So why does the RCCB trip in this case?


Postby sidewinder » Tue Jul 07, 2009 8:51 am

OK here goes (newbie to forums so please be gentle).
I have a split consumer unit with standard wiring setup. I have a MCB on the unprotected side (non RCCB side) and I have checked that it is connected to the unprotected neutral block. When I reset this MCB the RCCB trips! BUT not the MCB itself.

As this MCB is not on the RCCB side, why would this happen?
Do I have an earth leak on that MCB circuit?

Any help appreciated.
Thanks
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Postby summit elec » Tue Jul 07, 2009 8:24 pm

i would take a guess at the mcb supplying a lighting circuit. it used to be common practice to steal a neutral for the landing light from the downstairs lighting and this may be your problem try wiring the lighting circuits to the same mcb if this doesnt trip thats your problem. dont leave it like this as it needs sorting properly.
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Postby sidewinder » Wed Jul 08, 2009 4:49 pm

This circuit seems to run some sockets but it is not a ring.

In desperation I have contaced the previous owners of this house and managed to get some history. Here is the story. The MCB that trips the RCCB was on the protected side and worked fine except that Occationally the RCCB tripped. In an attempt to do some fault finding the previous owner moved one MCB at a time to the unprotected side and left it for a while to see if the RCCB tripped. I asked how occational it was and he said once or twice a month. This MCB was the second MCB out of 4 that he had moved (putting the first one back to the protected side when the RCCB tripped again).

I have just moved the MCB in question to the protected side and everything seems to work fine. I guess the RCCB will probably trip once or twice a month as it did for the previous owner. Atleast I can now test the circuits.

I am still confused why a non protected MCB would trip the RCCB!
I take the point of a shared neutral somewhere and I think persueing that is a good start point.

Thanks for the help. Not sure exactly how to progress but I am away working till next week so will think about it in the mean time. Any help still appreciated.

Thanks
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