Discuss Megger 1720 in the Electrical Forum area at ElectriciansForums.net

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hutch6447

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Right think I have enough of this machine thinking very hard and long about going back to Fluke.

Can anyone tell me a reason for me getting a calculated Zs: Ze+(R1+R2) of 0.68ohms then getting a measured Zs of 0.94ohms which is considerable higher than my calculated Zs, now I know it still passes and is within limits but this discrepancy does not happen all the time plus I never got any discrepancy with my fluke so does someone who has used a Megger 1700 series got any idea.

Doing the Ze on high current L-L L-N for none RCD protected circuits.

Doing the Zs Measurement on L-PE so it doesn't trip the rcd.

Now I know I can use links and bypass RCD but surely you shouldn't have to do this to get a correct result as it is a no trip test and the pam say's switch to L-PE and test on Terminals L1 and L2 Phase and Earth.
 
M

MarkieSparkie

Your Zs measurement is probably higher than the calculated value because the low current employed in the No-Trip loop test method cannot break-down the oxide and tarnish of the in-service joints and contacts. Normally one would expect fortuitous parallel earth paths (eg. via bonding) to reduce the measured Zs to a reading similar or less than the calculated value. However, if the non-electrical services (water and gas etc.) are provided via plastic or plastic covered piping, or not available, and there is a lack of structural steelwork, fortuitous parallel earth paths may be minimal or non-existent.

You should not blame you Megger MFT, using the No-Trip test on a Fluke or any other tester would likely as not produce similar results. It’s most likely the combination of the test method employed and nature of that specific installation that has caused you to doubt your tester.

In lab conditions both No-Trip and Hi loop test measurements are likely to produce similar results, certainly within the calibrated accuracy of the MFT, but add in the oxidized joints and contacts of a typical real world installation and result is likely to be completely different. My money is on the Hi current loop test giving the more accurate result because the higher current is likely to be able to break down the oxide or tarnish film.

If your Megger MFT1720 has a firmware version earlier than 2.01, it would be well worth getting it calibrated and the latest firmware update installed (usually inclusive in the calibration charge), as this has addressed virtually all the previous variability and RCD test issues. Two wire loop testing is still less than successful, but each firmware update does seem to produce a slight improvement.
 

Deleted member 9648

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I too have a 1720 and have sent it back once,because in practice 5 no trip tests on the same point will produce 5 different results...(sometimes wildly different)
It has been suggested on here that this is due to RCD's,but I've experimented on non rcd circuits with the same random results.
Markie's explanation makes a lot of sense,but I dont accept it should be the case....we are expected to ensure the accuracy of our meters with expensive regular calibration and documented check boxes,and yet manufacturers provide us with a wildly inaccurate meter and we are supposed to accept it?
I've said it on here before,and have said it to megger...the no trip test is not fit for the purpose.
 

Risteard

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Doing the Ze on high current L-L L-N for none RCD protected circuits.

Doing the Zs Measurement on L-PE so it doesn't trip the rcd.
I'm confused. Ze cannot be measured line-line or line-neutral. It is only line-Earth.

As with Zs - and it's the line-Earth which is likely to trip the RCD hence the non-trip loop setting (which is unreliable on all testers, not least Fluke).
 
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hutch6447

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L-L L-N is high current test to test the Ze because its not protected by rcd
L-PE on megger is no trip test for measuring Zs of rcd protected circuits.
 

Guitarist

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Basically, everything Markie says is spot-on. I would just add that the 2-wire no-trip test will not be accurate because of the way the machine does its calculations. As Megger told me though, if you need a no-trip test then a few ohms won't matter as you have an RCD...
I have a 1730 with 2.01 and I have to say that I am very happy with it, now that I know its limitations. :)

Btw, if you are close to your EFLI limits, then always test at the socket terminals and not with the 3-pin adaptor, as it always adds a little resistance.
 

