Its very unfortunate and a little dangerous that 95% of heating installers use twin and earth cable to the HW stat.
The green/yell wire is then used as one of the three signal wires from the stat. It then confuses many afterwards because they assume that its an earth wire !
I would have no problem with that if only they would hide the green/yell colour with a suitable colour of insulation tape so that it no longer looks like an earth wire!
Tony should have a problem with that because it is wrong. All three conductors on a correctly wired cylinder thermostat can be live, and the earth continuity conductor should not be used as one of them, even with green/yellow sleeving.
Yes, I agree that its wrong! But its what 99% of installers do.
Its also potentially dangerous if left marked to look like an earth wire!
I have had to come to accept that most installers do this bodge with three core cable. Coming from BBC Engineering where we did everything properly, that was a shock!
But at least it would not be potentially dangerous if seperately coloured at each end.
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