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Location of Isolation Switch for Extractor Fan

Postby Star35 » Thu Jan 14, 2021 1:45 pm

The diagram for wiring a Timed Fan unit on this site (at https://www.diydoctor.org.uk/projects/t ... n_unit.htm) shows the 3-pole isolation switch fitted BEFORE the Junction Box and Switch but it seems to me that it would be better BETWEEN the Junction Box and the Fan. Is there some reason that it has to go there?

Seems like the Switched Fused Unit would do the trick if you wanted to make the whole circuit (including the on/off Switch) safe and the 3-pole isolation switch could be used just to make the fan safe?
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independent-switch-fan-wiring.jpg
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Postby Mr White » Thu Jan 14, 2021 6:45 pm

What you say is true, but in practice extractor fans are rarely wired as the drawing shows.
Most of the time the supply is from the lighting circuit which is why a 3 pole isolator is required, it isolates the live supply from the lamp and the switched live as well as the neutral.
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Postby Star35 » Thu Jan 14, 2021 8:00 pm

Yes, I appreciate that, but I'm running a fan from a spur off a ring main, which is why this diagram is just about right for me. As I understand it, the 3-pole isolator is there to enable you to work on the fan safely. I would like to install the isolator near the fan (seems to be the most practical place) so I'd rather put it after the junction box. Just wanted to check there wasn't any reason that prevented that (or that it didn't break any regs).
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Postby mckaymmc1 » Sun Jan 24, 2021 6:27 pm

There is nothing in the wiring regs to say that TP isolators are required but for special locations ie bathrooms you should install in accordance with the suppliers instructions.

The fuse in the circuit is to protect the fan motor and not the cables so its location isnt important.

No idea why there is a JB in the diagram as this is just a point of failure in the circuit. Just wiring from the TP isolator
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Postby Star35 » Mon Jan 25, 2021 10:25 am

Thanks, I appreciate your input. All makes perfect sense.

I think the JB is in the diagram just to make the wiring simple, it was mentioned in the article that you could go direct from the Isolator switch.
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