What is this fuse-protected electrical panel with on-off switch and indicator light? It bears the label MAINS WATER SUPPLY.

I have tried leaving it switched on or off for long periods of time (weeks or months) and can detect no effect either way.

The switch is in my UK kitchen on the 8th floor of a tower block. The building dates from 1965 but apparently had extensive renovation in the 90's. The kitchen certainly does not look out of date.

I have looked under the sink and at the back of cupboards but can find nothing electrical that it might connect to. The switch is in plain view above one of the work surfaces.

I asked a plumber - he had never seen one before.

I am mystified, can you help?


I discovered that, on a quiet day, I can hear a faint humming sound when the switch is turned on. It sounds like an electric motor. However I can detect no difference in water flow.

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  • 1
    Is this an older house that at one time used a well pump for water, instead of city/town water supply?
    – crip659
    Feb 28 at 13:14
  • 2
    The landlord/maintenance company/people should have all the electrical/plumbing plans for the building. They should have the answer. Might also be an electrical water shutoff valve(odd/rare) that has failed in the open position.
    – crip659
    Feb 28 at 13:30
  • 2
    Pilot light should be after the fuse...
    – Ecnerwal
    Feb 28 at 15:35
  • 2
    I'm going to say that someone on the 3rd floor has been extremely irritated (and dirty) when you switched that off for months!
    – FreeMan
    Feb 28 at 15:36
  • 1
    I was thinking possibly a booster pump on the 8th floor they may not have had good city water pressure and now they do and the pump was removed with the remodel in the 90’s.
    – Ed Beal
    Mar 1 at 17:49

2 Answers 2


https://www.google.com/search?q=uk+"mains+water+supply"+switch produces results like:

It's a switch that controls a valve to turn off your water supply. Since the light still lights, power to the switch is still good. But since it doesn't cut off your water flow, probably the actuator has failed or possibly there is a wiring problem between the switch and the actuator.

  • Yes, I found that linked device online. The problem is, it's non-electrical and operated by water-pressure alone. Quote "use nothing more than water pressure to isolate the incoming mains water supply (so no batteries or electricity)". My device is definitely mains-powered. Mar 3 at 0:22

Purely a guess, but perhaps something (a pipe heater) to keep it from freezing in cold weather. That might not be very obvious, depending on the type - for an "in the pipe" heater (the good ones) you'd have to spot the point where it entered the pipe.

Given the updated location, the "failed open electric water shutoff valve" mentioned in comments now seems more likely, perhaps.

  • No reason for a down vote, reversed.
    – Ed Beal
    Mar 1 at 17:48

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