Chapter 51: common rulesWhat colour should the switched line be?

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Chapter 51: common rules

Chapter 51 opens with compliance with standards, then 512 — operational conditions and external influences: voltage, current, frequency and compatibility.

UK frequency is 50 Hz — fifty complete cycles of the AC waveform every second. It's why a phone camera picks up flicker from a light or a screen that your eye can't see.

512.1.5 — the 70 °C rule

§Switchgear, protective devices, accessories and other equipment shall not be connected to conductors intended to operate at a temperature exceeding 70 °C unless the equipment is suitable for it.

This matters in practice. SWA comes in 70 °C and 90 °C versions depending on the insulation. The rule is about the conductor's operating temperature: a 90 °C cable sized to run above 70 °C must not be connected to an accessory built for 70 °C, because the heat at its terminals could break the accessory down. Either size the cable so it runs at no more than 70 °C, or use equipment suitable for the higher temperature.

Compatibility, again

Compatibility appeared in Part 3 earlier, and here it is again in Chapter 51. That's the pattern for the whole of Part 5: it constantly refers back to Part 3 and Part 4. The parts are not separate silos.

512 · operational conditions and external influences 512.1.5 · equipment not connected to conductors above 70 °C

Identifying conductors by colour

Identification isn't only colour — the book also allows alphanumeric labels, ferrules and clips. But colour is what you'll be asked about.

Conductor	Colour now	Old colour
Line	Brown	Red
Neutral	Blue	Black
Protective (CPC, bonding, earthing)	Green-and-yellow	Green-and-yellow

%Green-and-yellow is a 30/70 split. One colour must cover between 30 % and 70 % of the surface, the other the remainder. Either way round — it doesn't matter which is which.

The switched line, and why sleeving matters

In a loop-in-at-the-ceiling-rose lighting circuit, the switched line in the cable to the switch is a blue conductor being used as a line. It must be identified as a line — brown sleeving over the blue.

✋Leave it blue and you're not following the regulation, and you've created a trap. A householder changing their own light switch later finds a blue conductor, assumes it's a neutral, and has no idea where it goes. Brown tape is acceptable but less durable than sleeving — it can pull off when conductors are disturbed.

514 · identification and notices Table 51.1 · identification of conductors

Colour is not the only means of identification — the book also allows alphanumeric labels, ferrules and clips. But colour is what you get asked about.

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