Mains electricity: live wire – brown

The idea

The live wire in a modern UK mains cable is covered with brown insulation. It carries the alternating potential difference from the supply and is the wire that is switched and fused for safety. In a three-pin plug the brown core connects to the terminal marked L, which links to the fuse and then to the live pin. The live wire is dangerous because it is at a high potential difference relative to earth, so it must never be touched when the circuit is live. For example, in a table lamp the brown core goes to the live pin, the blue core to the neutral pin and the green-and-yellow core to the earth pin. Older cables may use red for the live wire, so the colour must be checked rather than assumed.

What earns the marks

• The live wire is identified by brown insulation in cables made to the current UK harmonised colour code.
• The live wire carries the alternating potential difference from the mains supply and is the conductor that is switched for safety.
• In a three-pin plug the brown live core connects to the live terminal, which is linked to the fuse and the live pin.
• The live wire is at a high potential difference relative to earth, so touching it can give an electric shock.
• Older installations may use red insulation for the live wire, so the colour must be confirmed before work is carried out.

Common mistakes and how to put them right

• Calling the brown wire the neutral wire.

  Put it right: Brown is live and blue is neutral.

• Thinking the live wire carries a steady direct current.

  Put it right: Mains is an alternating potential difference, so the live wire alternates.

• Assuming the live wire is safe to touch when the switch is off.

  Put it right: The switch must be in the live wire and the circuit isolated before touching conductors.

Exam tips

• Write the colour and the wire name together, for example brown live, so the examiner sees both parts of the answer.
• When describing a plug, state that the brown core goes to the live terminal and passes through the fuse.
• Use the phrase potential difference when explaining why the live wire is dangerous, not just the word electricity.