Borrowed neutrals identifying where the error is?

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I have had a new consumer unit fitted, all seemed to work OK until night came and then lights tripped, I will guess until the two security lights were energised there was not enough current drawn as LED's don't draw much.

So found in the hall behind the 4 gang intermediate switch there are 4 black wires in a connection block and removing one stops hall lights working and stops the RCBO's from tripping, I will guess since hall lights on the wall they have been wired different to rest, but there is the standard red, yellow, blue going to two switches so there seems no reason why it needs borrowed neutral to work.

The consumer unit is not even in the house, so not easy running outside to reset or on/off and the RCBO do not switch neutral so trying to think of a way to test to find what does what.

I can take the easy way out and combine upper and lower floor lighting, but I must be having a senior moment just can't think how to correct the problem. Can't find clamp on and it would not measure 30 mA anyway.
 
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I have had a new consumer unit fitted, all seemed to work OK until night came and then lights tripped, I will guess until the two security lights were energised there was not enough current drawn as LED's don't draw much.
Doesn't it only require 4 - 7 Watts if it is a borrowed neutral?
Wouldn't a borrowed neutral cause the other circuit RCD to also trip - if more than 4 - 7W?

So found in the hall behind the 4 gang intermediate switch there are 4 black wires in a connection block
Shouldn't there be 5 (or 6)? If so, which one is missing?

and removing one stops hall lights working and stops the RCBO's from tripping,
Ok. It would, wouldn't it?
Are you now saying that both RCBOs are tripping?

I will guess since hall lights on the wall they have been wired different to rest, but there is the standard red, yellow, blue going to two switches so there seems no reason why it needs borrowed neutral to work.
Do you mean only the RBY to two of the switches?
That still leaves the other two? So there might be the correct number of neutrals.

I can take the easy way out and combine upper and lower floor lighting, but I must be having a senior moment just can't think how to correct the problem. Can't find clamp on and it would not measure 30 mA anyway.
You could buy one that does.
 
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Doesn't it only require 4 - 7 Watts if it is a borrowed neutral?
Wouldn't a borrowed neutral cause the other circuit RCD to also trip - if more than 4 - 7W?
Yes it did, lost all lights
Shouldn't there be 5 (or 6)? If so, which one is missing?
There were two three core and earth and four twin and earth not wired as expected.
Ok. It would, wouldn't it?
Are you now saying that both RCBOs are tripping?
Yes
Do you mean only the RBY to two of the switches?
That still leaves the other two? So there might be the correct number of neutrals.
It seems there were two line feeds, one from a triple and earth, the other from a twin and earth, the upstairs lamp from triple and earth and outside lights from twin and earth and the supply for the hall lights had been taken from triple and earth instead of twin and earth, once swapped all worked.
You could buy one that does.
Yes I could, however then I have to wait for it to arrive, my error was thinking borrowed neutral how do I connect to correct neutral, where in real terms it was borrowed line, only had to move one wire, but finding which wire to move, entailed over 5880 steps, was not wearing watch to start with, consumer unit in flat under main house, no direct connection so out back door around outside of house and down a level to flat under house every time I wanted to turn power off/on. Not all bad, think I have lost 1/2 a stone!
 

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