This note covers the dangers of mains electricity and the devices that protect people and equipment: fuses, circuit breakers, earthing and double insulation. It also shows how to choose a suitable fuse rating from the power of an appliance.
Key ideas
Hazards. The mains supply is high enough to cause a dangerous or fatal electric shock. The main hazards are:
- Damaged insulation. Bare wire may be touched, or a live wire may touch the metal case of an appliance.
- Overheating of cables. If too many appliances are plugged into one socket, the current in the cable is large and the cable becomes hot. This can start a fire.
- Damp conditions. Water conducts electricity. Wet skin has a much lower resistance than dry skin, so a larger current passes through the body. Electrical appliances should be kept away from water, and switches should not be touched with wet hands.
The three-pin plug. A standard plug has three wires.
- Live (brown): carries the alternating p.d. from the supply, about 230 V.
- Neutral (blue): completes the circuit. It is close to 0 V.
- Earth (green and yellow stripes): a safety wire connected to the metal case of the appliance. It does not carry current in normal use.
The wires are covered in plastic insulation, and the plug body is made of plastic or rubber because these are good insulators.
Fuses. A fuse is a thin piece of wire in a small cartridge. If the current gets too large, the wire heats up and melts. This breaks the circuit and stops the current.
- The fuse is placed in the live wire. If it were in the neutral wire, the appliance would stay connected to the live supply even after the fuse blew, and it would not be safe to touch.
- A fuse is chosen with a rating just above the normal working current. Common ratings are 3 A, 5 A and 13 A.
- A fuse of too large a rating will not melt when it should. A fuse of too small a rating will blow in normal use.
- A blown fuse must be replaced. This takes time.
Circuit breakers. A circuit breaker is an electromagnetic switch that opens when the current is too large. It can be reset with a switch once the fault has been fixed, so it does not need replacing. It also acts faster than most fuses. A residual current circuit breaker (RCCB, or RCD) compares the current in the live and neutral wires. If they differ, current is leaking through some other path, possibly through a person, and the device switches off the supply very quickly.
Earthing. Appliances with metal cases need an earth wire. Suppose a fault allows the live wire to touch the case. Without an earth, the case would become live and could electrocute the next person to touch it. With an earth wire, a large current flows from the live wire through the case to earth. This large current melts the fuse (or trips the circuit breaker) and disconnects the appliance from the supply.
Double insulation. Many appliances, such as hairdryers and phone chargers, have a plastic case and a second layer of insulation around the wiring. The user cannot touch any live part, even if a fault happens. These appliances do not need an earth wire.
Switches. A switch must be in the live wire, so that when it is open the appliance is not live.
Definitions and formulae
| Term or formula | Meaning / when to use |
|---|---|
| I = P ÷ V | Normal working current of an appliance from its power rating and the mains p.d. (230 V in these examples). |
| Fuse rating | The current at which the fuse is designed to melt. Choose the next standard value above the normal current. |
| Live wire | Brown. Carries the supply p.d. Fuse and switch go in this wire. |
| Neutral wire | Blue. Returns current to the supply. |
| Earth wire | Green and yellow. Connected to the metal case. Gives a safe path to ground if a fault occurs. |
| Double insulation | Two layers of insulation so the user cannot touch a live part. No earth wire needed. |
| Circuit breaker | Resettable switch that opens when the current is too large. |
Worked examples
Example 1 A toaster is rated 800 W and is used on a 230 V supply. The fuses available are 3 A, 5 A and 13 A. Which should be fitted?
Find the working current: I = P ÷ V = 800 ÷ 230 = 3.48 A.
A 3 A fuse is too small and would blow in normal use. The 5 A fuse is the next value above 3.48 A. The 13 A fuse would allow a current far too large before melting.
Answer: a 5 A fuse
Example 2 A hairdryer has a power of 1.8 kW and runs from 230 V. Which fuse from 3 A, 5 A and 13 A is suitable?
Convert the power: 1.8 kW = 1800 W.
Current: I = P ÷ V = 1800 ÷ 230 = 7.83 A.
This is greater than 5 A, so the 5 A fuse would blow. The next value up is 13 A.
Answer: a 13 A fuse
Example 3 A kettle (2300 W), a heater (1500 W) and a toaster (800 W) are all plugged into one extension lead on the 230 V mains. The plug has a 13 A fuse. Explain what happens.
Total power: 2300 + 1500 + 800 = 4600 W.
Total current: I = P ÷ V = 4600 ÷ 230 = 20 A.
This is above 13 A, so the fuse wire overheats and melts. The circuit is broken and the appliances stop working. This protects the cable from overheating.
Answer: the current is 20 A, which blows the 13 A fuse
Common errors
- Putting the fuse or switch in the neutral wire. They must be in the live wire.
- Saying the earth wire "carries electricity to the ground all the time". It only carries current if there is a fault.
- Choosing a fuse rating exactly equal to, or much greater than, the working current. It should be just above it.
- Forgetting to convert kW to W before using I = P ÷ V.
- Saying that a plastic-cased, double-insulated appliance has no protection. It is protected by the extra insulation. A fuse is still fitted.
Exam tip: When asked to explain how an earth wire and fuse work together, give the sequence in order: live wire touches the case, large current flows to earth, fuse melts, appliance is disconnected.
Check yourself
- A 920 W appliance is used on the 230 V mains. What is its current, and which fuse from 3 A, 5 A and 13 A should be fitted?
Answer
I = 920 ÷ 230 = 4.0 A. The 5 A fuse is the one just above 4.0 A. - A 2.0 kW heater is used on the 230 V mains. Which fuse from 3 A, 5 A and 13 A is suitable?
Answer
I = 2000 ÷ 230 = 8.70 A. This is above 5 A, so a 13 A fuse is needed. - Why is a switch placed in the live wire rather than the neutral wire?
Answer
With the switch open in the live wire, no part of the appliance is connected to the live supply, so it is safe. In the neutral wire, the appliance would still be live when switched off. - Why does a plastic-cased hairdryer not need an earth wire?
Answer
It is double insulated. The user cannot touch any live part, so the case cannot become live.