Light pendant Live when switched off

[TicklyT]

Cables have many properties, including:-
Resistance
Capacitance
Inductance
Leakance (inverse of insulation resistance)

If we start taking those into account, a pair of conductors in a cable starts to look something like this:-

Untitled

• TicklyT
• 12 Nov 2009
• 1

Many of the characteristics are of little consequence at low AC frequencies, such as 50Hz, and can generally be ignored, but increase in significance with frequency and cable length. That's why wiring TV aerials with T&E doesn't work.

Modern digital voltmeters have a much higher input impedance than older moving coil meters, so they are able to detect some of these insignificant properties.

 

[bongos]

That's why wiring TV aerials with T&E doesn't work.

Scary when they got it to work the other way round though!

(Sorry I know this has been posted again recently...)

 

[Skeee]

Thanks Bongos, the simplest answers are always the best!

So it seems

a) 'Capacitive Coupling' is common in domestic electric wiring and
b) an earthed centre CPC in T&E will greatly reduce the effect and
c) the phenomen is more detectable with a very high impedance meter (DMM).

I was indeed using a Fluke DMM, I have on old 2KΩ/V moving coil meter so perhaps checking with that may give some reassurance.

I'll also check the earths at the switch and the rose but I don't think I'll put in a bridge or capacitor there.

Its just that I don't consider 228vac (when it should be zero from a 239vac live circuit) 'insignificant,' rather it seems a little excessive.

All I was trying to do was satisfy that the pendant is safe (Dead) when switched off only at the switch like the rest of the house appers.

Probably best to assume not.

When I first saw the reading my instinct was to do a current flow check with the bulb (SORRY filament! ) removed and switch switched off. Thinking it may be an open circuit supply voltage.

But experience has taught me not always to go with the first instinct. Some thought is always useful.

Thanks also for the pic, I have a couple of pending TV rewires, perhaps I can save on cable and just use the 75Ω coax for the TV amp loft supply socket as well then!!!!!!

 

[Skeee]

Thanks TicklyT but that looks too much like an LC circuit to me!

I hated doing those
I hated skin effect and waveguides too!

I don't remember Leakance. (Not measured in Mhos is it?)
Sounds like conductance to me.




Just checked lightswitch three cables with CPC all 1ohm to earth.

How does this affect TicklyT's cable properties?