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Fluctuations in reading ZLine Ipscc

Adam Kuzack

Hi,

 

It's been a long time since I have posted a question. However, have a ' live'  situation where I could use the advice of members, experts and sages on the IET forum.

 

Stated to work in a domestic property single phase supply TT system and when I tested the PFC on the short circuit, the test instrument gives fluctuations in reading from 0.01 ohms to 0.07 ohms

 

The meter has recently been callibrated and despite this: the thing is jumpy and  cannot give consistent readings. I accept that MFT have tolerances and this is a metrel with a resolution of 0.01 with an accuracy rating of -+5digits. So at low ohms it can fluctuate but still be in callibration

 

I have measured the PFC on the short circuit with a loop tester Martindale which is not  callibrated and this is giving a reading of 0.07 ohms.

 

A consumer unit has been installed which has 6ka breakers and I have checked with the manufacturers Contactum  and the board has a conditional rating of 16ka.  The supply fuse is Bs1361 type 2.

 

I have made enquiries with the DNO and they have confirmed that the substation is 50 meters away from the property, it is a 500kva transformers and the supply cable is a 0.1 concentric cable cable of 70mm

 

This is going to be my assessment job for the NICEIC

 

The question is this: from the DNO information how do I calculate the pfsc short circuit current

 

The second question, and this is gleaned from other forums, to achieve greater accuracy  in the  MFT, it has been suggested to introduce a known resistance. I have made test probes where I have placed a 10watt wirewound resistance of 0.68ohms and used this on the neutral conductor to undertake the test. It has given more consistent readings on the jumpy Mft  of 0.06 to 0.08 ohms when I have  deducted of the resistor value placed in the test lead

 

From a gs38 and guidance note 3 basis, would the above practice of placing a known resistor in the test leads be acceptable

 

I do not intend to undertake the measurement of the PFC with the make shift test probes with the resistor before the inspector, but do feel that in the field it would be method of attaining greater accuracy in testing

 

Apologies for the length of post but would like some advice.

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  • 0

    they work out you (sic) shopping bill comes to £1,765.76 when you only asked for two bags o` crisps and a chocolate bar.

    The bane of the computer age.

    Instant high precision beautifully presented *&*^%$ rubbish, un-moderated by any common sense that a human would apply.

    Also seen with word processors and power point style presentations, where no facts at all can be spread over several sides with impressive clip art and special effects to rob folk of time they could more usefully spend drinking beer and setting the world to rights.

    The 3 line sum on the paper bag was never that bad, because a sense of 'that can't be right, I'll just check that before I make a fool of myself  ' would have kicked in.

    Back to the OP.

    In the days of the moving meter needles it would have been near the 'here be dragons' end of the scale where infinity, 5kA and 10kA all bunch up together,  and in the other coloured ink 0 ohms was full scale the next division down was 0.1 or 0.2 ohms or something while 250A 500A and 1kA and the higher resistances were wider spaced. If the needle  wobbled a bit with local loads coming on and off you'd have probably gone 'mmm' and written down a single value - the flickering digits hide an internal  PSSC calculation that approaches a knife edge as the loop resistance approaches being too small to measure for that sort of tester.

    Mike.

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  • 0

    they work out you (sic) shopping bill comes to £1,765.76 when you only asked for two bags o` crisps and a chocolate bar.

    The bane of the computer age.

    Instant high precision beautifully presented *&*^%$ rubbish, un-moderated by any common sense that a human would apply.

    Also seen with word processors and power point style presentations, where no facts at all can be spread over several sides with impressive clip art and special effects to rob folk of time they could more usefully spend drinking beer and setting the world to rights.

    The 3 line sum on the paper bag was never that bad, because a sense of 'that can't be right, I'll just check that before I make a fool of myself  ' would have kicked in.

    Back to the OP.

    In the days of the moving meter needles it would have been near the 'here be dragons' end of the scale where infinity, 5kA and 10kA all bunch up together,  and in the other coloured ink 0 ohms was full scale the next division down was 0.1 or 0.2 ohms or something while 250A 500A and 1kA and the higher resistances were wider spaced. If the needle  wobbled a bit with local loads coming on and off you'd have probably gone 'mmm' and written down a single value - the flickering digits hide an internal  PSSC calculation that approaches a knife edge as the loop resistance approaches being too small to measure for that sort of tester.

    Mike.

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