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.
wrote in message news:fe6b1dd6-a871-4b42-9acc-8ccdd76323f5@m45g2000hsb.googlegroups.com... > 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. Earth loop impedance Zs = R1 + R2 + Ze. > 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. 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
On 15 Aug, thomasbarkley@hotmail.com wrote: > 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. 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. -- B Thumbs Change lycos to yahoo to reply
thomasbarkley@hotmail.com wrote: > 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. 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... > 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. 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. > 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. 0.01 does sound a bit low - what cable is the circuit wired in? > I dont know who is correct. Possibly neither! -- Cheers, John. /=================================================================\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \=================================================================/
