Twin and earth testing

elektrisien · Feb 24, 2015

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 · Feb 24, 2015

Are you a trainee ?

telectrix · Feb 24, 2015

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 · Feb 24, 2015

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 · Feb 24, 2015

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 · Feb 24, 2015

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

[ElectriciansForums.net] Twin and earth testing

GB.Kayak · Feb 24, 2015

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.

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 · Feb 25, 2015

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 · Feb 25, 2015

farmelectrics said:
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 · Feb 25, 2015

farmelectrics said:
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 · Feb 25, 2015

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 · Feb 25, 2015

Talking about 2.5 t& e murdoch

Farmelectrics · Feb 25, 2015

Or I am not the op lol

Octopus · Feb 25, 2015

farmelectrics said:
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 · Feb 25, 2015

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 · Feb 25, 2015

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

Twin and earth testing

elektrisien

Andy78

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telectrix

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Octopus

ImpededLoop

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telectrix

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GB.Kayak

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Farmelectrics

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ImpededLoop

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Octopus

Farmelectrics

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Farmelectrics

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Farmelectrics

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Octopus

Farmelectrics

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TJ Anderson

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