Ring Final discussion

Sabo · Feb 26, 2022

Hi all, I had a fault on site today I've not encountered before. if anyone could shed some light on a similar fault they may have encountered before that would be helpful.

I was changing some damaged socket fronts over and I noticed the cables had signs of over heating so I thought I would do a couple of tests from the socket as the CU was rammed, and this is where I have a problem.

LL- complete ring
NN- complete ring
CPC- open ring

So I go to the CU to make sure the connections are good there 1st, purely because the board was rammed full of cable making the earth bar virtually inaccessible. the circuit in question was installed not too long ago by a previous contractor, so I was assuming he might have struggled putting the cpc's away.

I tested the ring final again, this time at the CU and the CPC's are complete. Could this be the classic case of a ring within a ring, and if so, how was I getting NN LL at the socket?

Thanks for reading.

littlespark · Feb 26, 2022

There’s a back box somewhere where the cpc’s have separated. The complete ring showing at the board is maybe through another path. If it’s choc full at board, maybe they’ve mixed up a cpc with another circuit.

Pretty Mouth · Feb 26, 2022

When you come across problems like this, note the readings you get.

Were the readings for L-L, and N-N, the same when tested at the CU as when they were tested at the socket? If so, it's a fair bet that it's just one ring, with a broken CPC somewhere.

If they differ, then there could well be a connection across the ring, or some other weird wiring configuration.

If it's the former, then as @littlespark says, there is probably a path somewhere linking both legs of the CPC. Perhaps both legs share the same metal backbox, or some boiler wiring/pipework etc making a path across

Sabo · Feb 26, 2022

littlespark said:
There’s a back box somewhere where the cpc’s have separated. The complete ring showing at the board is maybe through another path. If it’s choc full at board, maybe they’ve mixed up a cpc with another circuit.

That was along the lines of what I was thinking. But I spent 30 odd minutes finding the two legs in the fuseboard to make sure they was part of the right circuit.

telectrix · Feb 26, 2022

start by dissing all 6 conductors of the RFC from the CU . then meter from these to all other circuits in turn. 1st on continutiy, then IR @ 250V. if there are no cross connections, then you have traced the fault to the RFC alone.

Sabo · Feb 26, 2022

Pretty Mouth said:
When you come across problems like this, note the readings you get.

Were the readings for L-L, and N-N, the same when tested at the CU as when they were tested at the socket? If so, it's a fair bet that it's just one ring, with a broken CPC somewhere.

If they differ, then there could well be a connection across the ring, or some other weird wiring configuration.

If it's the former, then as @littlespark says, there is probably a path somewhere linking both legs of the CPC. Perhaps both legs share the same metal backbox, or some boiler wiring/pipework etc making a path across

The readings were the same at the socket as they were at the board, and the cps when x at 1.67 were bang on. I had both legs out of the db and in my hands to make sure it was definitely the same earth's. The clinic had to be back in use this morning so I will need to go back and investigate it more next week. All the plumbing has been done in plastic, and the ring serves 1 medium sized room. Thanks for your reply mate.

Pretty Mouth · Feb 26, 2022

Sabo said:
The readings were the same at the socket as they were at the board, and the cps when x at 1.67 were bang on. I had both legs out of the db and in my hands to make sure it was definitely the same earth's. The clinic had to be back in use this morning so I will need to go back and investigate it more next week. All the plumbing has been done in plastic, and the ring serves 1 medium sized room. Thanks for your reply mate.

It sure sounds like 1 complete ring doesn't it? I can only think that one or both CPCs were loose at the board, when you tested at the socket. Presumably correctly terminated now, I bet if you go back and test again at the socket, you'll find CPC-CPC will be complete.

Sabo · Feb 26, 2022

Pretty Mouth said:
It sure sounds like 1 complete ring doesn't it? I can only think that one or both CPCs were loose at the board, when you tested at the socket. Presumably correctly terminated now, I bet if you go back and test again at the socket, you'll find CPC-CPC will be complete.

It's definitely a ring, I'm just miffed how it's a complete ring at the board and not at the socket, even once reterminated. It's a badgers arse to be honest with you. It leaves the board in 4mm and at the occasional socket it has 2.5mm connected to a 4mm at some sockets and not the others, not like a hybrid ring.

I can only think the ring has been extended, to make a ring within a ring and I have a break on the cpc where it has potentially been adapted.

Pretty Mouth · Feb 26, 2022

Sabo said:
I can only think the ring has been extended, to make a ring within a ring and I have a break on the cpc where it has potentially been adapted.

If it's a ring within a ring, it's unlikely your r1 or rn values would measure the same when taken at the board vs. when taken at the far ring, if you see what I mean.

Not impossible, but it would mean that the far ring and near ring would have to be of the same length.

Sabo · Feb 26, 2022

Pretty Mouth said:
It sure sounds like 1 complete ring doesn't it? I can only think that one or both CPCs were loose at the board, when you tested at the socket. Presumably correctly terminated now, I bet if you go back and test again at the socket, you'll find CPC-CPC will be complete.

Pretty Mouth said:
If it's a ring within a ring, it's unlikely your r1 or rn values would measure the same when taken at the board vs. when taken at the far ring, if you see what I mean.

Not impossible, but it would mean that the far ring and near ring would have to be of the same length.

Yeah I understand what your saying dude, maybe I just got extremely lucky at that socket. I'll r1 r2 every socket next week and break the circuit down, the room size is roughly the same size as master bedroom so it is within the realm of possibilities that I got lucky at that socket as its the only one I tested from.

But I can't see any other way of having a complete circuit at the board and not in the room in question.

Thanks for your input bud, much appreciated.