Loop Impedance reading

[EddieB]

Hi
I bought a holiday lodge last month and decided to test the circuits to see how they measured. The lodge itself is fed from a central point in the holiday village by a 16mm PVCSWA. The main earth wire terminates into a MET it looks to be a 10mm earth that is crimped onto the incoming earth, I cannot see what its crimped onto as the head the mains terminated into prevents this. Anyway I carried out the the normal tests on the circuits R1 R2 RN and these tested all within the expected parameters as an example the one of the rings measured R1.24 RN.24 R2 .39 and R1/R2 .25.
The other circuits all tested within expected parameters. The Ze measured .6, however when I powered up and did a Earth Fault loop impedance test the machine measured 23 ohms, this was repeated on all the sockets and the same measurement. Assistance required please as to the possible reason (s) for this.

 

[Mikegh]

Something not right

I would probably try checking the loop impedance at the outgoing side of the mcbs and try to narrow it down and recheck the resistance readings etc

 

[telectrix]

you could have errors caused by RCD or MCB. worst i ever saw was a Ze of 0.2 but measured on the outgoing terminals of the rCD it was 341.

 

[EddieB]

Something not right

I would probably try checking the loop impedance at the outgoing side of the mcbs and try to narrow it down and recheck the resistance readings etc

Thanks for the response.
Yep I agree something is not right, when you say check the loop impedance at the outgoing side of the mcb what do you actually mean, I checked both ring circuits and cooker circuit and they read 23 ohms,
I haven't yet checked lighting circuits, but if 3 circuits read around the same then where now?

 

[EddieB]

you could have errors caused by RCD or MCB. worst i ever saw was a Ze of 0.2 but measured on the outgoing terminals of the rCD it was 341.

Thanks for the response,
I'll give that one a try

 

[Mikegh]

Thanks for the response.
Yep I agree something is not right, when you say check the loop impedance at the outgoing side of the mcb what do you actually mean, I checked both ring circuits and cooker circuit and they read 23 ohms,
I haven't yet checked lighting circuits, but if 3 circuits read around the same then where now?

Could be the DB

You can check in a few spots, external, also where your socket wires connect into dB and along the final circuit

 

[EddieB]

Could be the DB

You can check in a few spots, external, also where your socket wires connect into dB and along the final circuit

I'll give it a go --Thank you

 

[Mikegh]

I'll give it a go --Thank you

Ya your Ze and R1 and R2 are normal so

I'd be sticking a probe on the earth bar and checking fault loop at main switch ,rcds, mcbs on the outgoing sides

Did you disconnect anything at the met

May not turn out to be that at all either

 

[EddieB]

Ya your Ze and R1 and R2 are normal so

I'd be sticking a probe on the earth bar and checking fault loop at main switch ,rcds, mcbs on the outgoing sides

Did you disconnect anything at the met

May not turn out to be that at all either

I’ll go through it as you describe and I’ll let you know, as an aside I disconnected the main earth for the Ze test but reconnected it after. ?

 

[SparkySy]

I would put my bets on RCD uplift as @telectrix suggested, I had some BG ones a while ago that made some circuits wildly high Zs.
If you can do a bypassed Zs on the circuit that will prove it one way or another. To do this I isolate the supply then put a R1+R2 link cable in between live and earth at the furthest point on the circuit in question. Then put the tester on high loop and place between the outgoing circuit feed and the non isolated incoming live, this will give you a non uplifted reading to compare with your current one!
Be very careful when doing this test!

 

[EddieB]

I would put my bets on RCD uplift as @telectrix suggested, I had some BG ones a while ago that made some circuits wildly high Zs.
If you can do a bypassed Zs on the circuit that will prove it one way or another. To do this I isolate the supply then put a R1+R2 link cable in between live and earth at the furthest point on the circuit in question. Then put the tester on high loop and place between the outgoing circuit feed and the non isolated incoming live, this will give you a non uplifted reading to compare with your current one!
Be very careful when doing this test!

Thank you for replying, funny enough these are BG RCD's, If it is the RCD I can bypass it by feeding a spare MCB from the incoming feed block the neutrals from the mains
and then test the ring.
I would appreciate a sketch showing the process you describe , you can either email it to me or put it on here whatever is the easiest if you wouldn't mind.

you could have errors caused by RCD or MCB. worst i ever saw was a Ze of 0.2 but measured on the outgoing terminals of the rCD it was 341.

Telectrix, thanks for reply, I appreciate it.

 

[telectrix]

here is a method.you turn off the RCD and/or MCB. short L-E at point you want to measure. them MFT on hi current Zs setting (2 leads).bridge to RCD.MCB with the meter. this will give you a true Zs.

don't forget to remove the short before re-energising.

 

[ELECNEWT]

In addition to Tel's excellent description and drawing see this link to Chris Kitcher carrying out this method:

 

[Mikegh]

here is a method.you turn off the RCD and/or MCB. short L-E at point you want to measure. them MFT on hi current Zs setting (2 leads).bridge to RCD.MCB with the meter. this will give you a true Zs.
View attachment 90663
don't forget to remove the short before re-energising.

Good method, can't say I've ever had to carry this type of check in practice,don't even have a 2-wire tester

The theory is routine enough though

 

[EddieB]

here is a method.you turn off the RCD and/or MCB. short L-E at point you want to measure. them MFT on hi current Zs setting (2 leads).bridge to RCD.MCB with the meter. this will give you a true Zs.
View attachment 90663
don't forget to remove the short before re-energising.

Thank you my man, I'll be doing this early next week and I'll let you know how it goes.

 

[Mikegh]

Is the high current reading like 20a ?

Better for troubleshooting those issues

 

[telectrix]

Is the high current reading like 20a ?

Better for troubleshooting those issues

something like that. the higher current helps to get past tarnished contacts, that's why low current test often gives slightly different results. ( well that's my theory, anyway ).

 

[Mikegh]

something like that. the higher current helps to get past tarnished contacts, that's why low current test often gives slightly different results. ( well that's my theory, anyway ).

Yes that's my theory on the low current testing , should be more reliable at the higher current here

Similar to IR troubleshooting at 1000v where it can be more effective at locating a fault, when safe to test

 

[Mikegh]

I would put my bets on RCD uplift as @telectrix suggested, I had some BG ones a while ago that made some circuits wildly high Zs.
If you can do a bypassed Zs on the circuit that will prove it one way or another. To do this I isolate the supply then put a R1+R2 link cable in between live and earth at the furthest point on the circuit in question. Then put the tester on high loop and place between the outgoing circuit feed and the non isolated incoming live, this will give you a non uplifted reading to compare with your current one!
Be very careful when doing this test!

Dont think you'll find these particular tests in any reference books will you ?

 

[SparkySy]

No but very useful at times!