I've got a 240V water flow switch connected to the hot water pipe driving the shower - it has 3 cables (COM, NO, NC). When I connect a permanent live to COM, both NO and NC are live. It doesn't make any difference whether the shower is on or not. I don't understand how is this possible.. is it most likely that it's a fault switch?

  • How are you determining that they are hot?
    – DoxyLover
    Jan 8 at 13:24
  • Use the ohmmeter.
    – user263983
    Jan 8 at 14:47
  • Did you disconnect all wires from the NO and NC terminals before testing? Jan 8 at 19:23
  • Is your load more than 16 amps? Jan 9 at 3:28
  • @DoxyLover I was using a non-contact pen.
    – James
    Jan 9 at 22:16

Most likely, your non-contact voltage tester is fooling you. Such devices are not perfect and everything inside the switch is probably so close together that the tester cannot really tell.

If you have a voltmeter, try testing with one lead connected to neutral and the other on NC and then on NO. (Note that with a high-impedance input volt meter such as most digital meters, you may het a phantom reading on the open contact though it should be much lover than mains voltage.)

You can also try temporarily wiring in a light socket and test with that.

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