induced voltage

[RMS]

hi all,

I was at a job today to repair a elv living room light fitting. It was controlled by 2 way switching. The light fitting was faulty but i was measuring a voltage of 81v on the switch live when the switch was off. Also 40v when you switched the other switch on and the same switch off again.

When either switch was in the on position i measured 240v. All insulation resistance readings fine and continuity etc. I fitted a new pendant fitting and when i inserted the lamp the 81v or 40v disappeared. Obviously discharged through the filament but the lamp did not glow at all. When either switch was switched on the lamp worked fine. I tried fitting a low energy lamp to see if the81v or 40v would try and start it up but it never.

Is this just an induced voltage? and why did the lamp not glow with it? Also could this have caused the original elv light to go faulty as it has only been in for 4 weeks?

any answers greatly appreciated.

 

[ricicle]

It sounds like an induced voltage, but in this instance that is all it is a voltage.As soon as you connect a resistance to it, it will drop away (the lower the resistance the more it will drop)
As your tester is probably of a high impedance it will still measure the voltage.If it were of lower impedance it probably would not measure a voltage at all.

 

[Spark123]

IIRC it is normally caused by capacitice coupling, the resistance of the fillament is enough to pull the voltage down. I take it you were using a high impedance DVM to measure this?

Beaten by Ricicle!!

 

[ricicle]

Yeah, to add it could be induced voltage if the switch cables are ran close to other power cables, but as Spark said it could also be capacitive coupling!

 

[RMS]

My meter is a Fluke (digital) so i take it this will have a high impedance.

Thanks for the replies.