Unusual RCD fused spur

Pretty Mouth · Jan 26, 2023

A recent EICR turned up a fault: an RCD fused spur was feeding a socket and a shower pump, and neither socket or pump had continuity on the CPC. The circuit was also showing low IR between live conductors and earth (0.09 Mohms).

I came to do the remedial work and found the reason for the faults: the RCD spur. Nothing unusual about the front, but the back is interesting. There are 2 separate earth terminals, one labelled 'E', and the other labelled 'SE' (an IR test between these 2 earth terminals confirms that they are totally separate). There is a red insulated conductor connected to the SE terminal, which disappears into the body of the unit.

The feed to the spur had its CPC terminated in 'E', while the outgoing cables to the socket and pump had their CPCs terminated in 'SE'. At the time of the inspection I was able to test the RCD through the socket, giving a fairly normal time of 33ms.

I removed the RCD spur and replaced with a normal fused spur (circuit is now RCBO'd), and in addition to sorting the continuity on the CPCs, it also eliminated the low IR problem too.

Has anyone ever come across one of these, or know any more about them? My first thoughts were that it was a dodgy ebay item, however an internet search suggests that it may be an old volex unit.

[ElectriciansForums.net] Unusual RCD fused spur

buzzlightyear · Jan 26, 2023

There made by ced, I have one in the van if you want to swop.

nicebutdim · Jan 26, 2023

Optional clean earth terminal, which should have a link to the other terminal?

Wondering if that red conductor is part of the test mechanism.

pc1966 · Jan 26, 2023

I would expect the two E & SE terminals are linked via the metalwork.

I have seen RCD sockets that had low (50k ohm ish) values of resistance from L-E and N-E and i think it comes from the device having provision to trip if L-E swapped or N open. I think that is probably what the thin red wire in the photos if for.

However, it might be something a bit bonkers like a VOELCB type of arrangement. Personally I think that C/CPC should always be sized to clear on the OCPD just in case the trip electronics fail and your last resort is the 13A fuse or 32A MCB feeding it, but maybe this is different.

Can you run a multi-meter ohms check between E & SE?

Pretty Mouth · Jan 26, 2023

pc1966 said:
I would expect the two E & SE terminals are linked via the metalwork.

I have seen RCD sockets that had low (50k ohm ish) values of resistance from L-E and N-E and i think it comes from the device having provision to trip if L-E swapped or N open. I think that is probably what the thin red wire in the photos if for.

However, it might be something a bit bonkers like a VOELCB type of arrangement. Personally I think that C/CPC should always be sized to clear on the OCPD just in case the trip electronics fail and your last resort is the 13A fuse or 32A MCB feeding it, but maybe this is different.

Can you run a multi-meter ohms check between E & SE?

E and SE are in no way connected, IR >1999 Mohms at 1000V between them.

pc1966 · Jan 26, 2023

Pretty Mouth said:
E and SE are in no way connected, IR >1999 Mohms at 1000V between them.

Interesting, can you check with the RCD on as well? (Assuming it was tripped off before removal)

Other possible explanation is it is capacitively coupled so 50Hz can trip but DC IR testing shows nothing. But again, it seems a strange approach in principle.

Pretty Mouth · Jan 26, 2023

Ok, if I put IR 250V between L and N, the central neon lights up. Also, if I put the same between SE and L/N, the central neon lights up.

pc1966 · Jan 26, 2023

Pretty Mouth said:
Ok, if I put IR 250V between L and N, the central neon lights up. Also, if I put the same between SE and L/N, the central neon lights up.

That is interesting, I wonder if it is intended to show leakage off an appliance?

Pretty Mouth · Jan 26, 2023

pc1966 said:
That is interesting, I wonder if it is intended to show leakage off an appliance?

It's very odd. I've wired it to a plug, with the earth in E. Seems to work as expected, however, sometimes when I plug it in my monitor goes black!

timhoward · Jan 27, 2023

I have a memory of a daft situation with one of those where it protected something in a bathroom and the CU was later changed to a dual RCD board but the spur was left in situ (partly because it was in a crazy difficult place in airing cupboard I suspect)

My memory is that I couldn’t just trip it alone using the earth probe to supply neutral trick.
In the end I removed it.

I’ve been suspicious of them ever since! I think CEF still sell something that looks identical btw.

snowhead · Jan 27, 2023

It's a Volex VT1092,

Here's text that explains it, found on another site, no info in the Volex documentation.

nicebutdim · Jan 27, 2023

snowhead said:
It's a Volex VT1092,

Here's text that explains it, found on another site, no info in the Volex documentation.

View attachment 105693

Thanks for confirming this - it seemed the most logical explanation.

Odd that it isn't pre-fitted with a link from the factory as is the case with a lot of accessories that provide a clean earth option.

nicebutdim · Feb 12, 2023

Cleaning images folder in phone and came across a picture I took the day after this spur was discussed.

Couldn't understand why the spur doesn't come fitted with a removable link - perhaps it did and someone removed it...

timhoward · Sep 15, 2023

So to wake up an old thread, I didn't do a CU change today as I ran out of time. I got global IR of 0.04 Mohms. Digging deeper I found I had 0.04 Mohms both L-E and N-E on the one and only sockets ring final circuit in a large bungalow. I was quite tired and while I recognised that identical result for L-E and N-E was a bit strange I couldn't turn that thought into a clue or shortcut.

So I broke down the ring, and eventually twigged there was an RCD socket and this was the cause. Embarrassingly it was right next to the consumer unit and had been in sight the whole time....
Now it's happened to me I'll probably remember it, I just wish I'd remembered this thread earlier!

SparkySy · Sep 16, 2023

Had the same thing happen to me a couple of days ago with an RCD socket of some vintage. Well hidden in a conservatory, had to move a shed load of stuff to find it, I was initially thinking as the I got closer to it, ere we go another dodgy conservator install!
The best one I found on that job was a live switch box in one of the bedrooms, it had screw caps which were the only things providing a barrier from it.