Megger MFT1502 blowing fuses and tripping MCB's

[chronyx]

I had this calibrated recently, yet every time I try a 'No trip' Zs test it either pops the neutral fuse in the leads, or trips the breaker.

The only breakers that trip are the 6A breakers, the rest it will complete the test.

Using the socket adapter also works fine.

It does a Ze reading fine.

No doubt it's shagged but I thought it was worth asking, thanks for any help.

 

[leesturgess]

zs test is a 25amp test so it will trip 6amp mcb 3-5 times for type b

 

[telectrix]

zs test is a 25amp . ​not on no trip, it ain't.

 

[chronyx]

I tried the 25A loop and 'No trip' (Two and three leads repsectively) and it tripped on both.

 

[chronyx]

zs test is a 25amp . ​not on no trip, it ain't.

Cheers I was doubting myself for a sec!

 

[telectrix]

trhink it's 15mA on no trip, otherwise it would trip RCDs.

 

[leesturgess]

i have a megger 1552 mft and when i try to use the no trip on a 6mp mcb (mk) it trips the mcb. yes it does send 15 m/amps down the cpc

 

[chronyx]

That's interesting Lee. I wonder if our testers are faulty? I have definitely done a Zs on a lighting circuit with a 6A breaker using works Fluke testers before without any problems.

 

[leesturgess]

as with yours mine test fine for everthing else im sure this subject has come up on here before might be worth a quick search

 

[leesturgess]

i have all my calabraition certs for it as well

 

[Deleted member 26818]

Seems wierd that it's blowing the fuse in the neutral probe though.

 

[chronyx]

I have bought them from RS and also a wholesaler, SIBA branded. Same problem.

500mA as per the specification in the user manual.

 

[leesturgess]

i do not have fused leads with mine even though gs38 requires them, like i said before trips 6 amp mcb and thats it. this is a quote from niceic snags and solutions inspection and testing.AVOIDING UNWANTED TRIPPING OF RCD,S AND CIRCUIT BREAKERS WHEN MEASURING ZS, testing at a reduced current this technique uses atest current at or below 15 ma such that rcd,s having a rated residual current of 30 ma and greater will not trip. this technique will also mean that overcurrent circuit breakers of type b with a rated current less than 10 amps should not trip when measuring zs. when using my megger1552 mft it does trip 6 amp type b mcb? they say should not rather than will not?

 

[sparky1502]

make sure the test leads are in the correct positions on a zs test its 3 test leads black is top green is middle and red at the bottom. it can get confusing because when you changed to do end too ends you take out the green and put the black in its place

 

[leesturgess]

leads are all correct no trip selected still trips 6amp mcb good connection so no problem there?

 

[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.