Learning outcomes
- Identify live, neutral and earth conductors.
- Wire and explain a three-pin plug.
- Explain fuse, circuit-breaker and earthing protection.
- Calculate normal appliance current.
- Describe electrical hazards and safe practices.
15.1 The mains supply
Mains electricity is an alternating supply designed to deliver substantial power. The exact nominal voltage and frequency depend on country, so an examination question normally provides values when calculations are required. Because mains voltage can drive dangerous currents through the body, classroom investigations must use low-voltage supplies.
The live conductor alternates in potential relative to Earth and provides the hazardous voltage. The neutral conductor is maintained near Earth potential at the supply, but it should still be treated as a conductor that may carry current or become unsafe under fault conditions.
15.2 Live, neutral and earth wires
In modern colour codes, live is brown, neutral is blue, and earth is green-and-yellow. The live and neutral wires carry current during normal operation. The earth wire normally carries no current; it is a safety connection to an exposed metal case.
The earth conductor must be firmly connected and longer or arranged so it is the last wire pulled out if the cable is strained. Cable grips hold the outer insulation, not the individual inner wires.
15.3 Three-pin plug
A three-pin plug contains live, neutral and earth terminals, a fuse in the live conductor, and a cable grip. The fuse is placed in live so that if it melts, the appliance is disconnected from the dangerous supply potential.
No bare copper should be visible outside terminals. Loose strands can touch another terminal and create a short circuit. The correct fuse is selected from the appliance’s normal current, cable rating and manufacturer guidance.
15.4 Earthing a metal case
If a live wire touches a metal case, the earth connection provides a low-resistance path. A large fault current flows, causing the fuse to melt or circuit breaker to trip quickly. The case remains close to Earth potential, reducing shock risk.
Without earth, the case can become live. A person touching it while connected to Earth may complete a path and receive a dangerous current. Earthing is therefore used with many Class I appliances having exposed metal parts.
15.5 Double insulation
Double-insulated appliances use an insulating outer case and additional internal insulation so no accessible part can become live from a single fault. They do not require an earth wire and often carry the double-square symbol.
Double insulation is not simply “thick plastic”. It is a designed system of independent insulating barriers. The live and neutral connections still require correct protection and strain relief.
15.6 Fuses
A fuse contains a thin wire that heats when current flows. If current exceeds its rating for sufficient time, the wire melts and opens the circuit. A fuse protects wiring and appliances from overheating; it is not primarily designed to prevent every possible electric shock.
A replacement fuse must have the correct rating and type. Bypassing a fuse with thick wire is extremely dangerous because fault current may overheat cables and start a fire.
15.7 Circuit breakers and RCDs
A circuit breaker automatically opens a circuit when current is excessive and can usually be reset after the fault is removed. Magnetic or thermal mechanisms are common. Circuit breakers act more repeatably than rewireable fuses and can be designed for different trip characteristics.
A residual-current device compares currents in live and neutral. A difference indicates leakage, possibly through a person to Earth, and the device disconnects rapidly. An RCD improves shock protection but does not replace correct earthing, insulation or overcurrent protection.
15.8 Common hazards
Damaged insulation, wet conditions, overloaded sockets, crushed cables, incorrect fuses and exposed conductors increase danger. Water and body fluids reduce resistance, so current through the body can become much larger.
Before work, isolate the supply and verify it is off. Never handle mains wiring as an experiment. Appliance repairs require qualified procedures, suitable tools and testing.
Worked examples
Appliance current
A 1380 W appliance operates at 230 V. I = P/V = 6.0 A. A fuse just above 6 A, consistent with standard ratings and manufacturer guidance, is selected.
Fault protection
A live wire touches an earthed metal case. The low-resistance earth path creates a large current, causing the fuse or breaker to disconnect the live supply.
Incorrect fuse
A 3 A appliance fitted with a 13 A fuse may allow a dangerous fault current well above normal without the fuse operating quickly enough.
Practical focus
Investigation
Use an unplugged training plug or enlarged model only. Identify wire colours, terminal positions, fuse location and cable grip. Calculate currents from appliance power ratings and select from stated fuse options. Do not dismantle a connected mains plug.
Examination guidance
- The fuse belongs in the live wire.
- Earth normally carries no current; it carries fault current.
- Earthing and a fuse work together.
- An RCD detects current imbalance, not simply high current.
- Do not describe neutral as always perfectly safe.
Check your understanding
- Why is the fuse connected in live?
- What happens if live touches an earthed metal case?
- Why does a double-insulated appliance not need earth?
Answers
- Melting disconnects the appliance from the dangerous live supply.
- A large fault current flows through earth and operates the protective device.
- Accessible parts are insulated from live by two protective insulation layers.
