- Reaction score
- 2,441
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?
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?