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Twin and earth testing

Discuss Twin and earth testing in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

E

elektrisien

Hi,
A quick question, maybe a stupid one .... but it baffles me ,and I cannot find any info on the net. If you do the continuity test on a lighting circuit to get your resistance values, wired in twin and earth, how do you the the value for the earth ? Obviously you link the earth and phase or neutral, you get your reading ..... but the earth is of a smaller diameter, so the resistance is higher.... you cannot just divide it by two. So what is the secret ? Let me just say, I am just starting to do testing, so excuse my ignorance ..
 
if you want the resistance of the cpc ( earth) on it's own, then you need to use a wander lead. either null your tester with wander lead on it's own, or subtract it's resistance from the reading.
 
Most test sheets require either R1+R2 or R2.

You could calculate it from R1+R2 as Murdoch says, but this could be incorrectly influenced by an circuit fault such as a high resistance joint on the line conductor which would increase the value of R1+R2 but wouldn't cause any increase to R2 on it's own.
 
telectrix said:
if you want the resistance of the cpc ( earth) on it's own, then you need to use a wander lead. either null your tester with wander lead on it's own, or subtract it's resistance from the reading.
Click to expand...

If you didn't have the wander lead, might you measure R1 + Rn and take half of this value away from R1 + R2, leaving you with a reasonably accurate value of R2. That said ... the neutrals do not always follow the entire route of the line and switched line circuit so scratch that one!
 

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