Discuss Low Zs readings on MFT1771 in the Electrical Testing & PAT Testing Forum area at ElectriciansForums.net

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Dartlec

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My main tester is a Megger MFT1711, which I bought second hand but came with a calibration certificate. Rather than annual calibration, I have a calcard and check it monthly(ish) and keep records, which my NICEIC assessor has been quite happy with for years...

I have an original MFT1552 which was recently calibrated at a CEF open day and I keep as my back up

My plug in socket lead had recently failed so bought a Kewtech socket adapter to use with my existing Kewtech leads....

To cross check it I tested Zs at a 'test' socket I installed some time back, (on its own RCD so can only use no trip tests).

All the tests were with the same leads, and same socket adapter...

With 1711 on 3 lead no trip test - Zs shows as 0.19 consistently....

My 1552 on 3 lead no trip shows as 0.30 consistently...

I then tested 1711 on the 2 lead no trip - and get 0.31 consistently...????

Testing the leads with a calcard, Resistance and IR tests all seem fine and within limits on the 1711....

I left the socket adapter in place between tests to try to avoid any loose connection causing issues - and the readings were taken several times each, and were consistent.

So does this show a fault with the 1711? Or just variety in the way Megger tests now? Is it something that is even adjustable by Megger or third parties, or is it all electronic inside?

The variation seems too large to be within normal variation to me, but it's also consistent - the only errors I've every previously encountered were inconsistency, which can usually be resolved with new leads...

The 1711 does have the option to change the inbuilt resistance of leads for Zs testing, which I checked is set to the correct resistance of the leads...

The 1553 does not have that option, so it may be using some inbuilt higher default? The leads were nulled on it on resistance prior to the tests, but I don't believe it uses that null reading when carrying out loop test?

The 2 wire test on the 1711 is one I generally only use on lighting circuits - at switches with no neutral, for example, and I know it can be a little in accurate.

RCD uplift can be an issue I know, so could this be a case where the 1552 and 2 wire 1711 are both suffering from that?

In recent EICRs I've tended to R1&R2 all circuits, and then calculate Zs for any but socket circuits, just to avoid the issues with testing live - but I have been noticing that the Zs tests have tended to be lower - some of that I assumed to be down to parallel earth paths etc.

Looking back at the recent calibration cert for the 1552 the No trip loop was only tested at 1, 18 and 180, so not tested at this sort of range, where it may be less accurate..

Thoughts, suggestions?
 
TL;DR
MFT 1711 seems to be reading low loop results compared to 1552, but everything else consistent - is this likely to be a fault, or just an variety in the methods of testing?
It would be interesting to know what both testers said the high current Ze was and what both said the R1+R2 is, to give an indication what the right answer actually is.
I'm expecting them to agree on a high current Ze test, and agree on R1+R2 as the Calcard suggests they are within calibration.
This would confirm that the 1711 is indeed reading low on the high current test.

I don't know the internals of the 17xx range, but I know the service manual of the 15xx range very well.
On the 15xx range, the no-trip Zs test calibration actually needs three things to be right:
-the RCD calibration
-the 25A high current calibration - this is three software based calibration adjustments.
-The no-trip Zs test has it's own software based calibration adjustments, 9 of them if I recall correctly.

On the 15xx tester it's therefore totally possible for the 25A high-current calibration to be spot on and the RCD and no-trip to not be. If the RCD calibration is out is adversely affects the no-trip too.

I have no idea if it's the same on the 17xx range as I've never managed to get a service manual for that model.

The short answer is its probably worth getting the 1711 fully calibrated at least once as the CalCard is only really checking half of it, and all you know from the test socket is that it has stayed consistent.
 
This is interesting, I have a 1552 and a 1741+ as my main testers and they are both consistently inconsistent with each other on earth loop no trip setting, on the Hi setting they are both within about 0.07 ohms of each other. I have not however tried the 1741 on the 2 wire no trip to compare that but between the two there is always a discrepancy, I guess that some of it is down to the way they confuddle the RCD to prevent it tripping, the 1741 must be slightly better at it than the 1552!
I would definitely agree with Tim, it is worth getting it calibrated as they may find something afoot.
 
I had an EICR today so was able to take both along and do some testing....

When testing Ze on the TNCS installation (High on 1552, 2Hi on 1711), I got 0.09 on the 1711 and 0.15 on the 1552. - Again the readings were consistent over several occasions... (Same leads on both)

Testing Zs at the boiler FCU (no RCD) - the 6A High on 1552 tripped the MCB every time, while 1711 2Hi gave me a reading slightly lower than calculated Ze+ R1&R2 (2LO gave me the same reading...)

Testing Zs at a different non RCD FCU (16A) breaker on HI mode, both meters gave me the same reading....

Then testing at a socket (RCD) on No trip, I got similar readings for both.....

So not sure that makes things any clearer... It's possible that it's inaccurate only at lower readings...but not sure if that's possible with these sorts of meters.

I did R1&R2 on each circuit and got similar results with both, so it's definitely the loop test that is slightly off - but the 1552 tripping 6A MCBs was one of the reasons I wanted to upgrade.... (The no trip on 1552 is 3 wire only and a real pain to do on most FCUs with their layouts...)

Looks like I need to send it off to a proper calibration company, rather than the just "tick the box" that plug it in and tick a few boxes...

Any recommendations on good mail order firms who are likely to take the care to check it properly? Can they be 'repaired' if there is a fault - I've heard that Megger are too expensive to even bother with older versions of their meter....
 
Your post above has reminded me of a similar situation I had a couple of years ago with odd readings on an installation with a really low Ze I think it was around 0.05 ohms. I could not get a consistent Zs reading on most of the circuits, they were all however adequate for disconnection times and R1 + R2 results good.
How close to the installation is the local sub station?
Can't recommend anyone for cal and fixing your tester, mine has never needed fixing fortunately! And for calibration I usually just wait for CEF or Edmondson to have a cal day and get it done then.hi
 

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