire — green and yellow stripes.
The live wire carries the alternating potential difference from the supply. The neutral wire completes the circuit. The earth wire is a safety wire, to stop the appliance becoming live.
The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, and it only carries a current if there is a fault.
- Live wire
- the brown wire, which carries the alternating potential difference from the supply
- Neutral wire
- the blue wire, which completes the circuit
- Earth wire
- the green and yellow striped safety wire, which stops the appliance casing becoming live
10.38–10.39, 10.41–10.42Why the live wire is dangerous
You should be able to explain that a live wire may be dangerous even when a switch in the mains circuit is open, and the dangers of providing any connection between the live wire and earth.
Why an open switch does not make the live wire safe: opening the switch breaks the circuit, so no current flows — but the live wire on the supply side of the switch is still at about 230 V relative to earth. The potential difference has not gone anywhere. Touch it and you complete a circuit to earth through your body, and a current flows through you.
Why any connection between live and earth is dangerous: your body, standing on the ground, is at earth potential. Any path from the live wire to earth has about 230 V across it, so a large current flows. That is what an electric shock is. The same is true if a fault connects the live wire to a metal case — which is exactly what the earth wire is there to prevent.
Worked example
A metal-cased toaster develops a fault in which the live wire touches the case. Explain what the earth wire does, and why the appliance is then safe to touch.
- Without an earth wire, the case would be at about 230 V, and anyone touching it would provide a path to earth.
- The earth wire connects the case directly to earth, so it is held at 0 V.
- When the live wire touches the case, a very large current flows through the earth wire rather than through a person.
- That large current melts the fuse or trips the circuit breaker, which breaks the live connection.
- So the appliance is disconnected, and the case never stays at a dangerous potential.
Answer: The earth wire holds the case at 0 V, and a fault sends a large current through it that blows the fuse and disconnects the supply — so the case cannot become live.
Common mistake
It is the potential difference between the live wire and earth that makes the shock, not the current already flowing in the appliance. That is why a switched-off appliance can still be dangerous, and why standing on an insulating mat reduces the risk.
Before you practise
- Direct current is a flow of charge in one direction, from cells and batteries; alternating current changes direction.
- The UK mains is alternating, about 230 V, at a frequency of 50 Hz.
- Live is brown and carries the alternating potential difference; neutral is blue and completes the circuit; earth is green and yellow and is a safety wire.
- Live is at about 230 V, neutral near 0 V, and earth at 0 V.
- The live wire is dangerous even when a switch is off, because it is still at a high potential relative to earth.
- Touching live while in contact with earth gives a large potential difference across you, so a current flows through you.
- A fuse melts and a circuit breaker trips if the current is too high, breaking the circuit.
- Switches and fuses go in the LIVE wire, so that isolating them cuts the supply at its source.