rewire or not rewire????

[woko27]

Hi, A question for you experienced electricains out there.

My house is having a new consumer unit and I would like the ring main to stay wired as it is. A test was put on it and the results were as followed, r1 was 0.58, rn was 0.56 and r2 was 1.38, I think the cable is 2.5mm with a 1.0mm cpc. Is this ok? I am concerned about the resistance of r2 because the OSG stays r2 should be 1.67 times r1 or rn which is 0.96 which makes 1.38 quite high. Does this mean the ring main will have to be rewired in 2.5/1.5 T&E.

 

[RF Lighting]

Personally if the Zs value is low enough, then I would not fail the circuit merely for having a 1.0mm² earth, but circuits old enough to be wired in this cable often have other problems anyway, such as a lack of sockets, and installation methods.

 

[Adam_151]

Do you have a figure for external loop impediance (ze?) chances are unless it is unusually low, a 2.5/1 ring on a modern breaker won't be a problem

The 0.67 think in the OSG is just a quick pointer to check you are getting expected values and assumes 2.5/1.5, of course with 2.5/1 the ratio will be different

 

[woko27]

Thanks so far. I haven't a figure for Ze but ZS has been measured at the sockets since and it is quite high. 1.9 at the highest R1+R2 point of the circuit. which I believe is higher than it should be. 1.12 for type B 32A breakers.

Looks like a new ring main!!!!

Thanks

 

[Adam_151]

Something doesn't seem right here

Z's = 1.9 r1 = 0.56 r2 = 1.38

I know it doesn't work perfectly in practice but with an r1 and r2 of 0.56 and 1.38, you have a R1+R2 (note upper case!) of 0.485 subtract that from the 1.9 and you are left with a value of 1.415 for the Ze, I'd measure the Ze, because it sounds like a dodgy TN-S ...