This is how a bathroom lightswitch is currently wired... it works. But how?? (There's also a fan isolator switch supposedly connected to an extractor fan in the bathroom, but it apparently does nothing -- the fan never ran, even after it was replaced with a new unit.)

Thanks for any explanation!

drawing of switch wiring

Here's a photo, showing this wiring in action. And yes, it works fine. We can't understand how, or what the electrician who installed it was attempting to do.

wiring from switch to wall


Due to interest, here's the extractor fan's wiring:

Fan switch wiring

And the fan isolator switch's wiring:

enter image description here

And as a photo:

isolation switch wiring

It remains a mystery how they're all connected.

  • Does seem odd if using standard markings it does not trip breaker and the light works. Imagine you are not seeing it right or someone made a mess.
    – crip659
    Mar 15, 2022 at 11:59
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    It's odd for a switch to have "Live" and "Neutral" markings. Usually the poles of the switch are symmetrical, they are "live" and "switched live" but only after you connect wires to them. Until then there is no distinction. (Your blue-with-black wires are probably switched live). Perhaps this is not a simple switch. Do you have the documentation? The model number? Is this two switches?
    – jay613
    Mar 15, 2022 at 12:15
  • @jay613 Did the photo not load for you?
    – Chuck
    Mar 15, 2022 at 12:16
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    Ok .... when I blow up the photo I can see the markings clearly.
    – jay613
    Mar 15, 2022 at 12:19
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    it begins to make sense if you think of the switch as two separate switches ... L side is one ... N side is the other
    – jsotola
    Mar 15, 2022 at 16:26

1 Answer 1


This is marked as a "double pole" switch, and the terminals are marked "supply L&N" and "load L&N".

I think, with a little guessing, the switch is designed to isolate both the live and neutral sides of a device from power. What kind of device, and why, is not clear to me. If you know any local experienced electricians you could ask them how they've seen this kind of switch used in practice.

But the way it appears to be used in your house is to switch two different lighting circuits. You have two switch loops there, operated together by one switch. I can only guess why this would be done, rather than using a more typical pair of switches:

  • Maybe the two circuits are on different breakers so cannot share a live wire.
  • Maybe one switch loop controls the bathroom light, and the other the fan. They are on two loops because that's how they were installed. They were combined onto a double pole switch because that helps prevent users from using the bathroom without the fan, and from leaving the fan running after leaving.
  • maybe the installer only had one of these switches in his van. That's often the answer to many mysteries, but this would be an odd thing to have bouncing around in your van.

You could disconnect each blue wire, one at a time, to see what the two loops each control and whether they are on the same breaker circuit.

  • Thanks for attempting to understand this. It's very confusing! It's just the bathroom light switch. As I say there's also an isolation switch (with six wires, indicating a permanent live -- but I've no idea where that goes or what it's connected to). There's an extractor fan, but it only have two wires: Blue and Brown. (No permanent live.) And the it doesn't work.
    – Chuck
    Mar 15, 2022 at 12:26
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    Wild guess: There are two switch loops to this switch, meant to be controlled by two switches but some previous user decided to put in a double pole switch for "convenience". Nothing really wrong with that. You could disconnect each blue wire in turn, to determine what is controlled by it. Perhaps one is the fan, and the fan is broken.
    – jay613
    Mar 15, 2022 at 12:33
  • @jay613 That is about the only thing that makes sense. Would not be surprise if that is the answer. Should have seen the black tape on neutrals as a clue.
    – crip659
    Mar 15, 2022 at 12:44
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    I've seen the installation manual for this fan and I'm pretty sure that label that says "permanent" is just misleading. It's for switched power and the manual clearly described that it's for switched power. Their other models require both permanent and switched power. I think for this model they just got the sticker wrong.
    – jay613
    Mar 15, 2022 at 20:21
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    The fan was connected to a transformer. That's why the cables didn't match and why the fan didn't work: It was the wrong type.
    – Chuck
    Mar 29, 2022 at 0:01

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