IR and EFLI question

[a1pacino]

Hi guys,

I have a couple of questions on my mock paper that I am unsure about and would appreciate any input with these questions, I have put my answers below in blue, many thanks in advance.

1. an insulation resistance test on a circuit indicated zero Megohms between line and neutral. explain why the result of the previous R1+R2 test could not  be used to confirm polarity for their circuit.

the IR test is between L/N and the previous R1+R2 test is between L/E so cannot be used to confirm polarity for this circuit.

2. State why earth loop impedance test result is likely to be less than the sum of the recorded values of (R1+R2) and Ze?

The test result would be at ambient temperature and the calculated value includes the 70 degree correction factor of 1.2 to raise the value of Zs during current flow and higher conductor temperature

 

[Andy™]

makes a change someone trying first

1: question does seem odd and flawed: R1R2 alone cannot prove polarity: also doing R1Rn or R2Rn will confirm polarity (but that was not mentioned in question), however, since there is an L-N IR fault, then there is no way to prove correct polarity without fixing the fault first

2: not quite... Ze is external earth on its own. Zs includes other parallel paths. not usually much difference on TN, but often a large difference on TT

 

[a1pacino]

Thank you for responding so quickly Andy