Voltage in circuit when breaker switched off.

[sparkmonkey]

Went to replace a broken socket today and found that with the appropriate circuit breaker switched off, I was still reading 79 volts. I switched all circuits off one by one, testing between each one and it still read 79 volts. Only when I switched off the main circuit breaker did the reading go to 0.

Can anyone explain what's going on? Is this a serious cause for concern?

Many thanks for anyone's time and input.

John

 

[RF Lighting]

If the cable feeding the circuit you were working on is run in with a bunch of other circuits then it is possible that a voltage was being induced into that cable. 79V sounds a little high for this sort of thing (I presume you got 79V phase to earth?) It is perhaps possible that the MCB may not be properly isolating the circuit, or that another curcuit may be backfeeding to the circuit you were working on.
I find it unlikely that induction or feedback are the cause though if the voltage remained when all the other circuits were off.
You could try testing for voltage at the top of the MCB with the circuit disconnected and the MCB off, but you must only do this if you really know what you are doing with electricity, as fuse boxes are not a good place for amatures to stick their fingers.


Oh and welcome to the forum

 

[Spark123]

I take it you used a digital volt meter (DVM) to measure this. As RF says it is likely voltage being induced into the circuit, possibly by capacitance. A DVM has a high impedance and does not cause this voltage to collapse, if you connect a low resistance across the circuit i.e. by plugging a normal lamp in then the voltage should disappear.

 

[sparkmonkey]

RF - Thanks for the welcome. I am an amateur, but a fairly capable guy. Will carefully look at MCBs later during the week (too busy today). I'll probably just swap over the suspect MCB with one of the good ones and see if problem persists. If there are any good step-by-step procedures you can think of for something like this, that'd be good. I'm capable, but not too capable to accept instructions from folk that know better than me.

Spark123 - I am using a DVM andI did try the lamp test this morning. Voltage dropped to 0. However, with reference to amateur status mentioned above, I don't understand capacitance. Are you saying that it's picking up voltage from the surrounding circuits? Is this possible with all other circuits switched off?

Finally, thanks to both for giving me something to work on. It's appreciated. Also sounds as if, whilst there is a problem to sort out, it's not an immediate danger...yes

 

[lookinn]

Because your digital volt meter is high impedance it will pick up volts everywhere. If you use a analogue meter which has a lower impedance you find probably find you will have zero volts.

 

[sparkmonkey]

DVM picks up zero everywhere else. Only picks up weird reading on the one circuit.