Mains electricity: Most electrical appliances are connected to the mains using three- core cable.

The idea

A three-core cable contains three insulated copper wires: live, neutral and earth. The live wire carries the alternating potential difference from the supply at about 230 V and is dangerous if touched. The neutral wire completes the circuit and is near zero volts in normal operation. The earth wire is a safety wire connected to the metal case of an appliance; it normally carries no current, but if the live wire touches the case, a large current flows to earth and melts the fuse or trips the circuit breaker, disconnecting the live wire. The cable also has an outer insulating sheath and each wire has coloured insulation, so the wires are identified and kept apart. Appliances with plastic cases may be double insulated and need no earth wire.

What earns the marks

• A three-core cable contains live, neutral and earth wires, each with coloured insulation inside an outer insulating sheath.
• The live wire carries the alternating potential difference from the mains at about 230 V and is the dangerous wire if touched.
• The neutral wire completes the circuit and is at or near zero volts in normal use.
• The earth wire is connected to the metal case and normally carries no current.
• If a fault connects the live wire to the metal case, the earth wire provides a low-resistance path so a large current flows and the fuse melts or the circuit breaker trips.
• Double-insulated appliances with plastic cases do not need an earth connection.

Common mistakes and how to put them right

• Thinking the earth wire carries current all the time.

  Put it right by stating it carries current only during a fault.

• Believing the neutral wire is always safe to touch.

  Put it right by noting that a fault can raise its potential difference.

• Assuming every appliance needs an earth wire.

  Put it right by explaining that double-insulated appliances are designed without one.

Exam tips

• Name each wire and give its function in one sentence, then explain the fault path from live through the case to earth.
• Link the large fault current to the fuse melting or the circuit breaker tripping, and state that this disconnects the live wire.
• Use a simple labelled diagram of the plug and cable to show where each wire connects.