Twin and earth testing

[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 ..

 

[Andy78]

Are you a trainee ?

 

[telectrix]

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.

 

[Octopus]

Work out the ratio of the difference of the cross section of the two cables, then using R1 + R2 you will get an answer.....

 

[ImpededLoop]

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]

ah..... the good old high resistance joint........

 

[GB.Kayak]

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.

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!

 

[Farmelectrics]

Just do your figure of 8 test (r1 r2) test , your CPc will be roughly 1.67 times greater than your phase to phase and neutral to neutral job done

 

[ImpededLoop]

Just do your figure of 8 test (r1 r2) test , your CPc will be roughly 1.67 times greater than your phase to phase and neutral to neutral job done

Not on a radial lighting circuit though!

 

[Octopus]

Just do your figure of 8 test (r1 r2) test , your CPc will be roughly 1.67 times greater than your phase to phase and neutral to neutral job done

Not if its 2.5mm/ 1.0mm

 

[Farmelectrics]

Your right impeded loop I must read posts properly just letting him now for when he moves on to his ring mains ha ha

 

[Farmelectrics]

Talking about 2.5 t& e murdoch

 

[Farmelectrics]

Or I am not the op lol

 

[Octopus]

Talking about 2.5 t& e murdoch

The OP was refering to lighting.

My point wasn't to be smart, simply to point out, for anyone else brousing this thread, that different cable sizes have different ratio's

 

[Farmelectrics]

Yes I appreacate that Murdoch was giving him an example myself only I didn't read post properly its a radial he was asking not a ring

 

[TJ Anderson]

If it is 2.5/1.0 then the R2 is 2.5 x greater than R1.