Mains electricity: 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, it only carries a current if there is a fault.

The idea

In a domestic mains supply, the live wire alternates in potential difference relative to earth, and the stated value of about 230 V is the root-mean-square value, so the peak is higher, about 325 V. Earth is the reference at 0 V. The neutral wire is at, or close to, earth potential, about 0 V, because it is connected to earth at the supply substation; it still completes the circuit and carries the normal operating current. The earth wire is at 0 V and normally carries no current; it only carries a current if there is a fault, when a live conductor touches the metal case and a large current flows to earth, melting the fuse or tripping the circuit breaker. A multimeter set to a.c. voltage would show about 230 V between live and earth, near 0 V between neutral and earth, and 0 V between earth and earth.

What earns the marks

• The potential difference between the live wire and earth is about 230 V, and earth is taken as 0 V.
• The neutral wire is at, or close to, earth potential, about 0 V.
• The earth wire is at 0 V.
• The earth wire only carries a current if there is a fault.
• The stated 230 V is an alternating potential difference, so the live wire's potential difference changes direction and magnitude.
• During normal operation the neutral wire carries the circuit current while remaining close to 0 V.

Common mistakes and how to put them right

• Treating 230 V as the peak value.

  Put it right: It is the root-mean-square value, and the peak is higher, about 325 V.

• Assuming the neutral wire carries no current because it is near 0 V.

  Put it right: It completes the circuit and carries the normal current.

• Saying the earth wire always carries current.

  Put it right: It is at 0 V and carries current only during a fault.

• Confusing potential difference with current.

  Put it right: 0 V describes potential difference, not the absence of current in the neutral wire.

Exam tips

• State the reference clearly: earth is 0 V, the live wire is about 230 V relative to earth, and the neutral wire is at or close to 0 V.
• When asked why the earth wire is safe in normal use, say it is at 0 V and carries no current unless there is a fault.
• Use 'about 230 V' for the live-to-earth potential difference and avoid presenting it as an exact unvarying value.
• If a question gives a fault scenario, link the earth wire current to the operation of a fuse or circuit breaker rather than saying the earth wire 'uses up' the current.