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Topic Title: Zs testing
Topic Summary: accuracy or non-trip loop testers
Created On: 23 July 2011 10:18 AM
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 25 July 2011 09:17 AM
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deleted_2_tony30

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And keith is teaching 2391 when he isnt employed by hydro contracting doing testing for a living.
 25 July 2011 10:42 AM
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Legh

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Originally posted by: kj scott

Its a matter of propotion, the lower the test current, the lower the accuracy, that said; the high current ranges are not tha accurate either.

Field tolerance +/- 30%; if you want an accurate value you must calculate it.


I'm so glad we are now in agreement

Keith,
I can't see a problem with using a P-N loop particularly for a PME system where the final circuits are loaded with RCBOs/ RCDs and then either making adjustments through calculation or continuity measuring the R2 with parallel paths.

However, the most reliable, albeit, rather dangerous test would be to lob a spanner in the works - advanced subscribers only

Legh

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 25 July 2011 11:14 AM
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OMS

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However, the most reliable, albeit, rather dangerous test would be to lob a spanner in the works - advanced subscribers only


It would be a tad safer with "injection testing" though ?

I've regularly put several thousand amps through a transformer at 12 volts with a bolted short circuit at the terminal box for example and it's fairly easy to test say 4000A circuit breakers for performance - that should be enough for most domestics


Regards

OMS

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 25 July 2011 02:35 PM
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AJJewsbury

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I can't see a problem with using a P-N loop particularly for a PME system where the final circuits are loaded with RCBOs/ RCDs and then either making adjustments through calculation or continuity measuring the R2 with parallel paths.

I'd be in favour of L-N loop testing - just because we don't really test N for loose connections and we keep getting reports asking "why is it always the N terminal that overheats?" - but it would have to be in addition to L-PE, not instead of it.

The problem with L-N loop testing alone instead of L-PE is that you're not testing for bad connections in the c.p.c.. If you want to rely on R1 or R1+R2 (and there is a school of thought that that should be sufficient) then that's one thing, but to do a L-N test and claim it's the equivalent of L-PE is missing the point I fear.

- Andy.
 25 July 2011 05:42 PM
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Legh

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The problem with L-N loop testing alone instead of L-PE is that you're not testing for bad connections in the c.p.c.. If you want to rely on R1 or R1+R2 (and there is a school of thought that that should be sufficient) then that's one thing, but to do a L-N test and claim it's the equivalent of L-PE is missing the point I fear.


Probably, but it does take the 'full' loop test to the end of the line as far as R1 is concerned.
I suppose you could do a Ze and then continuity test R1+R2 or you could do the naughty test (without any RCDs in the circuit) but then that could be bridge too far

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 25 July 2011 05:51 PM
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Legh

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Originally posted by: OMS

However, the most reliable, albeit, rather dangerous test would be to lob a spanner in the works - advanced subscribers only


It would be a tad safer with "injection testing" though ?

of course it would - but then is current being injected by the instrument or does the instrument allow a window of current to flow from the installation circuit?

I've regularly put several thousand amps through a transformer at 12 volts with a bolted short circuit at the terminal box for example and it's fairly easy to test say 4000A circuit breakers for performance - that should be enough for most domestics

Yes, well that is ok for the breaking capacity but I was under the impression that MCBs also needed a certain voltage to trigger the instantaneous tripping effect?.

Legh




Regards



OMS


-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 25 July 2011 06:09 PM
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kj scott

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Originally posted by: GeoffBlackwell

This is what you need to get an accurate test of the strength and capability of the earth-fault-loop-path under fault conditions.



True Zs Tester



Just place it across the line and earth.



Regards

Geoff Blackwell


Looks more like the Mk1a RCD tester to me Geoff, are you sure that was the loop one, you haven't mixed them up in the test box have you?

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 25 July 2011 06:11 PM
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kj scott

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Originally posted by: Legh

Originally posted by: kj scott



Its a matter of propotion, the lower the test current, the lower the accuracy, that said; the high current ranges are not tha accurate either.



Field tolerance +/- 30%; if you want an accurate value you must calculate it.


I'm so glad we are now in agreement

Legh


Calculation for accurate loop values, but not for ring continuity.

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http://www.niceic.biz
 25 July 2011 06:18 PM
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kj scott

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Originally posted by: slittle

As to Keith's calculations, well I might as well ask my 5 year old for a number between 1 and 10, then divide it by 3 and multiply it by the number of times I've fed the cat today

Stu


To be fair Stu, if it were most circuits, you should start with a number between 0.1 and 0.9, then using your formula; you would probably be close to anything a loop test instrument could come up with.

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 25 July 2011 06:20 PM
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kj scott

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Originally posted by: tony30

And keith is teaching 2391 when he isnt employed by hydro contracting doing testing for a living.


First part correct, but who are hydro contracting?

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 25 July 2011 06:39 PM
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JonSteward

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KJ go back and read keithrepath's post
 25 July 2011 06:58 PM
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kj scott

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Also forgot, I'm Kevin.

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IET » Wiring and the regulations » Zs testing

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