I have deconstructed my bathroom fan and found three wires coming from the wall:

  • N -- neutral
  • A -- has 230V only when light switch is on
  • B -- has 230V only when light switch is off

I wonder how such a "device" can be constructed when powered only by standard household 230V wiring (line, neutral, ground). Does it have a name? What is the schematic of such device?

  • It's a bit unclear what you mean. All 1-phase household devices are powered with Live and Neutral, and with Earth/Ground for safety when necessary. – Justme Jun 5 '20 at 21:14
  • Where is the light with respect to the fan? Does the light work? Is there a switch for the fan? Is the fan switch in the wall box as the light switch? What worked and did not work before you removed the fan? It seems likely that there has been a wiring error somewhere rather than the "device" worked properly with the situation that you described. – Charles Cowie Jun 5 '20 at 21:31
  • @CharlesCowie I have removed the fan because it was old and ugly. It was working properly. When I switched light it started. When I turned light off then it kept spinning for few minutes and turned off. Out of curiosity I have checked how cables connected to the fan (N, A, B) behaves with respect to light switch. Now I wonder what kind of device can transfrom standard household wiring into (N, A, B) wiring. – Trismegistos Jun 5 '20 at 21:35
  • You should revise the question to add that description. I will post an answer with a diagram. – Charles Cowie Jun 5 '20 at 22:41
  • Can you post photos of the insides of the boxes please? – ThreePhaseEel Jun 9 '20 at 18:41

What seems to be happening is something like the function of the diagram shown below. The light switch is a single-pole, double-throw switch. When it is off, 230 the OFF contact supplies 230 V to the relay contact labeled CR NOTO. That contact is normally open, so no power is supplied to the fan motor. When the switch is turned on, power is supplied to the fan motor and the CR relay coil. When CR is energized, CR NOTO immediately closes. When the switch is turned off, 230 V is supplied through contact CR NOTO. That contact remains closed because the contact has a normally open timed to open function. It closes immediately when the relay is energized, but does not open immediately when the relay is de-energized. It opens only after the set time has expired. When the time expires, the contact opens shutting off the fan.

Such a relay should be available to buy. You may have difficulty finding one with a 230 V input or coil. You also need to be concerned about where and how to mount it in a manner that conforms with electrical safety regulations.

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Rather than restoring the original operating scheme, it might be preferable to just disconnect the wire wire that is live when the light is off and install a timer switch for the fan next to the light switch. There may also be such a thing as a combined light and timer switch that could replace the light switch. Two switches may require a larger wiring box for the switches. The available components and preferable configuration will probably be whatever is common in your part of the world.

  • In my installation light switch is just normal light switch. When it is off it breaks the circuit. When it is on it completes the circuit. – Trismegistos Jun 6 '20 at 14:31
  • "I have checked how cables connected to the fan (N, A, B) behaves with respect to light switch" and" A -- has 230V only when light switch is on B -- has 230V only when light switch is off" Say it is not just a normal switch. Have you looked to see how many wares are connected to the light switch? – Charles Cowie Jun 6 '20 at 15:28

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