Odd Non trip Zs readings

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Have been working on 2 flats in sunny Salcombe which is doing my head in. When I test for Zs using my Megger 1502, I keep getting a reading of 1700 ohms in both flats. Now Ze is coming in at 0.95 ohms ( I also tried this for N/L and got 0.1 Ohm), PFC is at @0.45 KA PSCC is @ 0.96KA. The supply is TN-s split from a cast iron box into upstairs and downstairs flats (though western power have now changed this due large hole in box - not had a chance to re-test since). One flat has new CU, the other a mini CU to supply single socket whilst building works carry on. Thinking that my test gear was knackered I have also had this re-calibrated and it was fine. Have also checked it against standard circuit in my own property - no problems. R1+R2 readings are fine on all circuits, tried replacing RCDs, and testing for continuity around CU and through RCDs - no problems. Also checked test leads and tried testing at back of sockets instead of using plug-in test lead - odd readings prevailed

So at the moment I'm at a complete loss as to how to explain the odd readings I'm getting when using the non-trip Zs function. It just doesn't add up, so if anyone can shed some light on this, it would be much appreciated.

 
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When you test Ze are you just using the two lead method. It could be that your green lead is knackered so when you test Zs using non trip you get odd readings. Could you try with another lead ? Just a thought.

 

paulh

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I don't quite understand how the pfc and the pscc figures can be correct if the volts are say 230 and the ze is 0.95ohms or l/n fault 0.1ohm? adding these gets a pfc of 242amps or 0.24ka, and pscc of 2300amps or 2.3ka.

moving on...

This sounds unlikely/odd but is there a good earth between the incomer and the cu,s particulary at the point where they split in the cast iron box?

confused me anyway

 
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earth has been checked by me and western power and seems Ok. As for test leads I always check before use and have certainly re-checked several times in this instance. As for PFc and PSCC numbers, they are from memory (not brill at my age) I should have written them down, its interesting though that western power guy got different readings at 0.56 ohms for Ze. Unfortunately I wont be back on this site for a while, so will be very interested to see if replacment box makes any difference. The only thing I can think of is that corrosion is causing some oddities given hole in cast iron box and salty sea air ?????

As I said above two lead test seems to reveal no problems, its only when I use no-RCD trip test I get daft numbers, which is certainly confusing me given that it seems to work fine elsewhere!!!

The only other thing I can think of is that the RCDs (Steeple) cause a problem when energised, but have swopped both RCDs out, and RCD in lower flat with temporary supply whilst re-wiring is a different make again. I only get daft numbers when testing through the RCDs. To check RCDs I did loop a wire between N and L to test continuity and got 0.001 Ohm so that doesn't make sense either :_|

 
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BinkyMore info needed on wiring system is it metal conduit being used as earthing.

Batty
Not in this case, but don't think earth is the problem, I suspect it is neutral fault. From cast iron box' which is a twin phase supply, split concentric cable feeds each property to modern consumer head which looks like PME but I am assured is TN-S. Salcombe is a strange place:D:D

 

septiclecky

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Are you saying that at the suppliers incomer the Ze is 0.96 ohms and at both CU the reading is 1700 ohms?

If this is correct then I think you should be looking at all the connections from the suppliers mains incomer to where you split the supply to each of the flats.

 
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No, 0.96 is at CU prior to RCDs, 1700 is testing downstream of RCDs whether at CU or at socket within property, this is why I suspect neutral problem, or something upsetting my test gear. Having said that I did use two lead Ze test on Neutral and Live and got 0.1 Ohm. The whole thing is bizarre and suspect cause is really obscure, hence I posted the question to see if anyone else has had a similar problem. Have tested properties on 3 sepaerate occasions - readings seem to fluctuate slightly, but only after changing RCDs did 1700 ohms drop to 600 ohms, yippee I thought I'm going in the right direction - next visit back upto 1700 ohms :_| :_| :_|

Does anyone believe in Gremlins ?:| ? :|

 

davetheglitz

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Hi Binky,

Sounds wierd!

