live earth and neutral when c/u switched off

[johnc86]

I installed an extra power socket for my mother for a plasma tv. It was a simple enough job, just a spur from a socket located about 3m away.

The problem is, Iv noticed that even when the circuit is isolated by the main switch, and there is no power to the live conductors, I am getting a small amount of voltage in the neutral and earth wires. Its not a lot, its like getting a static shock, barely noticable. It is intermitant too, it seems to build up, after it has given you a small tingle or lit up the bulb in a low voltage test pen it goes. As the screws on the face plates are connected to the earth terminals, you can also get a small shock from them.

I am pretty confused by this. What could be casuing it? We are going to get an electrician to test it as Im only level 2 qualified, (currently studying level 3).

They realy need a full re-wire, as only the extention to the house built 3 years ago has up to date wiring etc, and is fed from a second c/u. The one which supplys the circuit in question is an mcb type board, but does not have any rcd protection.

 

[sparkybird]

It could be induced voltage from other circuits perhaps? I wouldn't test it using your finger! Have you got a tester or multi meter? You could actually measure the voltage then.

You could try switching off the whole fuse board, to remove the induced voltage possibility.

SB

 

[RF Lighting]

Is the voltage still there when all the main switches are turned off?

Have you learnt about neutral feedback yet?

 

[johnc86]

It is there when all main switches are turned off yeah, thats what Im saying.

 

[disinfo]

Is the voltage still there when all the main switches are turned off?

Have you learnt about neutral feedback yet?

Do tell,

Interested but probably wont understand!

 

[davelx]

It is there when all main switches are turned off yeah, thats what Im saying.

What do you mean by "main switches"? MCBs? Main Isolator in each CU? Please be precise as it makes a difference: MCBs are generally single pole and leave the neutral connected. Main Isolators are double pole and therefore the results will be different.

PS: what type of earthing - TT, TNCS or TNS?

 

[johnc86]

Ok, I isolated using the main switch on each c/u. As I was pretty baffled I decided to switch off all the mcb switched too. I am fully aware that the main isolator switches are double pole but I thought it was a good idea to anyway. I will make sure tonight when I get home from work which type of earth the house has.

 

[RF Lighting]

Is the voltage still there when all the main switches are turned off?

Have you learnt about neutral feedback yet?

Do tell,

Interested but probably wont understand!

With the OP mentioning that he has more than one CU in the property, and having isolated one of the main switches some wiring remains 'live'

This can happen is a live is taken from one fusebox, and the neutral from another.

This is against regs, but is not unknown.

When the main switch for the fusebox supplying the neutral is turned off, it will break the neutral (return) wire for the appliance. As there is still live going into the appliance, it will pass through the appliance as normal, but will have no where to go as there is no path back to neutral.

This then makes all the neutral wire, and all the other neutrals connected to that same fusebox 'live' as well.

Hope that makes sense