induced voltage - when is it not??

[sparkybird]

Hi all

Just wondered if there is a level at which induced voltage becomes real voltage? Is it 230V?

Job today where I was getting 60V on a lighting circuit when a light on another circuit was switched on. And 11V when off. The cables run out of the fuse board together for some distance as far as I can tell....

Thanks

SB

 

[Spark123]

Voltages induced by magnetism can increase with current flow.
Capacitive coupling shouldn't change with current in the conductors.
The big difference between induced voltages and non-induced voltages in bog standard cables is that there is no current behind an induced voltage, with a small load it disappears.

 

[sparkybird]

thanks spark123

So now I see why it might go up when I switched on a light, but I suppose I should have asked - is 60V OK and nothing to worry about?!?! I am assuming of course that it WAS induced - how can I tell? Would I be getting 50 Hz on mains and 0 Hz on induced???

SB

 

[Spark123]

Nothing to wory about (providing it isn't open to touch).
I'd expect it to be the same as mains frequency, maybe a tad out of phase but same frequency. Certainly not DC or 0Hz.

 

[ban-all-sheds]

An induced voltage is real, no matter what it's magnitude. It's of little practical use though, as the effective internal impedance is so high that the voltage collapses if you try to draw a significant current. Often it is capable though of making LEDs glow, or slowly charging CFL ballasts.

And it will be the same frequency as the inducing voltage.

Get a moving-coil voltmeter, and you won't see induced voltages any more.

 

[Spark123]

Some digital multimeters made by Fluke have a low Z range for rejecting this. I think some isolation testers have too.

 

[sparkybird]

thanks for replies folks - you learn something every day, eh?

The voltage I was getting was on the load cables of a light, so not accessible and I was getting no glowing when the light off but the other switched on

Cheers

SB