Re: Megger test.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>" <scepp-at-mts-dot-net>

Hello You'all

Well I've never heard of such a thing happening, but I'm not one that uses
meggers and dielectic strength testers but I fixed them for about 4 years.
They do do insulation testing, between the windings and the case of motors
and transformers.  They usually have current limiters on them and warnings
if the current is out of spec.  Most of the dielectric strength testers us
variable DC voltage (10 to 20 Kv) and not 60Hz AC.  They are designed to
show a problem before it becomes a major fault, like insulation failure and
to verify a fault,  they are not made to cause a fault, just show that their
is comprimised insulation.

The AC Dielectric testers use a variac and a step up transformer to supply
0 - 2500 volts to the line to chassis AC power plug,  They are current
protected to about 95mA with a current sensing relay and latch cct.
Manufacturers use them to test to standards on new products, and they trip
very easily.   I've also seen some very old testers that only had a light to
show insulation failer..... these might power a transformer ???

I wanted to add this to give my 2 cents worth,..... why else :-))


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, October 06, 2001 12:04 PM
Subject: Re: Megger test.


> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>
>
> Hi greg,
>
>
> Tesla list wrote:
> >
> > Original poster: "Mr Gregory Peters by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
> >
> > Hey guys,
> >
> > As afore mentioned, I priced a reconditioned pole pig at $1045
> > Australian. However, they will sell me an unconditioned unit for $500
> > Australian (about $250 US). They say the only test they will do on the
> > transformer is a megger test. What is a megger test?
>
> As many others have said, The megger is a "megohm" meter that pushes
> upwards of 1500v into the windings and measures the leakage through the
> insulation.
> Not exactly sure how they do it, or why they cost as much as they do
> (big bucks)
> While i have used my meggers to check transformers that have been
> flooded or might be damaged, but this is usually on 550v- step-down
> transformers and It isn't a high enough voltage for the hv side of the
> pole transformer. Now i have seen some call there "high pot" testers
> meggers, and actually they work the same in principle, a high pot tester
> will test the primary windings at there normal operating voltages.
> Maybe ask these guys if there megger test is at the intended operating
> parameters?
> Also, if a 14kv pig is tested at say 30kv "high pot", It will actually
> damage the winding insulation. I remember one instance where a tech
> meggered the lv side of a 13,800v-550v transformer with his hand held
> megger at a setting of 1000v on the low side, this produced 25kv on the
> hv side and must have punched a hole in the insulation because the
> mildly suspect trans. turned into a very large arc welder at power up
> (and sorry to say, someone was hurt very bad loosing one eye).
> If your people are friendly, just ask them to power the lv side with
> 240v and leaving the hv side open will at least show if the windings
> will handle it? if no smoke erupts from the case, i would be safe in
> saying that you're all set.
> GE would come and high pot our transformers at a very high price tag,
> very thorough, and they were actually able to give suspected hours of
> operation left inside that magic smoke. Don't ask me how?
> Hope this helps,
> Marc
>
>
>  Is this a reliable
> > indication of the transformers integrity? Would you guys suggest saving
> > for the reconditioned unit or buying the "as is" unit? I figure I can
> > buy two of the "as is" uints for the price of 1 reconditioned unit, and
> > the chances of both being duds are minimal. What do you think?
> >
> > Cheers,
> >
> > Greg Peters
> > Department of Earth Sciences,
> > University of Queensland
> >
> > Phone: 0402 841 677
>