Both measurements are of course not really applying a short circuit  but rather a change in load that creates a small but measurable voltage drop, from which the current that might have flowed if the full 230V or whatever had been present,

The anti-trip passes less than 30mA RMS (or it would trip) and does a complex switching pattern to try and eliminate being fooled by system voltage changes during the test.

The traditional meter uses a much higher current - ten amps up to about 25 are common in testers for 32A circuits so the voltage drop from the test is much larger and the magnitude of any mis-read so much less.

However, 100:1 is well beyond this - so there are a few possible explanations.

1) a corroded contact that was high Z during the ATT testing, but then blasted clear and 'welded shut' by the higher test current

2) an RCD or RCBO coil - less likely as then the tests on the  non- ATT setting should have been cut short by it tripping. The coll inductance cancels for L-N loop (if it did not the RCD would trip) but not on an L-E loop  Certain ATT pulsed waveforms are high frequency and can generate a significant back EMF in that series inductor, yet to the 50Hz the inductance is not really significant, This can fool some staff into reporitng faults when there is not one. Certainly an un balanced final  circuit is a bad idea and  can fool certain types of test kit.

3) There is noise or waveform distortion that fools the meter, and another meter or even the same on another day, may read very differently.

Mike

Two different tests on Kewtech testers, high current measurement is the loop impedance whilst the low test measures is the loop resistance and as Mike pointed out the difference in the current makes a big difference. 

I have too many test meters, including six I can use for loop testing, my wife can't understand why some are not being sold on Ebay!

When I bought the last one, a Megger 1741, I phoned the Megger technical team to ask about a setting and said I should sell a few older testers, I was advised to keep some of the others as they all work in different ways and sometimes one will work better than another!

I have had the same conversation on trade stands at shows.

I have been thinking about this and loop testing just isn’t as exciting as it used to be,

My original tester a Robin KTS1620 uses a 25 amp current for a loop test on the 20 ohm range, whilst my latest tester a Megger MFT741 uses a 4 amp current for the high loop test and a lower pulsed current for no-trip loop testing.

You don’t get the arcs, sparks and crackling noises with the modern testers that test with less than a sixth of the current the older testers use.

Hi Sparkingchip. I also own the Robin and agree it was a satisfying feeling once the result was displayed. But with the use of residual current devices in domestic installations now renders loop testing redundant for final circuits. Instead, the values of R1+R2  added to Ze  and recorded.

Did you take a reading at the CU ?  Might be caused by dirty socket contacts. Try a dead test at the terminals of the socket.  Might be time for a new socket face. Garages are dirty places.

Gary

It could be a fault with the tester.  As Sparkingchip has pointed out, they are two completely separate tests on the Kewtech, which means a component fault can affect one setting but not the other.  They have three different measurement ranges on each setting, and a failure on one can lead to the tester giving the highest reading in that range.

Mine had the opposite issue - after a lot of use one of the non-ATT ranges failed and gave high readings.  Kewtech fixed it fairly quickly though.

I did a test on a couple of sockets in the garage, plugged and unplugged the tester a few times(as that often cleans contacts a bit), same readings. I didn't have time to do test at CU. My test meter is a Kewtech KT66, but quite new. Interesting thought about fault in meter though. There was a solar PV array on the garage, fed into the garage CU. Not sure if that could affect things. As said back there on Monday to investigate further, with some dead tests. Just never seen such a large discrepancy between ATT & Non ATT loop test. 

Do a test at home or in the office to test your tester before you go back.

It is indeed useful to have a known test socket.

Even better, have a test box.

The old Robin KTS1620 could be quite gentle, after the inspection and shouting to tell the plumber not to touch anything, the first test could be on the 2000 or 200 ohms range testing with a significantly smaller current, before dropping down onto the 20 ohms range.

Although in a TT installation the 20 ohms range might never be used, generally the 20 ohms range was only used in TN earthed installations.