Isolated Board but still live

[Brendo]

Any advice would be greatly appreciated on this.

Visited a job the other day where the light circuit had blown at the board. We Isolated the mains and tested but neutral bar remained live. Found that a rogue twin and earth with its neutral was on the bar and was live however the line conductor had been disconnected previously.
Could anyone's experience shed some light on what might be going on that although the mains was isolated this had happened.

 

[westward10]

What are you measuring the voltage with. The rogue neutral may still serve a purpose for a circuit from another source so you will get potential from neutral to earth with your board isolated.

 

[littlespark]

What are you testing with? Those non-contact pen things light up when a flea farts.

 

[Brendo]

It's a Duspol Voltage tester which I use to make sure the mains was isolated before getting my fingers stuck in. I did think the rogue TandE might be from the communal stairwell lighting

 

[westward10]

Can you access the board for the communal lighting.

 

[Brendo]

Can you access the board for the communal lighting.

I'll have a word with the building manager to check it

 

[westward10]

What I would do but not necessarily advise someone to do is disconnect the neutral and see what goes off.

 

[Pete999]

Are there any UPs on the insatllation, safe isolation, also includes backefeeds from other sources, could be worth checkng.

 

[Brendo]

Are there any UPs on the insatllation, safe isolation, also includes backefeeds from other sources, could be worth checkng.

No UPs on site at all. I've disconnected it now and all communal lights remain functional. Begs the question why it's there to begin with

 

[westward10]

Does the voltage disappear now.

 

[Pete999]

No UPs on site at all. I've disconnected it now and all communal lights remain functional. Begs the question why it's there to begin with

Need to find out where it goes/comes from, or find out what doesn't work when it's disconnected.

 

[Brendo]

Does the voltage disappear now.

Yes it's disconnect but still live but board is now isolated fully

 

[The Ghost]

Sure it's not a phantom voltage?

 

[Deleted account]

Are they emergency lights?

 

[Brendo]

Are they emergency lights?

Yes there are emergency lights in the communal stairwell.
Why would an alternate source be fed into the consumer board within one of the flats though and only utilising the neutral.