Wiring humidistat fan but only 2 core cable

Discussion in 'Electricians' Talk' started by squelch41, Aug 15, 2021.

  1. squelch41

    squelch41 New Member


    I am trying to fit a bathroom fan with a humidistat to replace an old, failed switched fan.
    There is only a 3 core cable (so live, neutral and ground - no switched live)

    There is an isolater switch outside the bathroom.

    I don't mind the fan being enabled all the time, switch off only by the isolator if the need arises.

    See the wiring diagrams attached.

    What I don't understand is how the unit detects the LS and L being bridged.
    Surely, when LS is live (because a light is on), it is equal to live?
    So LS and L being bridged is the same as having a LS switched to on?

    Bridging LS and L definately does disable the humidstat and timer - the fan just runs

    I think leaving LS disconnected does achieve what I want, but it might mean that leaving LS floating actually means the fan will turn on randomly?

    (I can't re-wire and run a cable with live-switched -- there is a floor above the room and I cannot access the wiring without removing all the flooring etc)

    Any insights would be appreciated!

    Attached Files:

  2. CraigMcK

    CraigMcK Screwfix Select

    The fan needs a switched live to activate the timer humidistat circuit, without that I don’t believe it will ever start.
  3. Xshaunyx

    Xshaunyx Member

    With the wiring you have you would have been better off getting a fan with a pull cord
  4. FlyByNight

    FlyByNight Screwfix Select

    Just leave LS un-connected. In normal use it will have several metres of cable running to it which will be "floating" and nothing ever happens even when that pick up stray voltages. It may well be designed with a high value (200k) resistor to pull it down.
  5. FlyByNight

    FlyByNight Screwfix Select


    The humistat operation will be independent of an incoming SwL.
  6. squelch41

    squelch41 New Member

    It does - starts up fine
  7. squelch41

    squelch41 New Member

    Hindsight is a wonderful thing!

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