Megger caused motor to explode!

[conny]

One of the apprentices at work was given a 3 phase motor to overhaul yesterday. It was off a tooling machine for a local car manufacturer so was covered in lubricating oil which had also penetrated to the inside the motor. Before we start dismantling we always do static tests, i.e. IR and phase resistance then visual check for any obvious signs of damage etc.
The Appo connected the earth lead to the motorbody and touched the terminal block with the other lead, as he pressed the button on the megger there was an almighty bang with a big cloud of smoke and he was splattered in oil and dust.

Has anyone ever heard of this type of thing happening before when using a megger? The only explanation I can think of is the oil, which was contaminated and filthy, had caused a build-up of some unknown gas and it had ignited when he pressed the test button.
The megger is model number BM223, just a basic megger/resisitance instrument with 250v/500v/1000v settings. He was using the 1,000v setting at the time. Any suggestions would be appreciated.

 

[jj4091]

I tend to agree with your theory, but fine dust can also be explosive under the right circumstances, do you not clean everything down before you start to overhaul motors

 

[holmslaw]

..

 

[ericmark]

I can remember we always cleaned the starter motors in petrol and then some bright spark decided it was dangerous and we were told to use paraffin instead but paraffin does not evaporate and quick as petrol so on rebuild it was still on the windings. OK to start with but on restart with warm engine. "Bang" all covers blown off.
And I would think something similar. I bet when motor came off it was worse then when it got to you and someone has cleaned it off first.
Eric

 

[conny]

Thanks for the replies lads.
It was one of our own guys who took the motor off and he didn't clean it first but that is not unusual. We have had motors in much worse states than this one and never had anything like this happen. When I worked on welding sets they used red diesel to try and clean motor windings! Convinced them eventually to buy some proper solvent cleaner.

JJ, well aware of the fine dust scenario. Had been on a safety course once and the instuctor demonstrated a 'small dust explosion'. However, something wasn't quite right and it was quite a big bang which took everyone by surprise including the trainer!

Holmslaw,
been there, had it done many a time as an appo and it does work! You learn pretty fast!!!
Nowdays if you even suggest something like that you get disciplined!

 

[vsynth]

Assuming it is a standard 3 phase motor you would megger it at 500V not 1000V...still, you wouldn't expect it to explode.

I guess the oil was contaminated with conducting material, possibly metal filings.

Way to gain experience

 

[holmslaw]

..

 

[conny]

vsynth,
our company test procedures are to test a motor at twice operating voltage. Therefore 415x2 =830 hence we test at 1,000v

Could be right about the filings but still never come across this situation and I personally must have meggered thousands of jobs over the past 30 odd years.

 

[uselessFCUK]

vsynth,
our company test procedures are to test a motor at twice operating voltage. Therefore 415x2 =830 hence we test at 1,000v

I thought it was twice operating voltage to earth therefore 500v for IR test.

You won't get 415v to earth.

I'll ask about that.

 

[conny]

****,

whilst I agree with what you are saying the winding companies I have worked for have all tested motors this way. As an apprentice I remember asking why such a high test voltage and was told "if it stands up to that it will stand up to normal running voltages." Even college lecturers took this view though in tests we had to state the 'correct' voltage.
We are sending a sample of the oil away for analysis to see what is in it and hopefully it will give us an idea as to why it happened. In the meantime we have been instructed to wear safety glasses at all times in the workshop. No problem to me as I need them for everyday use anyway.

why has your name been censored? I spelt it right!

 

[ricicle]

vsynth,
our company test procedures are to test a motor at twice operating voltage. Therefore 415x2 =830 hence we test at 1,000v

I bet I know which industry you work in

 

[conny]

I think from my posts that that is pretty obvious!

But go on then, which?

 

[uselessFCUK]

f*****,



why has your name been censored? I spelt it right!

You'll have to translate... it doesn't seem to start with Useless.... !