high end too end results

monkey5 · Nov 28, 2011

As the title really,I'm changing a board,and on the pre test ready for change I've got r1 and rn of 0.85 and r2 of 1.99. R1 + R2 is 0.89. Cable is 2.5/1mm2 t&e. Ze-0.11 Zs-1.11(on no trip setting)tncs system,mcb would have been 32A normally, and with As reading still within limits can't see there being a prob to he honest- but am all ears to the more knowledgeable. Its a biggish house.is there a limit on end to end readings?

Wayne

Revved Up Sparky · Nov 28, 2011

The readings seem OK to me. With a biggish house the ring main/s may cover a larger area and so more cable will be in circuit resulting in higher end to end readings. With a 1.0mm cpc you will get an r2 reading approx 2 and a half times higher than the r1 and rn. Your R1 + R2 should be approx (r1+r2)/4 so 0.89 seems reasonable.

monkey5 · Nov 28, 2011

thats what i thought mate, just the highest readings ive had, just got me thinking (dangerous) so wondered if the end to end have a limit like R1+R2.

cheers for quick reply mate

PaulSC · Nov 28, 2011

Hi Monkey,

Stating the obvious, but on that cable size r2 should be 1.67 times greater than r1 or rn. Not a million miles out, but you could retest (poor connection).

R1 + R2 from your results should be 0.71 ohms (r1 + r2 /4), so again not a million miles out though there are far more knowledgeable people on here than me.

Revved Up Sparky · Nov 28, 2011

Hi Monkey,Stating the obvious, but on that cable size r2 should be 1.67 times greater than r1 or rn.

That would apply for a 1.5mm cpc, it's 2.5 times for a 1.0mm.

AFAIK, there is no end to end limit, as long as the ring main does not cover a floor area of more than 100 metres squared. If r1 and rn are within 0.05 ohms of each other and r2 matches up OK (be it 1.67 times r1/rn (1.5mm cpc), 2.5 times r1/rn (1.0mm cpc) or r2 roughly equal to r1/rn (2.5mm cpc).... then it should be OK. Quite commonly r1/ rn will be less than 1 ohm.