Identify cabling and wiring practices

Since the harmonised colours were introduced, L1 is brown, L2 black and L3 grey. Blue is neutral and green-and-yellow is the protective conductor. Red, yellow and blue are the old colours, still found in older installations.

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

In the harmonised colours, line is brown, neutral is blue and the protective conductor is green-and-yellow. Black was the neutral colour in the old colour code, which is why it is the tempting wrong answer; in new wiring black is a line colour (L2). BS 7671, Chapter 51, identification of conductors.

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

On a 400 V three-phase, four-wire supply, 400 V is measured between any two line conductors and 230 V between any line and neutral. 400 V is the tempting wrong answer because it is the figure quoted for the supply as a whole. Basic electrical science: three-phase supplies.

Source: Basic electrical science (Three-phase line and phase voltages)

Before the harmonised colours, the three line conductors were red (L1), yellow (L2) and blue (L3), with black for neutral. Brown, black and grey are the harmonised line colours used in new work. Blue was a line colour in the old code but is neutral now, so never rely on colour alone: identify the conductor and prove dead. BS 7671, Chapter 51 (paraphrased).

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))

Green-and-yellow is used only to identify protective conductors, such as circuit protective conductors and earthing and bonding conductors, and must not be used for anything else. Although the neutral is connected to earth at the supply, it is a live conductor and is identified blue. BS 7671, Chapter 51 (paraphrased).

Source: BS 7671:2018+A4:2026 (Chapter 51, identification of conductors (paraphrased))