Borrowed neutral on split load board

[Brightonbill]

I have a client with what appears to be an intermitent fault on the lighting, but trips the power.

The installation has a 16th edition board with sockets on the RCD side, and lighting on the other side as usual.

Occasionally when the landing light is switched the RCD trips on the socket side. Would a borrowed neutral cause this. The lighting circuit itself has RCBO on it that doesn't trip; does the RCBO have anything to do with it.

I haven't checked for a borrowed neutral on the lighting yet, but even if there was one I can't see why that would affect the other RCD side.

Many thanks in advance

 

[mainline]

It could be a neutral that's borrowed off the other rcds neutral bar, which would cause an imbalance.

 

[Brightonbill]

It could be a neutral that's borrowed off the other rcds neutral bar, which would cause an imbalance.

It's a common neutral bar in this instance.

 

[oscar21]

It cant be a common neutral bar, the RCD has a neutral bar and the RCBO neutral goes into the unprotected bar.

 

[Brightonbill]

Good point. I was convinced it was common. Old Volex board. I’ll check it tomorrow.

 

[mainline]

Good point. I was convinced it was common. Old Volex board. I’ll check it tomorrow.

More than likely, the rcbo has its N in the Rcds N bar.

Does the rcbos lighting circuit stay on when the rcd trips ?

 

[davesparks]

Occasionally when the landing light is switched the RCD trips on the socket side. Would a borrowed neutral cause this. The lighting circuit itself has RCBO on it that doesn't trip; does the RCBO have anything to do with it.

When did this fault first occur?

If it occurred immediately after the RCBO was installed then it may have something to do with it.
If it occurred immediately after some work was carried out on the installation that could have created a borrowed neutral situation then it may be that.

If the fault first occurred recently without being obviously linked to recent electrical work being carried out then it is pretty unlikely to be caused by those things.
Borrowed neutrals don't occur on their own, nor do they sit there not causing tripping for months/years before one day suddenly causing problems.
Likewise an RCBO won't sit there for months/years then one day suddenly start causing a different RCD to trip.

What you have described sounds symptomatic of a N-E fault, I'd be carrying out testing for that first off.

 

[davesparks]

More than likely, the rcbo has its N in the Rcds N bar.

Only if the fault started to occur when the RCBO was installed, and it seems unlikely that this would cause it to only trip occasionally.

 

[Brightonbill]

Well, I’m the second person on this job apparently. Several other electricians from a contractor looked at it 3 years ago. Unfortunately the clients are elderly and it’s difficult to understand what the events and time lines are.

The CU has split neutral bars which I neglected to see previously. So apologies for the red herring there.

I did a wander lead on the hall and stairs lighting to see if there was a borrowed neutral but all appeared OK.

Then I combined all of the lighting circuits into one MCB bypassing the RCBO circuit, but it still tripped.

My last attempt was to temporarily move all lighting circuits over to the RCD side and I thought this had it cracked. But sadly after a while it tripped the RCD.

All IR tests are OK, not outstanding but lowest is 18meg, so I’ve assumed it’s not that.

Now baffled.

 

[MJPD29]

Have you got an earth leakage tester? You could clamp it and see what leakage is at any given time and ramp test the rcd to see what it is actually tripping at. You could find that it’s close to its limit and turning on a light in that lighting circuit is pushing it over the edge.