Re: Megger test.

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

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

Well, I am not arguing about a DC megger.  The one I had was DC and was meant
for testing the earth resistance in grounding rods.   It had very long leads.
One was connected to your ground rod and the other(hot lead) to a temporary rod
at about 20-30 ft away.  Then a third, temporary rod was placed about halfway
between them and a voltage drop was measured between your ground rod and the
mid point rod.  It came with charts etc. to aid in calculation of your ground
rod's earth resistance.  I wish I still had it. 
   My Hi-Pot tester on the other hand is meant to test dielectrics.  It has a
HV trannie basically the same as a OBIT and a resistor based voltage divider to
measure the output.  It also has two neon light indicators that indicate when
leakage current begins and when break-down voltage is reached.  This tester
output goes up to 10kVAC  .  My favorite use for it has been to set my (static)
spark gaps to fire at a desired voltage.  Works great for it.  Seems to work
great for that purpose.   
Mike 

>
> 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 :-)) 
>