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Fly lead or cpc? The one that catches people outWhich green and yellow is the cpc?

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Look at a metal accessory from the back and point out the cpc. Most people point at the short link from the back box to the front plate.

That is not the cpc. It is a fly lead.
What it isWhere it goes
cpcThe circuit protective conductorAll the way back to the board and the MET
Fly leadA short green-and-yellow linkBack box earth terminal to the front plate
How to make one. A metal back box has a small earth terminal inside it. The fly lead is green-and-yellow and the same size as the circuit cpc, run from that terminal into one of the earth terminals on the accessory.

What it is for: if a fault made the box or the front plate live, the fly lead gives the fault a safe path, so touching the metalwork does not hurt you.

Plastic boxes often have an earth terminal too, and it is not pointless. It is future-proofing. Plenty of people swap a plastic switch for a metal one without calling anyone — so if the terminal is there, terminate to it. If there is no terminal, leave the cpc safely in a connector block for whoever comes next.
✋Where it bites on an inspection. Upgrading plastic accessories to metal means the back boxes now need fly leads. It is a common addition and it gets missed.

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