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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

    Jumpy readings always make me think of a loose connection somewhere upstream - although when I've come across that, the readings have been a lot higher - but might be worth a look (and listen and sniff) around all the same. Or possibly a lot of electrical noise on the line - if there's machiney nearby it might be worth re-retrying at a quiet time.

    What do you need a PFC figure for? Presumably the DNO will give you a 16kA max figure out of their book - and so the CU will comply (as you confirmed) via annexe ZB - is there a reason you need to prove it's less than that?

       - Andy.

  • 0 in reply to AJJewsbury

    Andy, Appreciate the prompt response.
    No real reason to have PFC lower than 16ka.
    Worst case scenario accept the conditional rating of 16ka but would want a view on the questions that I have asked. It would be an opportunity to gain some learning and insight.
    Can the PFC be calculated with the information of the supply transformer, length and size of service cable?
    What about the idea of make shift test leads?

Reply
  • 0 in reply to AJJewsbury

    Andy, Appreciate the prompt response.
    No real reason to have PFC lower than 16ka.
    Worst case scenario accept the conditional rating of 16ka but would want a view on the questions that I have asked. It would be an opportunity to gain some learning and insight.
    Can the PFC be calculated with the information of the supply transformer, length and size of service cable?
    What about the idea of make shift test leads?

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