Faulty wiring on Bathroom Extractor ?

JimSReynolds · Sep 3, 2015

Hi all

I am replacing the bathroom extractor fan. It had been replaced within the last couple of years but died a couple of weeks back, tripping the switch for the circuit. When the switch was reset the fan was dead.

I bought a replacement fan (OK for zone 1) and isolated the circuit today to remove the old one. After removing the old, I realised that I was not certain which of the three wires was which. I isolated the fan and did some tests with my meter with the power back on and the bathroom lightswitch on and off. Now I am seriously confused.

The wiring colours are Blue, Red and Yellow. The truth table looks like this.

With the lightswitch on
B --> R = 240V
B --> Y = 240V
R --> Y = 0V

So I figure that Blue is earth with the other two wires as circuit and switch live.

However ...

With the lightswitch off
B --> R = 240V
B --> Y = 0V
R --> Y = 170 - 190V floating around.

This makes no sense to me at all. Am I just undereducated here or does this suggest some kind of fault?

The light switch, bulb and fan are all isolated by a switch on the wall above the bathroom door. With that switch turned off, there are no voltages present on any combination of wires. I am thinking the light switch is probably the next point of test. Sound about right?

Any suggestions folks?

PS (yes a qualified electrician will be brought in if things get much hairier.

telectrix · Sep 3, 2015

blue will be neutral. red and yellow will be perm L and switched L. try them one way in the fan. it should come on with the light and then over run after the light is switched off. if it don't work, the swap red and yellow.

trev · Sep 3, 2015

Jim, I think it's time for you to call an electrician now mate.

telectrix · Sep 3, 2015

trev said:
Jim, I think it's time for you to call an electrician now mate.

it's OK, trev. he's in essex, so it can be fixed with some fake tan and a bit of swearing.

trev · Sep 3, 2015

Sha Aap

davesparks · Sep 3, 2015

JimSReynolds said:
Hi all

I am replacing the bathroom extractor fan. It had been replaced within the last couple of years but died a couple of weeks back, tripping the switch for the circuit. When the switch was reset the fan was dead.

I bought a replacement fan (OK for zone 1) and isolated the circuit today to remove the old one. After removing the old, I realised that I was not certain which of the three wires was which. I isolated the fan and did some tests with my meter with the power back on and the bathroom lightswitch on and off. Now I am seriously confused.

The wiring colours are Blue, Red and Yellow. The truth table looks like this.

With the lightswitch on
B --> R = 240V
B --> Y = 240V
R --> Y = 0V

So I figure that Blue is earth with the other two wires as circuit and switch live.

However ...

With the lightswitch off
B --> R = 240V
B --> Y = 0V
R --> Y = 170 - 190V floating around.

This makes no sense to me at all. Am I just undereducated here or does this suggest some kind of fault?

The light switch, bulb and fan are all isolated by a switch on the wall above the bathroom door. With that switch turned off, there are no voltages present on any combination of wires. I am thinking the light switch is probably the next point of test. Sound about right?

Any suggestions folks?

PS (yes a qualified electrician will be brought in if things get much hairier.

Blue is not the earth, it must not be connected to the earth as this will create a potentially dangerous fault. The earth will be a bare copper conductor which needs to be sleeved with a green and yellow sleeve.

The floating voltage you have measured suggests that you are using a test meter which is not suitable for the job. You are measuring a phantom voltage caused by capacitive coupling within the cable.

Please stop placing your life and your property at risk and get the assistance of someone competant

Octopus · Sep 3, 2015

This should be in the DIY section

JimSReynolds · Sep 3, 2015

davesparks said:
Blue is not the earth, it must not be connected to the earth as this will create a potentially dangerous fault. The earth will be a bare copper conductor which needs to be sleeved with a green and yellow sleeve.

Apologies, bad terminology on my part. I meant neutral when I said earth. Aware of the difference.

davesparks said:
The floating voltage you have measured suggests that you are using a test meter which is not suitable for the job. You are measuring a phantom voltage caused by capacitive coupling within the cable.

Gotcha. I was aware of capacitive coupling in the context of line level signals and noise but didn't appreciate it could result in such high voltages in the context of mains wiring. So I would need a low impedance meter to test properly then.

Appreciate the feedback.