Measured and calculated Zs

[Dave1982]

Can anyone tell me how I could have from my dead test results for a lighting circuit obtained a value of R1+R2 which is greater than my live test Measured earth fault loop impedance for the same circuit.

According to the equation Zs= Ze +( R1+R2) that should be impossible.

Any help appreciated.

 

[Sparky Ninja]

Your live test would more than likely have included parallel paths, creating a lower reading than your calculation.

 

[Dave1982]

thanks for the swift reply. but i measured and obtained 4.11 ohm R1+R2 dead test. then my live test for Zs gave me 2.3 ohm. Surely even if parallel paths exist i this should not reduce the impedance overall. these were both measured values.. It just dosnt seem to add up...pardon the pun

 

[Guest123]

4.11ohms for R1+R2........are you sure???

Sounds like a very bad connection there somewhere???

 

[Sparky Ninja]

Is this a really long circuit, with really small conductors?

 

[Dave1982]

no its not a long circuit in fact its a bungalow rewire. used qite a few junction boxes. Could be a high resistance at a termination...but then how come i get a low Zs value on my final live test. Thats whats stumping me

 

[Guest123]

Is it a TT system???

 

[Dave1982]

TNS system measured Ze of 0.29 ohm

 

[Sintra]

Mate i would check out that circuit thoroughly as your R1 + R2 is very high.

 

[Guest123]

In that case, and sorry to be abrupt, but can you not see you have a serious problem there??? rather than worrying about the difference between measured and calculated Zs, I'd be more worried about the fact the reading is that high at all!!

A reading that high will only get worse over time and may end up earth continuity being lost all together. All connections on that circuit need to be checked without delay for soundness.

 

[Baal]

Is it possible you've recorded 4.1 Ohms instead of 0.41 Ohms and the Zs would be 0.23?

 

[Dave1982]

I think your right Lenny and Sintra. I'll be right on it tomorrow. thank you all for your feedback

cheers

 

[Dave1982]

whatever the case everyone i'd rather be sure and do the tests again and then systematically go through the circuit again.

 

[Sintra]

Best option mate.

 

[Sparky Ninja]

whatever the case everyone i'd rather be sure and do the tests again and then systematically go through the circuit again.

Best action.
I would link the cpc and line at the db then test through the circuit and see where the resistance increases. I'd suspect it to be something like a loose cpc in a JB.