Weird House Electrical problem. Stumped

[Tyre Tread]

This one has us stumped. Any help much appreciated.

Modern consumer unit

Have a short between live and neutral when the power is on at the main switch but not when main switch is off.

Swapped the wires into another breaker on the off chance it was an issue in that, but no joy.

My knowledge is basic but friend is an electrician and has him stumped too.

[miniman]

One one particular circuit?

[No ideas for a name]

Tyre Tread said:

This one has us stumped. Any help much appreciated.

Modern consumer unit

Have a short between live and neutral when the power is on at the main switch but not when main switch is off.

Swapped the wires into another breaker on the off chance it was an issue in that, but no joy.

My knowledge is basic but friend is an electrician and has him stumped too.

I am possibly misunderstanding... where are you measuring the short, and what with?
When you say the main switch, do you mean the main incoming feed? If that is closed surely you are seeing the voltage from the cut-out/meter.
A short there means the main fuse will have blown.

As I say, I probably have the wrong end of the stick.

[OutInTheShed]

Tyre Tread said:

This one has us stumped. Any help much appreciated.

Modern consumer unit

Have a short between live and neutral when the power is on at the main switch but not when main switch is off.

Swapped the wires into another breaker on the off chance it was an issue in that, but no joy.

My knowledge is basic but friend is an electrician and has him stumped too.

You 'have a short'?
What, exactly are you observing?
A proper 'short' here would blow the 'company' fuse.

[Tyre Tread]

miniman said:

One one particular circuit?

Yes. Lighting only.

[Spare tyre]

I’m probably misunderstanding it

But I’d start by removing all lightbulbs where easily done and seeing if any of them are the cause

Years ago we had a security light that would trigger the rcd but not always

It was also a security light that rarely got activated due to its location

[The Three D Mucketeer]

You haven't got two circuits (on separate breakers) using the same neutral by any chance ??? Hall/Stairs/Landing are often the culprit ... fine on old fuses .. but RCB need to only be one circuit (upstairs OR downstairs) ..

[Stu R]

Guessing it's an older house that's had a new RCD fitted and it's got shared neutrals somewhere, or you've got an appliance fault.

[Mr Pointy]

What do you mean by "the main switch"?

[hidetheelephants]

Tyre Tread said:

Swapped the wires into another breaker on the off chance it was an issue in that, but no joy.

No joy it did exactly the same thing(I assume trip the relevant circuit breaker, but you don't say) or no joy it didn't trip? Has any work been done to the electrics or has this fault spontaneously appeared?

[chrisch77]

When did it start? After doing electrical work or randomly after everything was preciously working fine?

If the latter than I’d expect either a faulty light fitting or a now fried rodent bridging the live and neutral of a cable in your loft space or under a floor board.

[Tyre Tread]

Spare tyre said:

I m probably misunderstanding it

But I d start by removing all lightbulbs where easily done and seeing if any of them are the cause

Years ago we had a security light that would trigger the rcd but not always

It was also a security light that rarely got activated due to its location

Thanks. Have tried that.

[miniman]

I had a similar issue - it was water ingress into an external light.

[Tyre Tread]

There has been a lot going on in the house recently with the owner self installing a new en-suite.

We narrowed it down to the cable from the upstairs to the downstairs lighting ring. We proved it by running another cable and isolating the original from fusebox to upstairs ring.

No idea why that piece of wiring has gone faulty as nothing has been done anywhere near it.

Oh well, at least it's resolved and it's relatively easy to run a new cable down a more permanent route

[Belle427]

If it was ok before the work odds on something has been drilled into etc or mice have been at work.
Lighting circuits are not on a ring, just for info.