Ring circuit testing problems

brizospark · May 21, 2011

Found myself in a bit of a predicament

Carrying out a PIR in a factory

Testing an existing ring circuit in an industrial distribution board which must be nigh on 35 - 40 years old, asbestos flash guards and all!

The problem with that the DB does not have cpc terminals, and all the cpc's are twisted together and then looping around and fixed via the one available earthing nut at the DB

I carried out the ring tests on phase and neutral which returned satisfactory results of around the 0.75 ohm mark each.

Now to test the cpc's I would have to disconnect every cpc to untwist them all from each other and find the correct cpcs and carrying out the test.

Because of this reason I informed the supervisor that the entire DB would have to be isolated as there would be no cpcs connected.

Low and behold the supervisor had a week earlier tested a similar DB about 20 metres away, exactly the same scenario. He carried out the tests live and spent an entire day disconnecting all the cpc, finding the correct ones, testing and then reconnecting them.

The DBs are due to be upgraded anyway by the way, and the ring tests could be confirmed then

So anyway after informing him that the entire DB would have to be isolated he said thats fine just go ahead and do it. I told him that I would require adequate lighting as the lights are fed from this DB and he handed me his torch!

So I thought about this a bit more, I had tested the phase and neutral which were satisfactory, I could see the cpc's from the two ring cables were connected to earth, surely I could just go to the middle socket of the ring, split the cpcs and carry out the tests from here

OK, I know that the standard practice is to do this at the DB, but in this particular circumstance, given the amount of time that would be required I decided that, in my professional opinion, I would be happy with this and if the ring was broken there would be no continuity between the two legs.

So I went to the middle socket, split the cpcs and returned a reading of 0.92, which would add up, slightly higher that the L & N given that it is Twin and earth (2.5/1.5)

My supervisor returned to see how I was getting on, I informed him of the way I had carried out the test to which he said thats no use that aint the way you do it. i tried to explain my reasons for doing this including the theory but he was having none of it and it ended with him saying he was going to report me to the big boss for refusing to carry out his procedure.

What do you guys think?

PC Electrics · May 21, 2011

Two observations:

# your method was perfectly sound. A ring is exactly that, a ring. You can break the continuity at any point and measure the end to end resistance and get the same result every time. Personally this is how I do it on domestic PIRs. Since I'm going to be inspecting behind most sockets anyway, I find one on each circuit that is easy to remove completely and carry out my testing there. I'd rather not dismantle the CU if I can help it, though i do check it over thoroughly.

# 0.75 end to end? That ring must be just about on the recommended length limit. Might be worth checking out the loading, load distribution and likely voltage drop. Indeed it might have been over length to previous editions when vd was still 4%!

guest119 · May 21, 2011

Hey Briz,
Clearly your gaffer has a management problem or he just dont like you but he does have a point.

Removing a socket for inspection purposes prior to testing is all in order as you subsequently (normally) then test the ring from the DB which tests continuity across the socket or sockets you have re fitted.

However, removing a socket to carry out figure 8 tests etc does not provide you with the final result as it does not include your work of fitting (re fitting) a socket.

Worst case scenario being that you have procured healthy results and then walked away and left a fault as you dont know if continuity is good after pushing back your socket, especially with solid core 2.5 as in t&e.

Boyd