Both rcd,s tripping with faulty light switch

[Topcat33]

I have consumer unit with 2 rcd circuits

Please see rather crude drawing. When I switch the second switch on it trips both rcd,s if there is no bulb in it doesn’t trip and also if I put a tester across the bulb holder that lights but doesn’t trip.

Why does it trip both circuits?

The neutral and earth for light 2 are from an unknown source buried in the wall

Also when I put a test lamp across l1 and com on switch 1 I get a light whether it’s on or off

Any help would be good

Thanks

 

[EFLImpudence]

No drawing - just copy and paste.

 

[EFLImpudence]

Has this just started to happen?

It sounds like a borrowed neutral or neutral in the wrong neutral bar.

 

[Topcat33]

All neutrals on the correct bars, looking at previous threads it looks like that neutral in the wall is probably the issue

 

[pete01]

Has it worked fine for years and it's only just started happening or has something been done/changed?

 

[Topcat33]

Yes it’s a new consumer unit installed in a house we bought by previous owners, my dad was an electrician many years ago so he checked the neutrals on the board

 

[SUNRAY]

Yes it’s a new consumer unit installed in a house we bought by previous owners, my dad was an electrician many years ago so he checked the neutrals on the board

I'll suggest checking again.

 

[Topcat33]

Would this happen if the neutral set in the wall is coming from a socket on the ring main which is on the other rcd?

 

[EFLImpudence]

Yes.

 

[ericmark]

The normal is the landing light.

Marked borrowed wire, what happens instead of using three wires between the switches, only two are used, and the line is borrowed from an adjacent switch. As long as the adjacent switch is supplied from the same overcurrent device, the only problem is it can stop some hearing aids working, but that's all.

However, as we started to use silly down lights, the load on the lighting circuits went up, and the ceiling rose is typically rated at 5 amps, and sometimes also the switches, so it was common to split the supply, down and upstairs. Which will require the twin and earth replacing with triple and earth, this was often omitted, which went unnoticed until the two circuits are put on different RCD supplies.

 

[Topcat33]

Update, this borrowed neutral is a spur from the nearest socket to this wall light, if I got an electrician to move the lighting circuit onto the same rcd as the house sockets, would this solve the issue?

 

[Murdochcat]

Thing is from what you described this wouldn’t just start doing it. The CU change would certainly identify this issue

 

[Risteard]

Update, this borrowed neutral is a spur from the nearest socket to this wall light, if I got an electrician to move the lighting circuit onto the same rcd as the house sockets, would this solve the issue?

No - you must remove the erroneous connections between the circuits. Putting them on the same RCCB doesn't remove the danger.

 

[Topcat33]

No - you must remove the erroneous connections between the circuits. Putting them on the same RCCB doesn't remove the danger.

Can I ask what is the danger? Thank you

 

[Echo the husky]

The main danger is to someone working on the circuit the neutral has been 'borrowed' from. If someone was to isolate the socket circuit, test it dead, it would appear dead. If they then disconnected the neutral wires at some point on the circuit between the consumer unit and the light, say to replace a broken socket, the neutral wire on the light side of the disconnected point will become live (if the light was switched on). The danger being electrocution.