6.3 Domestic uses and safety — Learn
Prerequisites: Section 6.1 and Section 6.2, especially current as a flow of charge, potential difference, and how components are connected. Chapter 1 for energy transfers.
By the end of this section you can: describe the difference between direct and alternating potential difference; recall the frequency and potential difference of the UK domestic supply; describe the live, neutral and earth wires; and explain the dangers of the live wire.
1. Direct and alternating potential difference
There are two kinds of supply.
- A direct current (d.c.) flows in one direction only. The potential difference stays the same way round. A battery supplies direct current.
- An alternating current (a.c.) keeps changing direction. The potential difference swaps ends many times each second. The mains supply is alternating.
The number of complete changes each second is the frequency, measured in hertz. The UK domestic supply has a frequency of 50 Hz and a potential difference of about 230 V. The current reverses direction 100 times each second, because it changes direction twice in every cycle.
A graph of potential difference against time is a flat horizontal line for a direct supply. For an alternating supply it is a wave.
2. The three wires
Most appliances are connected to the mains with three-core cable. Each wire has a job and its own insulation colour.
| Wire | Colour | Job |
|---|---|---|
| Live | Brown | Carries the alternating potential difference from the supply |
| Neutral | Blue | Completes the circuit |
| Earth | Green and yellow stripes | A safety wire, to stop the appliance becoming live |
The potential difference between the live wire and earth is about 230 V. The neutral wire is at, or close to, earth potential, which is 0 V. The earth wire is also at 0 V. In normal use the earth wire carries no current. It carries a current only if there is a fault.
3. Why the live wire is dangerous
The live wire is at about 230 V. If you touch it, a current can flow through your body to the ground. Even a small current through the body can be fatal.
The live wire can be dangerous even when a switch in the mains circuit is open. The switch may be in the neutral wire, or between the appliance and the live wire. Either way, the wire that goes back to the supply is still live.
A second danger is a connection between the live wire and earth. The earth wire and the metal case of an appliance are joined. If the live wire comes loose and touches the metal case, the case would become live. Because the case is earthed, there is instead a low-resistance path to earth, so a large current flows to earth and the case does not stay at 230 V. This is why the metal case of an appliance is always connected to the earth wire.
4. Try it: a fault and the fuse
The figure shows the live, neutral and earth wires of an appliance, with a fuse in the live wire. You can switch between normal use and a fault, where the live wire touches the metal case.
Predict first. Do not change the switch yet. Write down your answers.
- In normal use, which wires carry a current?
- When the live wire touches the metal case, which wire carries the large current?
- Which wire does the fuse protect?
Test. Switch on the fault. Watch the current in each wire and the fuse.
Explain. The metal case is joined to the earth wire. Explain why the case is safe to touch, and why the fuse is placed in the live wire. Write your answer in two or three sentences.
5. Worked example
Worked example 1. A mains appliance takes a current of 5.0 A for 2.0 minutes. Calculate the charge that flows through it.
The same 600 C flows through the live wire and back through the neutral wire. The current is not used up by the appliance.
6. Comparing direct and alternating
The table compares the two supplies you are most likely to meet.
| Battery | UK mains | |
|---|---|---|
| Kind of current | Direct (d.c.) | Alternating (a.c.) |
| Direction | One direction only | Changes direction |
| Typical potential difference | 1.5 V to 12 V | About 230 V |
| Frequency | 0 Hz | 50 Hz |
Watch out. The neutral wire is at about 0 V, but it is not the same wire as the earth wire, and it is not safe to touch. It is part of the working circuit, so it can carry a current. The earth wire carries a current only in a fault. Also remember that the live wire is dangerous even when an appliance is switched off. If the switch is in the neutral wire, the live wire is still connected to the supply. Re-check which wire carries the current in normal use before you describe a circuit.
7. Progress check
Return to the chapter problem. The cabin heater and lamp are connected to the mains supply.
- State the frequency and the potential difference of the supply.
- Explain why the metal frame of the cabin is connected to the earth wire.
You should now be able to
- describe the difference between direct and alternating current;
- recall that the UK domestic supply is alternating, at 50 Hz and about 230 V;
- name the three wires in a mains cable, their colours and their jobs;
- state the potential difference between the live wire and earth;
- explain why the live wire is dangerous even when a switch is open;
- explain why the metal case of an appliance is earthed.