Electrical inspection certificate technical question

[thomasbarkley]

My domestic test report schedule has Z (e) at origin of 0.17 ohms.

For one of the circuits continuity R2 ohms is 0.66 and Earth Loop impedence Z (s) is 1.31. Another electrician says that there should be a relationship where the 1.31 number minus the 0.17 number should give the 0.66 result - but this equation is not giving the right result. Is there a reason for the discrepancy.

Also for an immersion heater circuit the certificate has R2 as 0.01 and Z (s) as 0.41. The second electician says that the 0.01 would be correct only if the heater was right next to the fuse box - its actually at least 15 foot away.

I dont know who is correct.

TIA

Thomas.

[ARWadworth]

Earth loop impedance Zs = R1 + R2 + Ze.

R2 is indeed unlikely to be 0.01 and my guess is that someone has done a measurement with the immersion earth connected and got parallel earths from the earthed metalwork from a hot water cylinder and any supplementary bonding. Your R1 + R2 on this circuit is .24 If the circuit uses 2.5T&E then R1 is 1.6666 times R2.

Adam

[<me9]

If it was measured with the equipotential bonding in place (10mm^2) to copper pipes connected to the HW cylinder it could be that low.

[John Rumm]

Well say that was a lighting radial in 1.5mm^2, that has a resistance of

12.10 mOhms / m on the L/N and 18.1 mOhms / m on the CPC. So that would be 0.66 / .0181 = 36.6 meters, and hence R1 would be 0.44 ohms.

So 0.66 + 0.44 + 0.17 = 1.27

close...

Yup, he does not know his fault loop impedance from his elbow! ;-)

The fault loop impedance will be the round trip sum of resistance of the cable (so R1 + R2) and the external total impedance of the supply and earth.

0.01 does sound a bit low - what cable is the circuit wired in?

Possibly neither!