Bathroom Extractor Fan - Wiring

I am installing an integral pullcord-operated bathroom extractor with no timer overrun. The fan is powered from the mains lighting circuit via a triple-pole isolating switch.

I want the fan to work independently of the light. Therefore, I have taken the permanent live and neutral from the light into the supply terminals of the isolating switch using T/E cable, then connected live and neutral from the output terminal of the switch to the fan itself (which is double insulated so no earth).

The problem is that the fan operates perfectly when the light is OFF but does not operate at all when the light is ON. Can anyone explain to me what I have done wrong ?

Where exactly did you get the supply from?
My guess is ether you have taken it across the light switch and return wires or from the opposite side of the light’s 2 way switch.

Also where did you terminate the earth? It should run all the way to the fan where it should be neatly terminated.

The supply is taken from the permanent live and neutral terminals on the light's ceiling rose.

The earth wire from the isolating switch to the fan is cut and terminated at both ends.

The light itself is operated from a two-way switch wired up for one-way use.

The earth should be carried through to the fan.

If the fan supply is coming from the loop supply and going back to the loop neutral then as you say you would have a perm supply. In which case, the light status on or off would not affect the operation of the fan. The problem is it is affecting it.

I’d re-check your wring in the rose. If its light is an ES or BC fitting remove the bulb and see if the fan still works.

Don't understand about carrying the earth to the fan - there is no earth terminal in the fan and the instructions do say that, being double insulated, it does not require an earth.

I'll carry out the checks you suggest.

BotchItYourself said:

Don't understand about carrying the earth to the fan - there is no earth terminal in the fan and the instructions do say that, being double insulated, it does not require an earth.

Click to expand...

Yes the current fan doesn't need the earth. But a replacement might in the future. You know the earth is not connected but a future DIYer might not.

Up to the early 70's it was common practice not to carry the earth around lighting circuits. This is still causing problems with metal face plates and fittings today.

Let us know what you find.

if the fan only works with the light off it is possible that it has been connected between the live and switched live terminals (ie accross the switch)

if the fan doesn't work at all with the lightbulb out this is the most likely explanation

Fair point about the earth, brown-nought. There might be enough slack in the cable to rectify things.

Sure enough, the fan does not work at all with the light bulb out ! I've checked the wiring in the rose and everything looks OK - all red wires going into the loop, the switch neutral going into the switch live, other neutrals going into the N terminal etc. Could there be a fault in the rose itself ?

I’d be surprised if the rose was broken. I think if it was me I’d start again.

I’m assuming you have three or four cables.

1. Loop in
2. Switch cable
3. Fan Supply out.
4. Loop out (optional)

I’d firstly identify the light switch line. (Obviously with all the supply off) You can continuity test between red & black with the switch on and check for none when the switch is off. Put a red sleeve or piece of red electrical tape on the black. (This is required by the regs to identify the cable is not a neutral.)

Next I’d identify the fan supply. This is best done by linking each cables' red wire intern to its earth conductor at the rose and testing for continuity at the fan isolator switch between earth and the red wire.

Now there should be either one or two cables left. These should be the supply loops.

Join them back into the rose along with the fan cable. Put all reds in the middle connector and the black neutrals in the connector next to the rose flex blue wire. Bend the ceiling light switch cable out the way and carefully energise the system. The fan and all other lights on the same circuit should now work.

Next after isolating the circuit join in the switch line. Its red should join to the other reds with the black (red stripped) cable into the brown flex wire connector.

Now every thing should work - if it doesn’t come back to us.

I started again as you suggested and discovered that the loop neutral and switch live cables for the light were round the wrong way - the switch live that I thought I was picking up was the neutral from the mains and vice versa. Anyway, now everything is in its correct place and the fan is workin OK. Thank you very much for your help.