Low Ze, high PFC in industrial application

[alexcarnes]

Hi - just after thoughts on this please. I've been doing some testing on a 20 year old MCC. On the original EIC, Ze was reported as 0.04 R, PFC 5.75 kA.

Obviously this is a three phase supply, so shouldn't they have doubled the PFC and reported it as 11.5 kA, or is this business of doubling the phase fault current to estimate the short circuit current between all phases a recent thing?

The second problem - is there a reliable means of measuring such low Zes, or is it a fool's errand and you're best just calculating it? From a number of different tests, I'm gaining the impression that the real impedance might be nearer 0.03 R - a small difference, but that would make IPF 50% larger (230/0.03 = 7,666.7 x 2 = 15.3 kA). Even with all best practice my Fluke 1664 reads anything from 0 R to 0.03 R. Note that it's never given a higher reading, which is partly why I think the Ze on the EIC is suspect.

The third problem - I bet you know what I'm about to say! - there're a couple of 400v distribution boards full of 3 pole MCBs with 10kA breaking capacities. Zdb won't be meaningfully greater than Ze at the origin, one is fed off the bus bars, the other from a short run 16mm2 cable (Zdb is still so low I'm struggling to measure it - 0.02 R is commonly returned). Unless I've got something wrong or misunderstood something, the breaking capacity is inadequate - or was is OK to rate them on the single phase fault current 20 years ago? Even if the Ze on the EIC is correct, 2 x 5.75 kA = 11.5 kA.

Thanks.

 

[davesparks]

I don't think that works. Typically most meters are specified as something like +/-X% + N digits, your X percentage is the same on the total, once you remove the added R and its known range you get an even wider percentage of the result. Similarly once your R is subtracted from the reading, you still have the N least digits to consider.

Sadly none of the MFT I have ever seen even have the option for Kelvin cables, even as extra-cost option, which would get rid of a bit of the low R uncertainty at relatively low cost.

Yeah I have never been too certain of the method either but it was championed by the late Engineer54, you could probably find his posts on the subject with a bit of searching.

 

[Marvo]

I've also heard of adding a known resistance to bring the measurement more into the accurate band of the instrument but I've never seen the advantages of this where as PC1966 point out you're basically robbing Peter to pay Paul. Personally once you get much below 0.05 ohms I'd be derriving the PFC by calculation referencing the transformer published specs and known supply cable impedances unless I had access to a high res loop tester.

 

[loz2754]

I noticed that the new Megger MFT X1 has a higher Ze resolution:

The MFT-X1 has extended the low end of the loop impedance range from 0.01 ohms to 0.001 ohms resolution and 50kA current calculation.

Even so, I don't think I'll be getting one at that price.

 

[alexcarnes]

I noticed that the new Megger MFT X1 has a higher Ze resolution:

The MFT-X1 has extended the low end of the loop impedance range from 0.01 ohms to 0.001 ohms resolution and 50kA current calculation.

Even so, I don't think I'll be getting one at that price.

Yes I noticed that too, although the Fluke 1664 that I use claims the same precision (it has a milliohms mode in the high current loop impedance test settings). I think it's a bit of a gimmick to be honest - the accuracy is poor at very low impedances so you just get the wrong answer to another decimal place, basically. At higher impedances it's not really significant.

I wondered whether the MFT-X1 would work any better than a tool like the LTW425, and accordingly emailed Megger to ask, but they didn't even dignify it with a response.