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]

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!

 

[ELECNEWT]

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

See Chris Kitcher's reference book :

Practical Guide to Inspection, Testing and Certification of Electrical Installations​

 

[pc1966]

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 ).

It is one factor, but even then wiggling the probes/clips on low Z systems usually gets a slightly lower value.

More fundamentally the bigger the test current the larger the delta-V, and so it is easier to measure that and reject the wobbles on the supply that are usually there. For example, if test current is 20A then 0.3 ohms Zs is going to show a 6V change - easy to spot on 230V.

But if the test current is only 15mA to avoid tripping a RCD then you are trying to see a 4.5mV change in 230V or 0.002% which is why MFT take a long time to switch it on/off over and over so the supply noise gets averaged out but the disturbance reinforced.

 

[marcuswareham]

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.

Do you know the earthing arrangement, you say you cannot see what the earth is attached too so could it be a TT earth and the rod has resistance of around 23ohms

 

[Mikegh]

Do you know the earthing arrangement, you say you cannot see what the earth is attached too so could it be a TT earth and the rod has resistance of around 23ohms

He said 0.6 tns?

 

[marcuswareham]

He said 0.6 tns?

Did he, whoops my mistake

Ahh yes just seen the 0.6 Ze measurement, sorry about that

 

[Mikegh]

I dunno just know the uk tns is 0.8 max ,.we.dont have it

Could that be like a submain at 0.6 external loop, 16mm pvcswa sounds like it

 

[EddieB]

It is one factor, but even then wiggling the probes/clips on low Z systems usually gets a slightly lower value.

More fundamentally the bigger the test current the larger the delta-V, and so it is easier to measure that and reject the wobbles on the supply that are usually there. For example, if test current is 20A then 0.3 ohms Zs is going to show a 6V change - easy to spot on 230V.

But if the test current is only 15mA to avoid tripping a RCD then you are trying to see a 4.5mV change in 230V or 0.002% which is why MFT take a long time to switch it on/off over and over so the supply noise gets averaged out but the disturbance reinforced.

Guys, al

Do you know the earthing arrangement, you say you cannot see what the earth is attached too so could it be a TT earth and the rod has resistance of around 23ohms

Hi There,
I measured the Ze and it was .6 so I would assume not a TT, I have asked the facility manager the question to confirm it.

 

[marcuswareham]

Guys, al

Hi There,
I measured the Ze and it was .6 so I would assume not a TT, I have asked the facility manager the question to confirm it.

Yes sorry i misread it, and somehow missed the 0.6ohm ze part, ignore me !

 

[EddieB]

I started again from the MET and retested Ze, that tested out at .2 ohms, don't ask me, I tested it numerous times and it didn't move from.6. The R1 & R2 values stayed the same as previously, I re-tested the Earth Fault Loop and it tested out .8 as I would have expected on one ring and 1.17 on the other, the cooker tested out at .11. I have no idea what happened here as there was no sign of the 23 ohms it tested out at when I posted the original plea for help.
Thanks to all for their inputs

 

[Mikegh]

Hmm

Could be when you reset something at the board

Did you tighten any connections