Electrician-fitted part-load consumer unit.

Ronange · 2 Jan 2008

My son had his consumer unit changed due to having additional points fitted in his kitchen. This new unit included a RCD.

The year previously, I fitted some wall lights (3, with total load of 360W) controlled by a dimmer switch, in the dining-room for him and these worked well, until this new consumer unit was fitted. Now, when the lights are dimmed, the RCD trips!

I have to say that, for convenience, I took a neutral from the near-by ring main and the supply from a nearby lighting circuit.

Can anyone suggest what might be the trouble? I think it has to do with the new consumer unit as things were working o.k. before.

Thanks in anticipation.

Adam_151 · 2 Jan 2008

Can anyone suggest what might be the trouble? I think it has to do with the new consumer unit as things were working o.k. before.

Ahem... not quite, you are looking at it backwards, the problem was there since you installed the lights, it just didn't show itself up until the new DB was installed!

The problem is that you've created a borrowed neutral (non-compliant with BS7671 and for very good reason)

As neutral to the lights passes through the RCD, and the phase doesn't, its creating an inbalence and its shutting off exactly as it is supposed to. Only way to fix it is to have the lights on one circuit only, easiest way would probably involve an FCU off the ring final

JohnD · 2 Jan 2008

No, it has to do with the way you connected it, with your Borrowed Neutral from a socket circuit.

The RCD works by comparing current flow on the Phase and the Neutral. If there is an imbalance, e.g. a person is being electrocuted or water has got into a fitting, the the RCD trips to cut power to the faulty event.

In a split-load CU, there is a Neutral block for the RCD side, and another Neutral block for the non-RCD side. That enables it to compare Neutral current through the RCD block, with phase current through the RCD.

You have wired it so that Neutral load from your lamp goes through the RCD side (socket circuit), but Phase current goes through the non-RCD side) lighting circuit).

So it is bound to trip.

edited: bah too slow.

I wonder why this wasn't detected during test of the new CU installation?

luminaire · 2 Jan 2008

That's what all electricians fear "robbed neutrals"!

Why? Because "robbed neutrals" can kill!
I suggest you go back to your electrician and get him to put it right straight away.
Then I suggest you leave electricity in the hands of the professionals.
Electricity can and does kill people every year.

Chri5 · 2 Jan 2008

The year previously, I fitted some wall lights (3, with total load of 360W) controlled by a dimmer switch, in the dining-room for him and these worked well, until this new consumer unit was fitted. Now, when the lights are dimmed, the RCD trips!

Thing is, shouldn't the RCD trip permanently? The neutral being out of balance due to live from one circuit (light) and the neutral from a ring would keep the RCD tripped surely?

I can see why the dimmer would compound the fault, but am surprised the lights even work!

Could both the light and socket circuit be on the RCD, and it's the dimmer compounding the faulty live neutral set up?

Luminaire

I suggest you go back to your electrician and get him to put it right straight away.
Then I suggest you leave electricity in the hands of the professionals.
Electricity can and does kill people every year.

Does the fact that the fault wasn't seen during test, not indicate that going back to the current electrician might not be sensible? The fault would have shown up if the full tests required for a CU change were carried out correctly

ricicle · 2 Jan 2008

Which is why I think there should be an R3 test for Neutrals!

A borrowed Neutral would not show up on instrument tests, only on inspection.

Taylortwocities · 2 Jan 2008

In my day I think it was called rn ??

ricicle · 2 Jan 2008

Taylortwocities said:
In my day I think it was called rn ??

That's only on Ring Final tests.There is no requirement to continuity test Neutrals on other circuits.

ColJack · 2 Jan 2008

rn is for rings, not sure if you'd test for it on lighting..

you can't really compare it if you do.. the neutral is bound to be significantly shorter than the live / earth readings as it doesn't go to / through the switches..

also, (and slightly off topic, ) where do you test R1+R2 on a lighting circuit? as if you test at the light, you miss out the earth leg from ceiling rose to switch, and if you test at the switch, then you miss the switch wire out..

edit, beat me ricicle, but I did have more to type..

ricicle · 2 Jan 2008

ColJack said:
rn is for rings, not sure if you'd test for it on lighting..

you can't really compare it if you do.. the neutral is bound to be significantly shorter than the live / earth readings as it doesn't go to / through the switches..

also, (and slightly off topic, ) where do you test R1+R2 on a lighting circuit? as if you test at the light, you miss out the earth leg from ceiling rose to switch, and if you test at the switch, then you miss the switch wire out..

edit, beat me riccle, but I did have more to type..

You R1+R2 test at every point (light, switch) remembering to change two way switches over.Record the highest reading.

If you were continuity testing the Neutral from the CU (with the Neutral for that circuit disconnected from the N bar) then you would be able to check that that particular Neutral was at each point it should be.

This perhaps should be an essential test for anyone who is changing a CU as they possibly cannot physically check all wiring.

Ronange · 3 Jan 2008

Thanks to all who replied to my problem concerning the new consumer unit and the wall lights malfunctioning. I am much humbled (and grateful) by Luminaire's comments. I have done electrical work from time to time throughout my life and with no mishaps. I was an general engineer at one time and later changed my profession. Even though I have keep contact through the literature, I cannot recollect meeing the problem of borrowed neutrals. This, however, has given me food for thought and an intention of further learning! Thanks once again.

ban-all-sheds · 3 Jan 2008

Borrowed neutrals are not good.

If you take one from the different side of a split-load board you'll get RCD trips like yours, but this shows how they can bite you in a nastier way than that...

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Electrician-fitted part-load consumer unit.

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