Voltage on light circuit when off

[gazdkw82]

Friend of mine has phoned. He needs my help because his lights in the hallway keep flashing dimly whilst in the off position.

He has measured nominal voltage when the lights are turned on and 50v when the lights are turned off.

I dont have much information to work with yet but thought I would ask for a heads up on how to approach this. Iv not had a fault like this before.

The offending lights are via a 3 gang 2 way switch. Could this be induced voltage? If so, how would I confirm this and how can I remedy it?

He has said he changed the lamps for LED and they flash even more obvious now.

I will obviously check connections and test Zs, probably I/R switch live to neutral/earth etc...

I thought I'd just ask for a heads up on how to approach it.

My friend is an electronics wiz so he has some knowledge on the basics. I would imagine he bas tried the obvious

 

[Lucien Nunes]

No, only one per SL regardless of how many lamps it serves, as they are all in parallel. The snubber can be placed anywhere - at one of the lamps, in the rose, in a JB where the SL and N are accessible - to bleed that SL down to N and therefore stop any and all lamps on that switch flashing.

Does anyone have experience of what value of capacitor is typically needed? I would expect 0.1μF to be fine, Xc is about 32kΩ at 50Hz, so even 100's of μA of leakage would only produce a few volts at the lamps.

 

[gazdkw82]

One per switched line, it's bleeding off the leakage from SL to N instead of it passing through the lamp(s).

No, only one per SL regardless of how many lamps it serves, as they are all in parallel. The snubber can be placed anywhere - at one of the lamps, in the rose, in a JB where the SL and N are accessible - to bleed that SL down to N and therefore stop any and all lamps on that switch flashing.

Does anyone have experience of what value of capacitor is typically needed? I would expect 0.1μF to be fine, Xc is about 32kΩ at 50Hz, so even 100s of μA of leakage would produce a few volts at the lamps.

Does it make an difference that there are 2 circuits effected?

Should it be 1 snubber per SL and N, for each circuit?

 

[Lucien Nunes]

Yes. Each SL is a separate, floating 'leakage collector' when it is switched off. It needs to be bled down to its own neutral.

 

[Mark Wright]

Probably switch on the Neutral and not the live, quite dangerous really.

 

[DPG]

Probably switch on the Neutral and not the live, quite dangerous really.

Why do you think the switch is in the neutral? Apologies if I've missed something.

 

[Lucien Nunes]

Probably switch on the Neutral and not the live, quite dangerous really

Unlikely, since in post #9 they reported the switched feed to be 230V from earth with the lights on, and 79V from earth with them off.

 

[Mark Wright]

Sorry I thought he tested between Line and Neutral. I was going to suggest test between Live and Earth at light when switch off.

I have seen something similar and the Second switch was off the first Switch Live and Switch Line in T&E.

 

[gazdkw82]

Any advice as to what value would be needed?

 

[DPG]

Have a google on snubbers, there are ready made ones available that will have the values stated on them.

 

[Lucien Nunes]

0.1μF is common, I can't see that a larger value would be needed for any reasonable amount of leakage, and the larger case would be harder to fit in a back box or wherever it's going.

BTW did you ever do any insulation tests on the switching as I recommended? It would be good to confirm that you are not simply concealing an actual fault.

 

[gazdkw82]

0.1μF is common, I can't see that a larger value would be needed for any reasonable amount of leakage, and the larger case would be harder to fit in a back box or wherever it's going.

BTW did you ever do any insulation tests on the switching as I recommended? It would be good to confirm that you are not simply concealing an actual fault.

Thanks. I will grab a bunch of 0.1μF RC snubbers.

I'm hopefully going back round this weekend. I will IR test then.

 

[pc1966]

You should find them in various places, example here from RS:

PMR209MC6100M100R30 | KEMET RC Capacitor 100nF 100Ω Tolerance ±20% 250 V ac, 630 V dc 1-way Through Hole PMR209 Series | RS

uk.rs-online.com

The type like that would need legs sleeved of course, some come with insulated wires already:

https://uk.farnell.com/roxburgh/xeb1201/cap-0-1-f-250vac-20/dp/1187659

 

[gazdkw82]

I think I'll order the ones from cpc

https://cpc.farnell.com/ampohm-wound-products/fe-sp-hdr23-100-100/contact-suppressor-0-1uf-100r/dp/FT00715

 

[pc1966]

Yes, though I had posted another one earlier!

 

[gazdkw82]

Update on this......

I confirmed the insulation was in good order. Customer ordered and fitted the snubbers.

Problem solved.

Thankyou for the help and advice.

I wont be so confused if I come across this again.