Megger 1552 vs 1720

[bobby101]

Hi all just got a quick question for you all I've been using the megger 1720 recently and think its the best piece of kit going but I can't help to notice that when doing ipf test when compared to the 1552 the readings are considerably lower when tested on the same part of an installation also I've noted that when taking a ze at DB,s the reading from a 1552 are much lower than from a 1720 which when testing a large installation with two guys makes for an inconsistent report any ideas would like to hear from megger re this issue also RCD test results are completely different from both machines but what I did notice is that 2 separate 1720 machines as you would expect have near same results is it just a case of the 1720 being more accurate or vice versa please note I had the 1552 calibrated pre experiment on installations thoughts please

 

[phawk]

I've been told that the firmware on the new megger is better than the 1552 (which I have). This came to light in another post regarding Zs testing, and how the new megger can break down the oxides on the cables better for a more accurate reading (from what I remember).

 

[richy3333]

Ring Megger technical and put the question to them?

 

[MarkieSparkie]

The 3 wire loop test methods used by both models of Megger MFT are similar.
The fact that two calibrated MFTs produce the same results is purely coincidence, that probably shocks you, but it's true. The best that can be expected is that all the calibrated MFTs produce results which are similar and within specification.

MFTs made to BS EN61577-3 are not the precision instruments that most electricians imagine; the results displayed on their digital displays, in most cases to two decimal places, seduce and beguile their users to believe them implicitly.

The specifications of both Megger MFT series quite similar, but not identical, if we compare the two loop specifications in our normal working range:

Megger 17xx MFT 0.1 to 39.9 ohms ±5% ±5 lsd (± noise margin No-trip)
Megger 15xx MFT 0.01 to 9.99 ohms ±5% ±0.03 ohms (± noise margin No-trip)

So, if we just take the basic accuracy of ± 5% in both cases, then if your calibrated 15xx series MFT is close to the lower limit of -5% and your 17xx series MFTs are close to the upper limit of +5% then you can see that the readings can differ by nearly 10% and yet still be within BS EN 61577-3 specification.

This is of course assumes laboratory type conditions, in the real world if we through in a few tarnished or corroded joints, No-trip testing can produce results way outside these limits because it's unable to breakdown the oxide film due to the low current or short duration of the test method employed.

[FONT=&amp]If you thought that was bad, even wilder things happen when attempting to test PFC on installations with very low earth fault loop impedance.[/FONT]

 

[bobby101]

If possible please elaborate in terms regarding whether or not I can use readings from both sets of instruments in which the results don't numerically match or make sense for example the mft 1720 records a reading of ze 0.24 at origin but the mft 1552 records a reading of 0.09 at the main section panel would this be acceptable to submit and would the niceic or other governing body's accept this not to mention certain clients who have an eye for details also it brings in to question installations pre tested with the mft 1552

 

[richy3333]

If possible please elaborate in terms regarding whether or not I can use readings from both sets of instruments in which the results don't numerically match or make sense for example the mft 1720 records a reading of ze 0.24 at origin but the mft 1552 records a reading of 0.09 at the main section panel would this be acceptable to submit and would the niceic or other governing body's accept this not to mention certain clients who have an eye for details also it brings in to question installations pre tested with the mft 1552

I think Markiesparkie has answered the question and advised you about the difference in tolerances between the machines? Surely the assurances/questions you now have, need to be put to the NICEIC and/or Megger.
If you only had a 1720 and showed the NICEIC you calibration certificate and system for routinely checking your machine to show it doesn't vary by more than 5% then the NICEIC are going to be 'happy'? Any assessor isn't going to 'whip out' a second meter to test alongside your own? The issue has only arisen because you are comparing 2 machines.
If you think the 1720 is out of calibration or the software might not be the latest then speak to Megger, they will answer all your questions and arrange any work required to the machine?

 

[bobby101]

Hi thanks for the reply I wasn't disputing the information only trying to get full clarification for several reasons I don't dispute one installation one tester one calibration cert no problem or now as I understand 1 installation 2 different type of meters and results for one installation as long as they are calibrated and comply with the afore mentioned BS number. I have contacted megger also for there input thanks for all your help

 

[Tim]

Call meggers technical department and speak to ben, very helpful bloke. I just sent my mft1730 back to get the new software as my readings were all over the place.

They send fedex to collect and deliver it back with the latest software uploaded completly free.

Tim

 

[MeggerTSG]

Hi all just got a quick question for you all I've been using the megger 1720 recently and think its the best piece of kit going but I can't help to notice that when doing ipf test when compared to the 1552 the readings are considerably lower when tested on the same part of an installation also I've noted that when taking a ze at DB,s the reading from a 1552 are much lower than from a 1720 which when testing a large installation with two guys makes for an inconsistent report any ideas would like to hear from megger re this issue also RCD test results are completely different from both machines but what I did notice is that 2 separate 1720 machines as you would expect have near same results is it just a case of the 1720 being more accurate or vice versa please note I had the 1552 calibrated pre experiment on installations thoughts please

Hi bobby101

As MarkieSparkie has already mentioned, the basic accuracy for both MFT1500 and MFT1700 Series testers is ± 5% so that different testers can give readings that differ by 10%, well within the EN 61557-3 standard.
Based upon this information, for example, you may have 4 MFT testers, 3 giving almost identical readings with the other 1 giving a seemly odd value. Spurious noise on the system can also give rise to odd readings between testers especially if a one of comparison test is performed.
You mention the RCD test results are completely different, how much do they differ?

Kind Regards
Megger Technical Support