Mains electricity: neutral wire – blue

The idea

In a three-core mains cable the neutral conductor is identified by blue insulation. It completes the circuit: current flows from the live wire, through the appliance, and back along the neutral to the supply. In normal operation the neutral stays close to earth potential, so it is not the wire that delivers the dangerous driving voltage; the live wire does that. The blue colour is a safety code, letting anyone wiring a plug or inspecting a cable connect the correct conductor to the correct terminal. A common example is a table lamp: the blue core is clamped into the neutral pin, the brown core into the live pin, and the green-and-yellow core to earth. If the blue core were swapped with the brown in the plug, the appliance could still run, but the appliance's switch would sit in the neutral conductor, leaving parts live when apparently off.

What earns the marks

• States that the neutral wire is identified by blue insulation in a three-core mains cable.
• Explains that the neutral completes the circuit, carrying current from the appliance back to the supply.
• Notes that the neutral is near earth potential in normal operation, so the live wire provides the driving potential difference.
• Links the colour code to safe wiring practice, for example connecting the blue core to the neutral terminal in a plug.
• Recognises that reversing live and neutral in a plug is dangerous because the appliance's switch would no longer isolate the live conductor, leaving parts live when switched off.

Common mistakes and how to put them right

• Thinking the neutral wire carries no current.

  Put it right: It carries the same current as the live wire in a series circuit, so the correction is that it completes the circuit rather than being dead.

• Believing blue means earth.

  Put it right: Earth is green and yellow, so the correction is to learn the three colour codes as a set.

• Assuming the neutral is always safe to touch.

  Put it right: A fault or reversed wiring can raise its potential, so the correction is to treat all mains conductors as live until proved otherwise.

Exam tips

• Quote the colour exactly as blue, not as black or dark, since the specification names blue.
• When explaining a plug, name the terminal as well as the colour, for example blue core to the neutral pin.
• Use the phrase completes the circuit to show understanding of current flow rather than just naming a colour.