To my mind if Ze at the point of measurement is OK and R1 + R2 from the same point is OK then Zs should be OK - but I'm sure that's where you came in!

Here's the scattergun approach!!!!!

Does the RCD trip properly?

Do you have a heavy load that you can run (shower/cooker)? V. small voltage drop should confirm good neutral connection.

Silly - but have you tried replacing the RCD with a main switch and retesting on a 'safe' circuit?

Are you close to a substation, transformer or transmitter? I wonder if there is some electromagnetic interference causing a problem with the test gear? I once did a test on RCBO's at a nightclub on the Barbican which would not trip at the CU, but would work perfectly when tripping downstream - presumably further away from the interference. We always tend to rely on our meters - but they aren't exactly shielded. This was repeated with both a Fluke and a Metrel.

It strikes me that a no RCD trip measurement would be more susceptible to interference than a dc measurement. The Ze and low ohms R1 + R2 re still valid measurements and Zs can be calculated. If the RCD trips out OK then arguably all is well.

You could spend a long time getting to the bottom of this if you haven't got the test gear to check your test gear in that situation.

Sometimes engineers have to shrug their shoulders and say 'Funny isn't it!'.

If you want to borrow another tester to confirm give me a ring.

Cheers

Dave

 
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Yeah wouldn't ming borrowing your spare gear Dave just to confirm whether I'm going mad or not :p (Actually I know the answer to that - do you know the tune to ''they're coming to take me away ha ha'' ) ( you young uns won't have a clue what I'm on about there :D )

RCDs trip fine, have tested the continuity through RCD by linking with a bit of wire between Neutral and live before fitting replacemnts to the CU, and checked replacements in same way before fitting - 0.01 Ohm

Was planning to use calculated numbers for certificate as it strikes me that the problem is external to property, and therefore beyond my control, especially as it affects both installations, and main supply to Salcombe went bang the other day though test results didn't alter :(

But having seen antiquated nature of western power gear in this area, I'm not too surprised, which gives me a thought - the cast iron box also supplies a cast iron wall mounted street light. Does street light equipment cause problems ?????? Ere admin, can we have a smiley with a light bulb over it head?

 
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In the office again.
I am here:

"Where life is beautiful all the time,

And I`ll be happy to see those nice young.........."

That`s enough of that!

what happens if you link out the RCD and do a 2 lead test phase-earth?

and do one on the supply side of the rcd> may shed some light perhaps?

Odd one Binky, you have. (yoda strikes back!)

 
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i used to get this a while ago when i had an old seaward tester,always always gave weird readings on triplock......

I gave up on it and just started linking RCDs out.

no idea why it happened tho.
I'm glad its not just me Steps, but its funny how it only happens on this one installation. I'll be back there next week to finish off some stuff for Xmas, so will be interesting to see if changing the outside box alleviates the poroblem.

 
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Just aquick update as promised. Returnef to Salcombe today and re-tested circuits with nice shiny (for plastic) box fitted by Western Power - hey presto fault has disappeared snf things are testing as per normal, and in line with calcultaed Zs values. Conclusion, there was something wrong with supply neutral, or I am mad and it was a figment of my imagination ?:| :p :eek: :^O

Merry Christmas to you all, I'm off for some well earned Guiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness Drink

 

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Merry Christmas to you too Mate. And enjoy that hard earned beer. :D

 
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Just aquick update as promised. Returnef to Salcombe today and re-tested circuits with nice shiny (for plastic) box fitted by Western Power - hey presto fault has disappeared snf things are testing as per normal, and in line with calcultaed Zs values. Conclusion, there was something wrong with supply neutral, or I am mad and it was a figment of my imagination ?:| :p :eek: :^OMerry Christmas to you all, I'm off for some well earned Guiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness Drink
well if you are....

so am I!:DBlushing

Guiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness Drink

hippody bippody chrimboldy crispmas!!! :p ] :)

 
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