New Spur Tripping RCD

[NeilJones]

I’m installing an outdoor wall light by spurring off an internal double socket. I’ve wired it as:

2.5mm T&E from socket to switched fused connection [supply] (which I’ve fitted with a 3A fuse), then 1mm T&E from connection [load] to light.

When I switch power back on, the RCD immediately trips and I can’t work out why. With the fused connection switched off, it still trips suggesting a problem between the socket and the connection, but a continuity test on that cable doesn’t show anything amiss. I’ve even tried another length of cable there, but still the same problem. However, if rather than connect to the back of the socket I put a 13A plug (fitted with a 3A fuse) on this cable and plug it into the socket to test it, everything works fine. I’ve reconnected everything multiple times, even replaced the double socket, but it still trips every time it’s wired as a spur.

I’d really appreciate some advice please!

Neil

 

[EFLImpudence]

Just to say you are not being ignored.

From what you describe, I can think of nothing.

 

[terryplumb]

Fit grommets in the galvanised box's cable entry points ,and sleeve the bare copper earth conductors.
Remove the switched FCU from all cables. And measure resistance between its earth and neutral terminals .

 

[oldbutnotdead]

Make sure you haven't got the L & N ends too deep in their holes (if that steel plate is continuous that could be your problem- done similar myself )

 

[NeilJones]

Thanks for the responses. My "proper attempt" had been done properly with grommets and earth sleeving, the photos were just my quick-hash trouble-shooting.

I had another go at it today with the same wiring and components, and it worked fine first time. The only thing I think I did any differently was trimming back the L&N ends so they didn't sit so deep as suggested above, so maybe that was it - I can't think of any other reason. I'm glad to say light number 2 installation went much smoother, just two more to go now!

Thanks again,

Neil

 

[EFLImpudence]

I find it hard to believe that the L and N terminals have no 'bottom' and allow the conductors to protrude and contact something they should not or the earth bar which only goes around the top of the socket.

 

[oldbutnotdead]

Yup, got caught out by that with some metalclad switched FCUs a few years ago, can't remember the brand but apart from that little flaw the engineering was good.

 

[Jackrae]

... but apart from that little flaw the engineering was good.

Really ?

 

[NeilJones]

I decided to test the FCU and the socket I'd had the problems with (I replaced them both, but will need to use this FCU for one of the remaining lights), but neither have any 'oddities' which would lead to a L or N earthing if inserted too deep (the FCU is an MK, so I wouldn't have expected it but the double socket was just a generic trade one), so I'm still left no wiser why this happened. Given that it worked when I stuck a 13A plug on the end of the supply cable to the FCU, it must have been something with the spur connection on the back on the sockets, but I'm lost for an explanation.

 

[flameport]

Faulty sockets are not impossible, I had one once on an old installation which tripped the RCD as soon as anything was plugged into it. The socket was wired correctly, replacing it eliminated the problem, and testing the socket after it had been removed didn't reveal any fault.
Assumed that it must have been some internal problem, where in a particular position with a plug inserted, parts inside made contact where they shouldn't have.