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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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 23 July 2011 10:18 AM
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andrew331

Posts: 110
Joined: 21 December 2008

Using my loop tester (Metrel MI 2122) to do low current tests on rcd protected circuits, the Zs readings I get are always higher than if I measure Ze, R1 and R2 and add together. If I and keep repeating the test I get a big variation in results, e.g 0.7,1.4,1.1,0.9,0.7,1.1.

Is it time for me to get a new tester or is this the best to be expected wit a 15mA test?
 23 July 2011 10:26 AM
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kj scott

Posts: 2144
Joined: 02 April 2006

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.

-------------------------
http://www.niceic.biz
 23 July 2011 10:34 AM
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John Peckham

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Andrew

That sort of variation is common to all low current non-trip loop testers. They have a tough job to do and carry out numerous (100ish) measurements and then display the most common result. The more complex the instrument the more prone it will be to noise and harmonics on the circuit under test. Anyway it does not really matter as you are testing RCD protected circuits and the permitted max. Zs allows for a wide band of tolerance. Plus or minus 1 ohm is said to be acceptable by a well known club.

The most consistant loop tester I have is my old Robin 4120DL. It gives a fast no trip readings on RCD protected circuits which are reasonably consistant. The tests are done at high current with a DC injection (Dlok) to prevent RCDs tripping. The downside is they will trip nearly all RCBOs and some electronic RCDs. Also a good dose of current, 25A ish around the circuit. Nice big arc flash if you don't press hard on the test probes.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 23 July 2011 10:59 AM
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dg66

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I've always used Megger(LRCD220)and found that on the 'no trip' function the results were always lower,so calculation was the order of the day.

-------------------------
Regards

Dave(not Cockburn)
 24 July 2011 07:31 AM
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keithredpath

Posts: 415
Joined: 30 March 2002

To get a more accurate reading with my meter. I dont put it on "No Trip" but simply test L to N. Then I calculate the L to E based on what type of earthing is in place. e.g. If it is a Twin and earth cable, I calculate the earth to be L to N x 2.5 / 1.5.

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keithredpath
 24 July 2011 07:49 AM
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GeoffBlackwell

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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
 24 July 2011 08:10 AM
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jleltd

Posts: 199
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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


Does the spanner come with PPE, GS38 finger guards and a calibration certificate.

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James
 24 July 2011 08:16 AM
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GeoffBlackwell

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It doesn't need calibration as it is a use once device .

Regards

Geoff Blackwell
 24 July 2011 08:54 AM
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dg66

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But its not gold plated

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Dave(not Cockburn)
 24 July 2011 09:19 AM
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dickllewellyn

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I was chatting to our local Megger rep recently about the new MFT1730 and hearing the virtues of this, that, and the other... He went on to mention the two wire no trip loop tests the machine can perform. When I quizzed him on accuracy, he suggested if a result seems wild, try reversing the probes.... Quite what that will achieve I don't know, but that's what he said...

I'm off to the tool sale in the Guild Hall now to buy some spanners!

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 24 July 2011 02:14 PM
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mossep

Posts: 335
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From my experience with megger testers they are the worst culprits when it comes to throwing out wild low current Zs test results. I sent two testers back to them as they were hopeless. My old Robin as someone stated above in this thread was superb. I have a Fluke multi jobby now and although not as good as the robin, it does give consistent reults.

-------------------------
www.mossep.com
 24 July 2011 02:25 PM
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seeker

Posts: 319
Joined: 10 March 2007

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


One was allegedly dropped across the 50v busbars in the old York telephone exchange - most of the spanner was vapourised I am told (this may be an urban myth)
 24 July 2011 03:26 PM
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alancapon

Posts: 5745
Joined: 27 December 2005

I saw a spanner dropped across 50v busbars in Dartford - there was nothing left that could be described as a spanner - a better description would be "a lump of metal"!

Regards,

Alan.
 24 July 2011 03:45 PM
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napitprofessional

Posts: 368
Joined: 08 March 2008

Originally posted by: keithredpath

To get a more accurate reading with my meter. I dont put it on "No Trip" but simply test L to N. Then I calculate the L to E based on what type of earthing is in place. e.g. If it is a Twin and earth cable, I calculate the earth to be L to N x 2.5 / 1.5.



I despair, I really do ....
 24 July 2011 04:04 PM
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daveparry1

Posts: 6090
Joined: 04 July 2007

I despair, I really do ....
----------
So do I napit, how on earth can that give a true (or even meaningful) result?

Dave.
 24 July 2011 06:55 PM
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alancapon

Posts: 5745
Joined: 27 December 2005

Originally posted by: keithredpath
To get a more accurate reading with my meter. I dont put it on "No Trip" but simply test L to N. Then I calculate the L to E based on what type of earthing is in place. e.g. If it is a Twin and earth cable, I calculate the earth to be L to N x 2.5 / 1.5.

Good grief! You would be just as accurate thinking of a number at random and writing it down, leaving your tester in its case! I hope you are joking here!

Regards,

Alan.
 24 July 2011 07:50 PM
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slittle

Posts: 3453
Joined: 22 November 2007

Geoff,

I feel you may have been mislead, the picture clearly shows battery discharge tester not a Zs one

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
 24 July 2011 10:49 PM
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napitprofessional

Posts: 368
Joined: 08 March 2008

Originally posted by: alancapon

Originally posted by: keithredpath

To get a more accurate reading with my meter. I dont put it on "No Trip" but simply test L to N. Then I calculate the L to E based on what type of earthing is in place. e.g. If it is a Twin and earth cable, I calculate the earth to be L to N x 2.5 / 1.5.


Good grief! You would be just as accurate thinking of a number at random and writing it down, leaving your tester in its case! I hope you are joking here!



Regards,



Alan.


How I hope he`s registered and his assessor views this forum ....
 25 July 2011 04:38 AM
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ebee

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"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"

The Schrodinger method?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 25 July 2011 05:28 AM
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Jaymack

Posts: 4585
Joined: 07 April 2004

Originally posted by: andrew331
If I and keep repeating the test I get a big variation in results, e.g 0.7,1.4,1.1,0.9,0.7,1.1.

Are you using fused test leads?

Regards
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