Deleted member 9648

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Basically, everything Markie says is spot-on. I would just add that the 2-wire no-trip test will not be accurate because of the way the machine does its calculations. As Megger told me though, if you need a no-trip test then a few ohms won't matter as you have an RCD...
I have a 1730 with 2.01 and I have to say that I am very happy with it, now that I know its limitations. :)

Btw, if you are close to your EFLI limits, then always test at the socket terminals and not with the 3-pin adaptor, as it always adds a little resistance.
Well if it's not accurate and it doesn't matter anyway whats the point in doing it?
 

Guitarist

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Well if it's not accurate and it doesn't matter anyway whats the point in doing it?
That was my point. It's either right or it's not...
Anyway, I only use the 3-wire no-trip test and I'm happy with that so I don't lose any sleep over it. As I've said in a few other posts, my only gripe is that this is not made clear by Megger from the start, and so many of us waste time and energy finding it out.
 

sparks1973

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Basically, everything Markie says is spot-on. I would just add that the 2-wire no-trip test will not be accurate because of the way the machine does its calculations. As Megger told me though, if you need a no-trip test then a few ohms won't matter as you have an RCD...
I have a 1730 with 2.01 and I have to say that I am very happy with it, now that I know its limitations. :)

Btw, if you are close to your EFLI limits, then always test at the socket terminals and not with the 3-pin adaptor, as it always adds a little resistance.
and why not use the three pin adaptor?..i assume you mean the BS1363 plugtop adaptor?....i would be using that as its the LAST POINT OF CONTACT on the fixed install...before you go onto portables/fixed/moveable equipment etc....surely you would be wanting a Zs from there....hmm?
 
T

terry5880

we just got a brand new megger in work never done pat testing before but they got me doing it and im sure there rating for fuse against wattage is wrong but they wont have it lol
 

Guitarist

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and why not use the three pin adaptor?..i assume you mean the BS1363 plugtop adaptor?....i would be using that as its the LAST POINT OF CONTACT on the fixed install...before you go onto portables/fixed/moveable equipment etc....surely you would be wanting a Zs from there....hmm?
Don't get me wrong, I use the 3-pin adaptor most of the time. Occasionally though, due to the fact that it is such low current on a no-trip test, the resistance of the pins can add appreciably to the Zs reading and make it appear that the circuit doesn't comply. As I said, not usually a problem, but can be important to remember if your Zs is close to the limit. All that said, if you have an RCd in circuit (which you would with a no-trip test), then you might consider that you have oodles of ohms to play with.
 

sparks1973

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Leeds
Don't get me wrong, I use the 3-pin adaptor most of the time. Occasionally though, due to the fact that it is such low current on a no-trip test, the resistance of the pins can add appreciably to the Zs reading and make it appear that the circuit doesn't comply. As I said, not usually a problem, but can be important to remember if your Zs is close to the limit. All that said, if you have an RCd in circuit (which you would with a no-trip test), then you might consider that you have oodles of ohms to play with.
now i aint getting dragged into that one guitarist..lol....you know what i think about 1667 and `magic` cure all RCDs....lol!!
 

Guitarist

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Norfolk
now i aint getting dragged into that one guitarist..lol....you know what i think about 1667 and `magic` cure all RCDs....lol!!
Same here mate. That's why I always design a system so that the RCD is additional protection. :)

Anyway, was just pointing out that sometimes due to old sockets, coupled with the low current of the test, measuring at the terminals is sometimes a better option than using the adaptor. Not trying to start a debate, just something I've found....
 

Zorro

Regular EF Member
Messages
70
Well I had a student who has a megger mft1720 and megger mtb7671/2 calibration checkbox. I got the student to check the mft1720 with the checkbox, but had a slight problem the new test leads are not compatible,the probes don't get any contact at all, the new probes are also not compatible with the kewtech socket interface
adaptor again could not get any contact when testing ring final circuit. I then decided to use the old megger 6220-796 leads and these worked fine? It looks like megger have made another bulls up.
 
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