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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.

Parents
  • 0

    I don’t think standard instruments will be of much value below 0.1 ohms. You should also consider the implications of Appendix 14 of BS7671 which clearly indicates that prospective fault current measurements are not required in domestic installations where the consumer unit is to BSEN 61439-3 and the DNO declares a maximum of 16KA. If someone was injured as a result of testing when such tests were not necessary then you would have a difficult case to answer.

  • 0 in reply to lyledunn

    Consider how the tester works - it flicks in and out a test load, of perhaps 10-20A in the bigger machines, and  tens of mA in the no trip RCD safe mode, and it looks for the voltage drop. Perhaps unsurprisingly, trying to measure a drop of fractions of a volt on a real supply shared with the neighbours, it fruit machines a bit. To understand the datasheet for your tester,  lists the accuracy and it will be plus-minus so many milliohms plus minus so many least signifcant digits of the display.

    Now both the readings you provided as different (0.01 and 0.07) are both really more or less identical when this is properly allowed for.Let that sink in properly  0.01 on the display means is is something  between +0.07 and - 0.04, and 0.08 means anytihng between 0.14 and 0.02. These extremes also agree with your other meter, 

    Averaging many readings may smooth things a bit, but the only solid way to get a less noisy reading is to use a higher test current. also be aware that the state of probe tips and what you push them onto can have an effect at the few 10s of  milli-ohm level.

    Calculations - rule of 16 - 1m of 1mm2 copper is 16 milliohms cold more like 19 when hot. So 70 m length of  70mm2 cable would also be 16 milliohms or more, and 50m of it would be more like 2/3 of that, just under so say 10 milliohms. However, the electrons need to go there and back, so really it is 100m round trip, so 20 milliohms for the street cable. (you could look up exact voltage drops for 70mm cable, it will change the answers by between 10 and 20%  but  assuming hot or cold cables also tweaks it a bit.)

    As a very, very rough and ready thing you may think the transformer drops 2 or 3% at full load current (500kVA /230 over 3 phases so any one phase is 170 kVA, and at 230V call it 800A ) and work back to an extra series impedance there as well (800A drops at least 5V, off the 230, so 4 more milliohms).

    The company fuse dissipates 4-5 watts at rated load, so that is another resistance to add in (it's late you can do that one), and then the meter tails and the meter, any henley blocks etc, but the rest  is all floor sweepings, the cable and transformer alone say it must be at least 25-30  milliohms, so PSSC less than 10000 amps and could easily be more like 6000, and that would be well within the tolerance of the metered result too.

    Mike

    PS do please check my sums, I'm not good at the best of times and late at night can introduce spurious factors of pi, root3, or division by -1 etc. The overarching idea is the important thing, hence all the sweeping rules of thumb and waving arms and rounding to the nearest 100A.

    The same low precision estimations allow me arrive at a place, and  to look up and in the first 30 seconds say 'the volt drop in that submain looks like it may be  a bit close - can we check it please ?' and let someone else run off for a copy of the right tables or more likely these days some electric fondle-slab and fire-up some bespoke software ...

    Bit like assuming ten inches to the foot for first estimates it makes sure you are in the right ballpark, and often that may be enough on its own . And if it isn't, then at least you an tell them the computer is using the wrong assumptions when its 5 digit precision answer is ten times higher or lower than the first estimation.

    Mike.

  • 0 in reply to mapj1

    Was that meant as a reply to me Mike? I doubt it! But as you say, late at night and that reply button confuses me even when fully awake!

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  • 0 in reply to mapj1

    Was that meant as a reply to me Mike? I doubt it! But as you say, late at night and that reply button confuses me even when fully awake!

Children
  • 0 in reply to lyledunn

    no that was to the thread . I thought they were going to fix that yesterday.

    I'll repost and delete.

    No  I'll repost and get some message about not having permission to delete children. OK so there are 2 copies sorry folks.

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