Issues with megger mft1720

[slacky1230]

Hi

I've got a megger mft 1720 that I was hoping any one can shed any light on



When performing continuity tests (even when nulling the leads) and earth loop tests I seem to get higher readings than normal.



After a while of testing the tester seems to settle down and gives more accurate readings.



The tester has been calibrated a few months ago but has been like it for a while now. I've also brought a new set of leads in case they was the issue but still the same problem.


Has any one had any simular issues with their megger?

Thanks
 

 

[buzzlightyear]

Possible your crocks or probes not connecting probably.how long have you had it.

 

[slacky1230]

I've had it a few years now and purchased it second hand so unsure how old it actually is. Even when nulling the leads I get something like .40 but never the same reading it fluctuates then eventually calms down to around .20 when been used for abit

 

[Risteard]

I've had it a few years now and purchased it second hand so unsure how old it actually is. Even when nulling the leads I get something like .40 but never the same reading it fluctuates then eventually calms down to around .20 when been used for abit

Try a new set of leads.

 

[slacky1230]

I have lol. That's what I thought it would be but still the same

 

[Risteard]

I have lol. That's what I thought it would be but still the same

Is it fully charged?

 

[slacky1230]

It uses batteries but yes tried a new set of batteries

 

[timhoward]

The sockets the test leads plug into are actually themselves connected to plugs which plug into the motherboard.
I'd suggest giving them a clean:

Remove these two screws:

And prise the socket block out using a screwdriver as shown.
Clean up the contacts both on the sockets:

and inside:

You can also try shorting out the red and green motherboard sockets to check for consistent repeatable results, ruling out the socket block, leads, clips etc.
That will tell you if it's the tester itself or not.

 

[Risteard]

It uses batteries but yes tried a new set of batteries

Then as @timhoward suggests I'd be looking at the 4mm sockets.

 

[Chivers]

If the leads are giving 0.4 or even -.2 that's higher than they should read. I'd expect about 0.1. Suggests that it needs to go back to get looked at by the calibration place if you've tried a new set of leads. Unless you try the above and it works

 

[buzzlightyear]

If you have calibrated it should be OK
Send it back,.
I have a 1720, few years ago it had been calibrated by a company and it came back ok ,next thing i was not getting loop testing ,sent it to the collaboration center Manchester .

 

[timhoward]

I actually don't think this one is a calibration issue per se as the readings improve with time.
I think it's either poor contacts as per above or a failure on the measurement PCB.
But either way a decent calibration center should sort this.

If it's 1st generation PCBs then all 3 PCBs would need changing and it's cheaper to send it to Megger, as for a fixed repair fee ~ £250 it will basically become a 1721 and gain 2 wire lo-current loop tests (though I'd never actually trust the results of these!)

But first things first - I should clean the socket block first and then short out and use known resistances directly on the PCB contacts to see if you get consistent results.

 

[slacky1230]

Hi guys

Quick update. Took the terminal block out tried a few resistors straight across the motherboard and was getting pretty consistent results. Put the block back in and the testers seems a lot better and more consistent readings. When nulling leads its around 0.17ish and doesn't fluctuate like it did before.

Still thinking about upgrading to a new tester and thinking of sticking with megger as i find them very easy to use with quick rcd tests.

Quick question how do you guys get around rcd uplift when doing earth loops? i do quick a few EICR's and find myself doing both dead tests and live tests just because the live tests aren't true readings due to RCD uplift.

 

[timhoward]

Well done. Leads sound a bit high btw mine are 0.07 when warmed up.

Rcd uplift - I either calculate Zs, or use the kitcher trick.

See