High earth loop impeance at Transformer

How are you measuring this - very low loop impedances can be easily thrown off with the wrong kit, and I'd be wary of anything with conventional test leads and probes when looking for things much below 100 milliohms, You need a lot of test current and probably a four point connection.

Mike.

You are indeed correct in your expectation of a loop impedance of  around 0.02 ohms. I have calculated the figure for you and it comes out at 0.024 ohms. As has been said, best to not rely on everyday loop testers for this level of work.

Regards, UKPN.

It appears that according to your test results the 800 kVA transformer cannot be used to supply a 5.4 kW under sink water heater. 

engx.theiet.org/.../en-61000-3-11-and-en-61000-3-12

Certainly I agree about testing properly first. That's probably the main thing. Measure voltage independent of test-current, four-point as Mike said.

Presumably it's a 400 V secondary, not e.g. 690 V (for motors) which would go some little way to explaining the difference.

If the primary supply is among the weakest in the world it could add the extra 0.1 ohm seen at LV.  Like 3.3 kV primary with 10 miles of tiny overhead conductor such as 40 mmsq ACSR.  It would not be normal or reasonable to connect a 800 kVA tr to that.  At the more likely 11 kV you'd be super hard pressed to add much impedance.

0.024 ohm sounds high, perhaps an  ESI thumb-rule with an assumed source impedance and LV conductors.

This is a strange thing to attempt to test. Your private transformer I presume? Where is the N-E link? Your test instrument is useless for this job close to the transformer, and conceptually wrong. Essentially you are attempting to measure the output impedance of the transformer, which at 800 kVA is very small indeed. A loop tester has neither the resolution nor accuracy to give any kind of result. UKPN has given the only sensible result available, using the transformer data. The PSSC at the terminals will be around 24 kA N-Phase or 35 kA P-P with any kind of useful HV supply.

Ignoring upstream impedance, 24 kA for 3-phase at LV terminals; less for P-P; similar or more for P-N.   Agree 0.024 sounds about 2.5x too high.   (But all this isn't very important ... the essential point is already well made, that more than a simple two-wire tester is needed for sensible result at low levels.)

Further to this, you'd not normally measure it in situ at all unless there was evidence of a problem and even then you may be better to look at voltage drops on switching of really big load banks  of  a decent fraction of the rating.This is in effect all the tester does, but given its size it can only switch a small test current, so in a low impedance circuit, the voltage drop it tries to detect is 10% of SFA and small compared to lots of other voltage changes due to other things like oxidation and contact resistances, so its gives a funny answer,  a bit like a fruit machine - you pull the handle get some random digits, and perhaps a lemon

Mike.

Both these machines are made and have application. The one  on the left is used for filling out EICR  forms on cheaper rented property I think....  

But joking aside, it is important to understand the limits and likely variation from the desired result in both cases. - failure to do so can cause a rapid loss of profits and credibility.  Over the range  of Zs for which it is designed to be used the basic  loop testers are very good, outside that, they are not to be trusted. Testers, know your limits and if in doubt check the maker's catalogue data - usually the uncertainty is so many % plus a few digits in the last place, assuming shiny new leads and a following wind..

This is a private Transformer which sits on a PWN. As part of the commissioning process as the TX has just been installed they have tested it and the results are high. But i agree i believe this to be a meter issue. 

Assuming the transformer is tapped for the usual 433V and the HV can deliver unlimited fault current, which it cannot do, an infinite bus.

The maximum output current will be 800000/(400 x route 3) = 1154.73A rounded up 1155A.

The prospective fault current will be 1155 x (100/ 4.75) = 24.316kA.

The voltage to earth is 433/Route 3= 250V.

Ze = 250/24316 = 0.0102.

That is at the transformer terminals. The fault current will drop as you move away from the transformer terminals and the ELFI will rise. The fault current will also be limited by the current limiting effect of the circuit protection downstream of the transformer.

The so called high current testers usually have a test current of 5A or so which will not give an accurate reading close to the transformer but the accuracy will improve the further away from the transformer you test. I have the Megger LT300 high current, test current 10A, which is better than most MFTs. I also have the Kewtech KT65DL which delivers very good results when compared with calculated values under test conditions when I have tried it on one of my design jobs 5m from a transformer at the main panel. I have not had the opportunity yet  to try Kewtech's latest KTW 66DL which has a variable test current of up to 25A looks promising. 

If you want to do some serious testing up close and personal the Megger MIMS 1000 4 wire tester delivers up to 1000A of test current. I did a test with the MIMS at a transformer terminals with all my PPE on, including brown trousers, but was only brave enough to test with the MIMS set to 500A!

Hi John

It's a shame you've not had the opportunity to play with the KT66. I was looking forward to review as I fancy treating myself to a new multifunction meter. Any chance of you getting your hands on one in the new year?