Light fitting trips one RCD, but not if connected to a different RCD

[littlejohn]

Hi,

I have a lighting circuit which trips the RCD, I’ve got pared back so that if I take one brand new light fitting and wire it directly into the consumer unit with a brand new cable - and still the RCD trips. If I remove that wire from the consumer unit and put a 3 pin plug on it, and plug that 3 pin into the mains rings, one sockets ring works fine and the other trips out.

The lighting ring is a 6A MCB and the socket ring that also trips out is a 32A MCB, both on the same RCD; the socket ring that does not trip out is also 32A, but protected a separate RCD in the same CU.

The CU was brand new a few months ago (not fitted by me, it’s a properly certified installation) and came fully populated. Total current draw at the main fuse is around 2A so the MCBs are definitely not overloaded.

How is this possible? Dodgy RCD?

Thanks
John

 

[buzzlightyear]

my advice is get a spark in the sort the problem out .

 

[Pete999]

Hi,

I have a lighting circuit which trips the RCD, I’ve got pared back so that if I take one brand new light fitting and wire it directly into the consumer unit with a brand new cable - and still the RCD trips. If I remove that wire from the consumer unit and put a 3 pin plug on it, and plug that 3 pin into the mains rings, one sockets ring works fine and the other trips out.

The lighting ring is a 6A MCB and the socket ring that also trips out is a 32A MCB, both on the same RCD; the socket ring that does not trip out is also 32A, but protected a separate RCD in the same CU.

The CU was brand new a few months ago (not fitted by me, it’s a properly certified installation) and came fully populated. Total current draw at the main fuse is around 2A so the MCBs are definitely not overloaded.

How is this possible? Dodgy RCD?

Thanks
John

Borrowed Neutral,

 

[buzzlightyear]

The CU was brand new a few months ago (not fitted by me, it’s a properly certified installation) and came fully populated.

If I remove that wire from the consumer unit and put a 3 pin plug on it, and plug that 3 pin into the mains rings, one sockets ring works fine and the other trips out

I take one brand new light fitting and wire it directly into the consumer unit with a brand new cable - and still the RCD trips.

and you have wired it your self ?either you have a borrowed Neutral or its been wired up wrong .

 

[littlejohn]

ah, of course! (Borrowed neutral that is) I get it, thank you both

 

[shaun1]

From the description of what you have tried I cant see how it could be a borrowed neutral. You need to do an RCD test on both RCDs and also an insulation resistance test on the light fitting, and potentially a leakage test on the side that trips. It does sound like a faulty fitting and potentially faulty RCD. But it could also be that the side that is tripping has a high background leakage and that fitting is just tipping it over the edge. Is it some kind of LED fitting?

 

[littlejohn]

Finally got a chance to look again at this on Friday - it was a borrowed neutral. Thanks for all the help!