Why does it read 10V when it's off?

[Gone West]

We have a lighting circuit running through a 6A MCB. It has a junction box connecting just two cables which are supply cables. When the MCB is on it's reading around 239V and when the MCB is off, it reads 10V. Is that acceptable or should I be looking for some sort of fault?

[Alan Ambrose]

It's a bit odd - are you using an auto-ranging meter and is the number actually 10mV? These are lighting circuits? Does the 10V persist for a while e.g. a minute?

[Gone West]

15 minutes ago, Alan Ambrose said:

It's a bit odd - are you using an auto-ranging meter and is the number actually 10mV? These are lighting circuits? Does the 10V persist for a while e.g. a minute?

It's an old Mercury MTM01 multimeter and I've got it on up to 600V. The junction box just connects two lighting supply cables as the other circuit connecting to the junction box was removed some time ago. The 10V has persisted for around 10 seconds. I haven't tried it for any longer yet.

[MikeSharp01]

38 minutes ago, Gone West said:

t's an old Mercury MTM01 multimeter and I've got it on up to 600V. The junction box just connects two lighting supply cables as the other circuit connecting to the junction box was removed some time ago. The 10V has persisted for around 10 seconds. I haven't tried it for any longer yet.

It is not fool proof but perhaps test a couple of places WITH THE MCB OFF, the meter on 600V range and with safely insulated probes - see dia below, ?1 = what I think you are doing plus also try ?2 and ?3 to see if the neutral has got away from earth. Disconnect any lamps - ?4 as modern LED and tubes have electronics in them which may be charging, and try the voltage test on the DC range as well as AC to see if anything odd is out there. If it goes away when you disconnect the lamps then its coming from / via them somehow and you might be able to spot where the problem is given the 1,2 & 3 tests at both AC and DC.

 

[Alan Ambrose]

A couple of other easy tests to do:

 

+ Switch off the MCB and leave for 10 mins or so. If the 10V goes away, then it was likely just charge being held in some LED power supplies or similar.

+ If possible, turn the whole consumer unit off and see whether the 10V goes away. If so, there might be some cross connection e.g. on the neutrals, between circuits.

 

>>> the other circuit connecting to the junction box was removed some time ago

 

I might also be inclined to double check whether this was fully disconnected. Know what that circuit was used for?

Edited July 16 by Alan Ambrose

[Mattg4321]

Probably ‘ghost voltage’ being induced from other circuits the cabling for your circuit runs alongside. Impossible to say for sure without looking at it though 

[MJNewton]

Yes, as above it'll almost certainly be due to the ghost/stray voltage that is picked up when using high-impedence test meters (the MTM01 has an input impedence of 10MΩ).

 

Some further reading here:

 

How Stray Voltage Affects Multimeter Measurements

[Gone West]

@Alan Ambrose , @MikeSharp01 , @Mattg4321 , @MJNewton , thanks for the replies. I have carried out the various tests after leaving it off for half an hour. When MCB switched off L/N = 9V, L/E = 9V, N/E = 0V. Turned the whole CU off and L/N = 0V. So my conclusion is that, it is, as suggested 'ghost voltage'. Every day is a school day.

[Alan Ambrose]

>>> ghost voltage

 

I'm thinking it could also be a crossed neutral. You have RCDs in that consumer unit?

 

Another simple test - turn CU on and all MCBs off. Your stray voltage should be zero. Turn the MCBs on one-by-one leaving your suspect circuit til last. See when the ghost voltage turns up.