3 core t+e giving 45 volts when off

[mick b]

I have a 3 core t+e on a 2 way switch and when it is turned off there is still 45 volts at the light. I have stripped this down to a 1 way switch and the voltage disappears. I then meggered the 3 core and it is clear. There is continuity through the conductors and no obvious fault with the cable. The earth is connected but when I put the cable back on it goes to 45 volts again. Any ideas on induced voltages on 3 core cable.
Many thanks

 

[Marvo]

If you do the same test with a real mans voltmeter ie one with a needle instead of a digital display I bet there's no significant voltage


There's lots of threads around about the ghost or phantom voltages being a problem with the high input impedance of digital testers.

 

[D Skelton]

Any ideas on induced voltages on 3 core cable.

It's not induced. Copper is non-ferrous remember, you can't induce voltage copper to copper. The problem you have is capacative coupling, it is a phantom voltage and is harmless. Use an analogue multimeter and it won't register.

The most likely cause for this 'problem' for want of a better word, is degraded insulation.

 

[Darkwood]

3 core T&E ---isn't that an oxymoron?

 

[Silly Sausage]

I've developed a high impedance to these questions now, that's why I don't respond to them anymore.
I might give a false answer.

 

[davesparks]

It's not induced. Copper is non-ferrous remember, you can't induce voltage copper to copper.

Yes you can, induction occurs in any conductor if it moves through a magnetic field, or if an alternating magnetic field crosses a conductor.

 

[D Skelton]

Yes you can, induction occurs in any conductor if it moves through a magnetic field, or if an alternating magnetic field crosses a conductor.

You can't create a substantial magnetic field with a non ferrous material. Think! A ferrous material is needed to focus the induced magnetic force from one conductor to another

To induce a current in a copper conductor you need; alternating current --> copper --> ferrous material --> copper

If you want to induce measurable current from copper to copper, you would need miles and miles and miles of conductor with a gap between them so small it would be almost invisible to the eye! Even then you'd be talking microamps!

When I say you 'can't', what I really mean is that it's really really hard!

 

[Marvo]

I've developed a high impedance to these questions now, that's why I don't respond to them anymore.
I might give a false answer.

The frequency of this particular question does point to an across the board lack of understanding in this particular area. It's obviously something that's not being covered properly in the training and I'm not convinced it's just the limited scope training that's at fault here.