Slightly high CPC continuity on ring final

[1974stephen]

I'm just testing a new ring final circuit prior to third party inspection and have 0.56 and 0.57 on r1 and rn but 1.18 on r2, when calculated it should be around 094. Am I looking too hard for a fault that's probably not there?

 

[sparkwright]

Is it 2.5mm2 twin and earth with 1.5mm2 earth? I suspect it is, yet if 4mm2 T+E was used, the earth would be 1.5mm2.

It would suggest a bad earth connection somewhere on the circuit.

Then again, such faults can be discovered, no fault ever found, and it can't be explained.

 

[1974stephen]

Yes, 2.5mm T&E with 1.5mm CPC.

I'm thinking a bad screw-down connection or maybe just caught a little bit of sleeve, but is that discrepancy going to result in the tester who's putting his name to the certificate to want to investigate further? I guess the answer to that might depend on how thorough he is....

 

[sparkwright]

On most modern sockets nowadays, the terminals are square and quite large, and smaller earth wires just do not stay in place very well unless you double the ends over.

Might be worth a look if you want to be thorough.

 

[EFLImpudence]

Perhaps you could split the circuit around half way and see if you get an equal discrepancy on both sides or whether it is all on one side.

 

[1974stephen]

EFLImpudence - Would that involve linking all three conductors on each side of the ring at the CU, split ring in the middle, and then noting the relationship between r1 and rn and r1 and r2, on each leg, and proportionally it should be slightly higher on the 'faulty' side? I've been trying to rack my brains of a way of easily and methodically finding this rather than randomly taking sockets off; it's so much more difficult than finding an open circuit.

 

[plugwash]

Is this a 100% new circuit wired with new cable or is it made up of bits of existing wiring?

 

[1974stephen]

All new.

 

[sparkwright]

.

 

[EFLImpudence]

EFLImpudence - Would that involve linking all three conductors on each side of the ring at the CU, split ring in the middle, and then noting the relationship between r1 and rn and r1 and r2, on each leg, and proportionally it should be slightly higher on the 'faulty' side? I've been trying to rack my brains of a way of easily and methodically finding this rather than randomly taking sockets off; it's so much more difficult than finding an open circuit.

It would be easier with a wander lead. Just measure all six of the separated socket conductors to the CU.

Or you can link conductors, measure pairs and work it out.

 

[AdrianUK]

What are you measuring it with & when was that device last calibrated?

 

[JohnW2]

What are you measuring it with & when was that device last calibrated?

I don't really understand how calibration issues could result in the apparent disparity between R1 and R2 figures that has been observed.

Kind Regards, John

 

[1974stephen]

It's a Di-Log DL9110, calibration due in three months.

 

[1974stephen]

I split the ring at the two places it starts serving the actual rooms, so effectively removing the two legs that feed back to the CU, and then did a continuity test between those two points; and the higher CPC reading remains. I've then used a wander lead measuring from one end of the ring and worked round logically in the order it was wired and measured increasing readings on both r1 and CPC, but at no point did it jump hugely on CPC. I'm coming to the conclusion it is just one of those things!

 

[JohnW2]

I'm just testing a new ring final circuit prior to third party inspection and have 0.56 and 0.57 on r1 and rn but 1.18 on r2, when calculated it should be around 094.

I split the ring at the two places it starts serving the actual rooms, so effectively removing the two legs that feed back to the CU, and then did a continuity test between those two points; and the higher CPC reading remains. I've then used a wander lead measuring from one end of the ring and worked round logically in the order it was wired and measured increasing readings on both r1 and CPC, but at no point did it jump hugely on CPC. I'm coming to the conclusion it is just one of those things!

Well, there has to be an explanation

If, as you say, both R1 and R2 increase gradually, progressively, and presumably roughly pro-rata, with no 'jumps', as you move around the ring, then what you are seeing surely must be 'correct' in terms of the relative resistivities of the L conductor and CPC in the cable (and regardless of the calibration of your meter)?

Is the cable of a 'reputable' make? Do you have any significant amount left (or, ideally, a whole unused drum from the same batch) on which you could measure the R1 and R2?

Kind Regards, John