Megger MFT1502 blowing fuses and tripping MCB's

[MarkieSparkie]

Megger MFTs display PFC directly on the three lead no-trip loop test and therefore it is not necessary to test for PEFC and PSCC separately and select the highest reading for PFC. They do this by injecting a current pulse of <30A L-N for the PSCC loop and injecting a current pulse of <15mA L-E for the PEFC loop. The Megger MFT repeats this sequence 6 times, storing the readings in memory as it goes, on completion of the test it averages the 6 PSCC and 6 PEFC results, selects the highest average reading and displays this as the PFC. The reason for doing this is to maximise the noise immunity of the test. It is for this reason that Fluke fans often criticise the Megger MFTs as having a slow no-trip loop test, however, Megger users rarely, if ever, get a no-trip loop test inhibited and a noise warning displayed, unlike the Fluke MFTs.
From the explanation of how a Megger MFT does it's no-loop testing I think it's obvious to see that the L-N PSCC loop may well trip a particularly sensitive 6A MCB.
Because the <30A pulses have fast rise time leading edge with a peak current of hundreds of amperes it is essential that fuses used in the probes or meter are High Rupture Current (HRC) rated.

 

[leesturgess]

cheers markiesparkie

 

[telectrix]

Megger MFTs display PFC directly on the three lead no-trip loop test and therefore it is not necessary to test for PEFC and PSCC separately and select the highest reading for PFC. They do this by injecting a current pulse of <30A L-N for the PSCC loop and injecting a current pulse of <15mA L-E for the PEFC loop. The Megger MFT repeats this sequence 6 times, storing the readings in memory as it goes, on completion of the test it averages the 6 PSCC and 6 PEFC results, selects the highest average reading and displays this as the PFC. The reason for doing this is to maximise the noise immunity of the test. It is for this reason that Fluke fans often criticise the Megger MFTs as having a slow no-trip loop test, however, Megger users rarely, if ever, get a no-trip loop test inhibited and a noise warning displayed, unlike the Fluke MFTs.
From the explanation of how a Megger MFT does it's no-loop testing I think it's obvious to see that the L-N PSCC loop may well trip a particularly sensitive 6A MCB.
Because the <30A pulses have fast rise time leading edge with a peak current of hundreds of amperes it is essential that fuses used in the probes or meter are High Rupture Current (HRC) rated.

having read this highly informative post, i'll never again curse my 1552 for being slow. LOL.

 

[Q.S.]

Hi Guys, The Zs current depends on the age of the software, if you got an early one it could be 25 Amps. I would phone Megger and see if they can tell you what your test current actual is - they could up date the software for you, however on a no trip test it will not be this high but they have made lots of changes to the way the tests happen so a software update my help.
You have not said what MCBs you are testing, Megger has had problems with some makes and I think the 1700 series can test at 10amps now on high current test.
When you change the fuse make sure you fit the right ones (600 volt) or you can blow the tester up and make sure it is on the right setting before you connect as it does not have the same safety features of the newer testers. I think you can use unfused leads with the 1500's but I am a little rusty on them now.

 

[chronyx]

Hi all, thanks for all your replies, I also e-mailed Alpha Electronics but forgot to mention the popping fuses, I have sent another e-mail. Their original reply was:

Your unit is not faulty and there is not a way of overcoming the issue other than purchase a new more modern unit

I will have to calculate the Zs on these circuits and put LIM for the actual recorded value I assume.