In this answer a part was identified as a "3-pole fan isolater". It appears to me that this is like a "regular" switch that disconnects 3 lines instead of just one.

  1. Is my understanding correct?
  2. What is the advantage of this over a single-pole switch?
  3. Why would it be specifically a "fan isolater" and not a "{device} isolater"? i.e. are they used for fans only?
  • Does the switch control a 2 speed motor?
    – d.george
    Nov 30, 2018 at 11:42
  • @d.george - not a clue, that's why I'm asking. :) I use 3 position "dimmers" on all my ceiling fans at home. I have no idea if they are, internally "3-pole fan isolaters".
    – FreeMan
    Nov 30, 2018 at 14:50
  • from my google search it sounds like it is used in commercial environments when work is to be preformed on a fan while not halting power to the other devices on that circuit. Also: "is this a test?"
    – ojait
    Jan 7, 2021 at 20:16

4 Answers 4


It's common for extractor fans, especially in bathrooms, to be linked to the lighting circuit. When you turn on the light, the fan comes on. When you turn off the light, the fan keeps running for a few more minutes. Some fans have a humidistat, so they keep running until the humidity is low enough.

This requires three wires (plus an earth):

  • A a permanent live, which powers the fan.
  • A switched live, connected to the bathroom lights.
  • A neutral.

Safely isolating the fan for maintenance requires all three to be disconnected. In particular, if you disconnected only the permanent live, there would still be a live at the fan as soon as you turned to lights on to inspect the fan.

Fans that don't have any over-run capability can be isolated by a two pole switch.

  • I guess this must be it since other answers support this. If it were me, I'd turn power off at the breaker before working on a fan/light combo and would just use a regular, 1-pole switch to turn the fan/light on/off.
    – FreeMan
    Sep 4, 2023 at 19:59

It allows you to disconnect (isolate) the fan from the rest of the circuit so you can service it and leave the lights and other power on at the same time if needed.

Here are a couple of resources: https://www.tlc-direct.co.uk/Technical/DataSheets/MK/3_Pole_Fan_Isolator.pdf https://www.se.com/ww/en/faqs/FA166866/

  • 1
    Perhaps you could expand on this a bit with summaries of what you've linked? Jan 6, 2021 at 5:19
  • In what way is this different from a "switch"? Pretend I'm not an electrician, but just play one on various internet web sites (and around the house)...
    – FreeMan
    Jan 6, 2021 at 13:04
  • This looks an awful lot like a 3-pole switch that fits in a standard residential box. Such a thing would as weird as a 2-pole residential switch would be in the US. I'm not surprised it gets a very different name.
    – KMJ
    Aug 30, 2023 at 21:54

To answer your question the switch isolates 3 lines that being Live, Neutral and Switched Live, unlike most devices, a fan is required to run by the call from another device, (usually a light), so the switch isolates the fans main power source, (Live and Neutral), and the demand call from another source, usually a light switch, (Switched Live).

Some fans operate after their initial demand, some fans are required to work on their own in order to clear products of combustion, (kitchen), humidity, (shower room), some fans have built in devices humidistat’s, timers that run the fan for a preset time after demand, for this reason there are three wires for supply, Live, Neutral and switch Live.

  1. Your understanding if not really correct, omly light switches require single pole isolation as there only required to isolate Live, most devices require two pole that being Live and Neutral.

  2. All isolators should by design isolate all cable carring current nuetral included, thus most devices require douible isolation.

  3. A fan isolator as described has three inputs, most devices have two so yes mostly used for fans only.

  • Could you please expand on, "a fan is required to run by the call from another device, (usually a light)"? I'm not certain how that works. I've got ceiling fans in my house (for example) where I can turn the fan on/off entirely independently of the light. I must be totally misunderstanding you.
    – FreeMan
    Aug 31, 2023 at 17:09
  • will edit my post, thing of a bathroom fan some are required to run after the light is turned off, thus require a permanant supply, a feed is then supplied from the light to the switch to operate this feature via the isolator. Aug 31, 2023 at 18:20
  • @FreeMan Hope that clears that up for you Aug 31, 2023 at 18:49

Isolator switches are commonly found in industrial environments when equipment using high voltage needs to be worked on. High voltage lines can not use a common "on/off" switch due to the arc potential from the conductors.

Here's one electrician's unique summation on the use of an isolator fan switch. It sounds like he's going against the established view of how and when the isolator should be used. His statement also claims that a 3-pole isolator is not always needed for certain situations.

Depending on who you ask (or where you look) it doesn't appear isolators are a required device when maintaining proper code standards. I've read mixed answer's claiming "yea" and "nay". Here's one answer that quotes chapter and verse, but still seems ambiguous.

Thanks. I learned a lot about isolation switches today!

  • But nobody learned anything from your answer. Please include relevant bits from your links because links rot over time.
    – FreeMan
    Jan 8, 2021 at 11:40
  • @Freeman- So it is a test! The important point of asking a question is whether the OP is satisfied with the answer and has acknowledged the answer as appropriate. So let me ask you: "Did my answer's satisfy your questions?" I believe they did. No need to beat a dead horse as they say. If you need more detail please be more specific to what area of the question. You made a vague statement about others not learning. That's subjective and wrong as I stated earlier I had learned from answering your question. So nobody learning is a false claim.
    – ojait
    Jan 8, 2021 at 15:44
  • No, it's SE policy to not post link-only answers, and that's exactly what this is.
    – FreeMan
    Jan 8, 2021 at 17:39

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