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Topic Title: Measurement of earth loop impedance
Topic Summary: Testing completed circuits
Created On: 09 September 2007 02:56 PM
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 09 September 2007 02:56 PM
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Avatar for Mark Everson.
Mark Everson

Posts: 31
Joined: 09 September 2007

I have had a recent occasion to carry out earth loop impedance measurements on completed circuits and although I consider myself a confident and proficeint NICEIC QS for my company, imagine my surprise (nay astonishement) when testing earth loop impedance on ring main circuits given the foregoing - different readings (and thus conflicting readings) from either side of the twin switched sockets throughout the circuit.

These accessories were new and manufactured by MK (Profile range) and the installation had not been used or energised prior to the inspection and test procedure. On further inspection there were no defects observed within the cable terminations to these twin sockets and nothing apparent from other inspection of the rest of the circuit to explain how twin sockets could read differently when being connected to the same single pair of ring final circuit conductors.

I wonder if any esteemed members had come across this phenomenon or if someone could offer an axplanation?
 09 September 2007 03:10 PM
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GeoffBlackwell

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Yes and it proved to be faulty socket outlets (also MK).

Regards

Geoff Blackwell
 09 September 2007 03:25 PM
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intrinsic4225B

Posts: 1618
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What sort of difference in value are you getting between the two socket outlets?

Regards,

Ross Currie TMIET

-------------------------
Ross Currie TMIET
 09 September 2007 04:56 PM
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Mark Everson

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One side reading 0.32 ohms the other 0.22 ohms - across the whole circuit these vary between 0.20 to 0.32 but this is only measured acros a totoal of 5No sockets all mounted across the same wall within 4 m of each other?
 09 September 2007 05:00 PM
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Mark Everson

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I am amazed and shocked/disappointed that you have also had this experience - with what I would expect to be a high quality product with a tradition of consistent performance that is (almost) second to none.

Regards

Mark
(MIET)
 09 September 2007 06:26 PM
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ajelectrical

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Mark
I know this may be a long shot but have you tried cleaning your test plug with alcohol to remove any grease or oil that may be on it affecting the readings (say the day before in case of ignition risk)
Just a thought.

-------------------------
Andrew. But I don't want you to test anything. I just want the board changing !!
 09 September 2007 06:27 PM
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Legh

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Welcome Mark,
I find this regularly when rfc continuity testing DSSOs, sometimes switching the switches off and on several times and manipulating a plug in each of the sockets helps to get consistent results. There have been times with certain socket outlets when the values differ consistenly from left to right up to as much as 0.02 ohms. This I can only put down to poor manufacturing techniques.
I feel reasonably confident about the consistency of continuity readings of my instrument.
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."
 09 September 2007 06:48 PM
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Mark Everson

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Legh - thanks for confirming a similar experience - to confirm, there appears to be no real answer to this occurance as it seems to me the chances of manufacturing defects with new equipment will multiply with the increase in numbers of socket outlets used in the first place.

I will certianly try your suggestion of switching the DSSO switch on/off several times during testing but this also underlines another technical anomally with testing in general; that of not accepting the first test result taken when carrying out these tests?

This does,of course raise another question of how much testing of the circuit can be done without effectively "warming up" the CPC by carrying out the tests in the first place?

Any Thoughts?
 09 September 2007 07:55 PM
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Legh

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I have not noticed any difference in values once established when continuity testing dead circuits.
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."
 09 September 2007 08:09 PM
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Mark Everson

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Legh - as you indicate - "dead" testing prove consistent readings throughout circuit testing, however when carrying out "live" testing this phenomonem appears - could this be something to do with the level of the test current used acting upon the DSSO contacts?
 09 September 2007 09:25 PM
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Phillron

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Surely a variation although probably very small must occur when live testing because of possible variations in supply voltage at the socket and where on the sine wave the test was triggered. Inductive reactance must play a part in my opinion, but if anything significant then manufacture must be the main culprit.
Maybe not faulty as such but simple newness ie
switch contact spring tensions etc
 10 September 2007 10:30 AM
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Legh

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As Phillron says, there are all sorts of variables that might affect your readings such as Supply transcients and the ability of your instrument to measure acurately while these transcients are active.
Also, I have often had readings consistently higher than one would expect when testing RCD/RCBO protected sockets.
One test Instrument company suggests doing at least three tests and then taking the average.
If I get odd readings then I tend to varifiy those circuits by dead testing and adding that value to the external impedance
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."
 10 September 2007 04:34 PM
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Mark Everson

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Legh/Phillron
I think you have just hit upon a collection of possible solutions and I thank you for your time and consideration in this matter - I can confrim that these aforementioned DSSO circuits were all protected by newly installed RCBO's.

I think the answer will inevitably be when, as you suggest, that 3 readings are taken and the average of these recorded on test results sheets.

Regards

Mark
(MIET)
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