Re: Megger test.

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

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

Tesla list wrote, in part:

> Original poster: "Metlicka Marc by way of Terry Fritz
 
> 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,

	The classic megger used a hand cranked generator (alternator,
	actually...) and a trick meter movement to account for the
	variable input voltage.
	Modern ones use an inverter type supply.

> or why they cost as much as they do (big bucks)

	More intricate design & less sold.
	Care is needed in design to have HV AND not damage the
	equipment under test or the megger.

> 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?

	Probably:
	Look at the current leakage current, compare to the design
	value and some historical data on how fast leakage rises
	with time.

	meggers are handy because a leak which is undetectable at LV
	may be detectable, or even nonlinear (increasing with
	applied V) at high applied V.

	best
	dwp