Light pendant Live when switched off

Skeee · 11 Nov 2009

Whilst replacing a light pendant I noticed it was 228v ac when switched off.
Switched on was 239v ac. Light has worked fine for years.

Is this a borrowed neutral? It is only fed with one switch 2G which also feeds an outside light.

With the CU fuse pulled it drops to zero.

Aragorn84 · 11 Nov 2009

Does the light fitting have a loop connection?

bernardgreen · 11 Nov 2009

If you were measuring across the contact of the lamp holder then it is an induced voltage from capacitive coupling between Live and Switched live wires in the cable to the switch. And would not be there if there was a filament lamp in the holder.

ricicle · 11 Nov 2009

Sounds like a normal 'loop-in' system to me. The light will always have a permanent line at the rose which will only de-energise if the protective device is removed. The switch wire takes this line down to the switch and back up when the switch is made to bring the light on.
This is why the protective device should always be switched off/removed when working on lights and not just the switch turned off !

Skeee · 11 Nov 2009

Thanks for your replies but am sorry I didn't make it clear, with the bulb removed there is 239v ac accross the two bayonet spring terminals when switched on and 228v ac when the light is switched off. Only when the CU lighting ring fuse is pulled does it drop to zero.

Skeee · 11 Nov 2009

Thanks to Bernard Green. I think I understood what you've said... the light switch is capacitive coupling 228v across the input supply Live to the output load Live which should go when the bulb (sorry, filament!) is refitted.

Not sure why the filament would cancel the capacitance accross the switch.

Doesn't that mean the lightswitch needs replacing if the insulation is breaking down. The switch looks fairly new, 5-10 years old maybe.

Aragorn84 · 11 Nov 2009

how many wires are in the fitting?

Skeee · 11 Nov 2009

I think there was a few wires in the rose (could be a loop in) I'll have a look this weekend.

bernardgreen · 11 Nov 2009

Skeee said:
Not sure why the filament would cancel the capacitance accross the switch.

The amount of energy that can be passed across from Live to Switched Live by the capacity in the cable is very small and far to small to affect a filament lamp. The small amount energy goes as a weak current through the filament to neutral.

Doesn't that mean the lightswitch needs replacing if the insulation is breaking down. The switch looks fairly new, 5-10 years old maybe.

The insulation is not breaking down, capacitance occurs whenever two wires are close together as in a cable.

Aragorn84 · 11 Nov 2009

Surely that effect would only occur if theres a current carrying live wire near the switch wire?

The switch wire in isolation couldnt do it?

bernardgreen · 11 Nov 2009

Aragorn84 said:
Surely that effect would only occur if theres a current carrying live wire near the switch wire?

The switch wire in isolation couldnt do it?

Capacitive coupling is voltage driven. As the wire with an AC voltage goes positive it attracts electrons in adjacent wires, and when the polarity changes to negative the electrons in adjacent wires are repulsed. This creates a voltage in the other wires.

Magnetic coupling results from the magnetic current ( not voltage ) created by current flowing in one wire inducing a current in other wires in the magnetic field. ( how real transformers work )

LiamPope · 11 Nov 2009

Aragorn84 said:
Surely that effect would only occur if theres a current carrying live wire near the switch wire?

The switch wire in isolation couldnt do it?

No current flow is necessary for capacitive coupling which is a consequence of the voltage-generated electric field. You're thinking along the lines of inductive coupling which is caused by the magnetic field of a flowing current - a different kettle of fish.

EDIT: Beaten to it

bernardgreen · 11 Nov 2009

LiamPope said:
EDIT: Beaten to it

Great minds think alike. Some just work faster......but not always better,

Skeee · 11 Nov 2009

Thanks to you all for your ideas especially Bernard Green for your capacitance explanation. If I remember rightly (I often don't) for that much of a voltage (228v ac) to be seen across the switch then the wires would surely have to be microns apart for charge to develop at such a low volatge. I find it hard to believe that, that much capacitance could be charged across two insulated 1 or 1.5mm cables in the conduit.

I will put the volt meter accross the light switch (switched off) Live supply in and Live load out with filament fitted and then removed, to see if this capacitance idea is the correct way forward. If it is then I certainly won't be correcting this stray capacitance with any inductor or capicitor to earth in the rose fitting.

In my limited domestic electrical experience I've never known 230/240v cables capacitve coupling.

I'd welcome domestic sparky replies if this is a common phenomenon.

bongos · 12 Nov 2009

It's very common! Do a search on here for "capacitive coupling"

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Light pendant Live when switched off

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