Very low Ze and Zs

What Meter would you trust, a 10 year old Unitest one that has only just been calibrated or a brand new Kewtechnic one that came with a calibration cert.

Doing a Ze/IPF test I got 0.03/9kA with the kew technic and 0.14/1.75kA with the unitest.

The Ze seems too low on the kew meter to me although the instructions with it said it would be more accurate when testing close to the transformer, (it is right next door), due to reactance?

I tried them both at home and got pratically the same reading on both, why would they be so different when close to the transformer.


Also another quirk, the unitest failed a circuit on insulation, L-E was 0.00, however the kew meter passed it with 300+Mohms. All other circuits meggered ok with either meter. What could short the unitest but not the kew one, all I can think is the cable was getting charged up like a capacitor which the unitest couldn't deal with but the kew one could, they were long runs but the others were too and they tested ok.

 

It was actually bought off the shelf from Denmans. The Unitest has been brilliant, gave about £450 for it 10 years ago and it has never missed a beat. I only bought a new one as we have a big testing job to do and could do with 2 meters.

I have just done some proper tests at home, TNC-S supply.

Unitest 0.2 ohms Zs, Kewtechnic 0.1 ohms Zs - same result however leads are configured and on both high and low tests. Same lead set is used with both meters, again kew meter seems unrealistically low to me.

Zs test at a socket using plug in lead

Unitest 0.4 ohms, kewtechnic 0.42 ohms on low test, both trip RCD on high test.

All the other aspects are spot on, RCD trip times and continuity both exactly the same.

 

Well I was told it came uncalibrated but on opening the box there was a calibration cert inside so I suppose it could have been knocked in transit. Wouldn't have thought there was anything movable inside to change the readings anyway.

Denmans do calibration days by Fluke where it is half price, and when ever I go in there when its being done the bloke at the desk has test meters all round him but I have yet to see one with the back off and him fiddling round inside it. I suspect there is not much inside them to change.

 

I've sent Kewtech a email but have not had a reply yet, we had a big testing job to do but only one meter between two of us. We were going to pay someone for the week to help us out as we were on a deadline but I thought the money would be put to better use buying a second tester so we could both get on with it. However its not worked out that way as we have used both meters on a fair few loop tests to try and make sense of it all.

Take a twin socket in a room, kewtech 2.8 on one side 0.01 on the other, both clearly way out, connected unitest to same plug lead and get a reading of 0.64 on both sides, swap back to kewtech and its 2.8/0.01. Did this without removing the plug from the socket, as you can get different readings depending how good a contact the plug makes with the socket.

It is good for the auto RCD tests though and the continuity and Mohms is quick and accurate. However this brings me onto another question.

The unitest RCD test does not have a phase switch on it, you do one test at x1 and one test at x5 and get the results, I did query this at the time and was told by a calibration bloke it didn't need to do this for a reason I cant quite remember.
On a sample RCD tested I got 39ms (x1) and 19ms (x5) with the unitest and 39/29 (x1) and 29/19 (x5) with the kewtechnic one, so the worst case figures are 39 and 29 ms. I know everything passes but which is correct.

 

Meant to update this, I have returned the tester and got a replacement, it seems a lot more accurate, it still reads slightly lower then the Unitest but the results are a lot more stable and consistant. They are also pretty similar to the original installation cert readings, so I am happy to use this one.