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Topic Title: Reliability of PSC test results
Topic Summary: Different meters give us different results
Created On: 21 October 2013 02:22 PM
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 21 October 2013 02:22 PM
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Jonknight

Posts: 2
Joined: 21 October 2013

I have been doing some prospective short circuit testing in a 400V installation recently and getting very strange results. Different manufacturer meters give us very different results making the analysis of the information impossible. There is a 750kVA substation transformer in the building and the cabling details on site are known. Theerfore calculation of the PSC at various boards down stream of the transformer is straight forwards (i used IPSA to calc this). However getting the test meters to agree with the calculated data is near impossible.

Has anyone else had any issues and have found sensible solutions.
 21 October 2013 02:58 PM
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Avatar for Parsley.
Parsley

Posts: 1090
Joined: 04 November 2004

Quite normal with high fault currents and larger supplies, See page 55 of GN3.

Regards
 21 October 2013 03:20 PM
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AJJewsbury

Posts: 11784
Joined: 13 August 2003

or search this forum for the phase "fruit machine"

There is a 750kVA substation transformer in the building and the cabling details on site are known. Theerfore calculation of the PSC at various boards down stream of the transformer is straight forwards (i used IPSA to calc this). However getting the test meters to agree with the calculated data is near impossible.

If you know the cable size/length and can prove continuity (e.g. with dead resistance test), how much more do you feel you need to do?

- Andy.
 21 October 2013 03:27 PM
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OMS

Posts: 19900
Joined: 23 March 2004

How are you treating "reactance" in your calculations - that's going to have a significant impact in addition to all the otjher problems when using loop impedance testers close up to large transformers

Getting measured and calculated values to agree is pretty much a lottery.

Regards

OMS

-------------------------
Failure is always an option
 28 October 2013 02:28 PM
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Jonknight

Posts: 2
Joined: 21 October 2013

Thanks guys

Perhaps the instruments will one day do what it says on the tin!!
 30 October 2013 05:50 PM
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HarryJMacdonald

Posts: 262
Joined: 15 May 2002

I had this in a large installation, I even had negative readings but got some sense by taking the average of several readings.

My conclusion was that the tiny dip in voltage that these meters try to measure was being swamped by noise on the line so averaging was a reasonable way forward.

I'm sure I once used a meter that used a fairly substantial test current to avoid this problem, but when I looked for it again I couldn't find it.
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