Megger vs Hipot

Discussion in 'General Electronics Chat' started by metaltomato, Mar 4, 2013.

  1. metaltomato

    Thread Starter New Member

    Dec 14, 2012
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    I am working to create a manufacturing process to produce small brushless ac motor/generators.

    Recently I equipped the final testing station with a GW Instek 4 in 1 electrical safety tester. With the help of the manufacturer and several other fine folks, I set up the Insulation Resist and AC Withstand stand tests and started making pre-production units.

    I do not know the tests that well, and only have UL, IEEE, and other standards to go by. My question is this;

    Why would my motor pass the Megger and impedance test, but fail the AC Withstand test? Wouldn't the megger and impedance tests show the fault?

    Thank you in advance for any input you might have!
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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  3. PackratKing

    Well-Known Member

    Jul 13, 2008
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    They both have their uses, tho' where the rubber meets the road, I come out in favor of a tightly calibrated Hi-Pot...

    I repaired consumer-level power tools, a.w.a all types of electric motor service, as one of my duties in an HVAC bench-technician job the 12 years before I retired.
    OSHA and FedGov Consumer Safety protocols, demand several safety tests prior to releasing a power tool back to its owner.

    Contractors in general are notorious for beating the daylights out of their power tools. Lets not go where what consumers do to them...:D

    The Hi-Pot, as a final judge, often revealed electrical faults that I am convinced kept us out of a truckload of liability snafus.
     
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  4. gerty

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    Aug 30, 2007
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    I used to work in maintenance for a major air conditioning manufacturer, they used a hipot on every unit. They used different voltages ,depending on the rated voltage of the unit.
    We would, on occasion, borrow on of the hipots to check a cable pull that we made. We also used them from time to time to check a motor that would blow fuses for no apparent reason.
     
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  5. metaltomato

    Thread Starter New Member

    Dec 14, 2012
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    thanks guys. because Im designing a production cell, I want to use tests that would be needed to certify.

    Im meggering 500v with a lo-set of 20 megohms and hipotting according to ul (2x DUT rating + 1KvAC); So my hipot is set to 1.8KvAC with a hi-set of .500 milliamps for 1 second.

    My problem is lack of experience pure and simple. Ive started having better convos with guys who know what they are talking about as Ive started doing more and more testing.

    I just figured that anything that would trip the hipot would have also failed the megger.

    This leads me to another issue. Because I will hipot every unit that comes off the line, how much hipot is too much? We are now thinking because we cant find the root failure (passed megger, failed hipot), maybe we should do a low voltage hipot in the middle of the line, say before trickling, but we are afraid from the stuff we've read that the hipot is a destructive test.
     
    Last edited: Mar 5, 2013
  6. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Standard (min.) insulation values throughout industry are 0.500 Meg-Ohms.
    This can be verified by a .001 ampere reading on your Megger @ 500 volts.
    Make sure your Megger output is 500 volts! Disconnect all secondary wiring (after transformer) before using your Hi-pot. 2 x DUT rating + 1000 Volts is standard for 1 sec read (to chassis or Gnd screw.)

    Cheers, DPW [Everything has limitations...and I hate limitations.]
     
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  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    As for "why did it pass the megger but not the hipot test?" I would assume that there is maybe pinhole faults that cannot be measured at 500v, but can be measured at 1800v. If your megger goes up to 1800. V, I bet you get the same fault. Does your megger go that high? Can you try it?
     
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  8. metaltomato

    Thread Starter New Member

    Dec 14, 2012
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    My megger only goes up to 1kv, however, sure enough, the fault was detected.

    My presumption has been that the megger is used more frequently because it is not a destructive test. Now some of the questions that have been nagging at me come back up like; How destructive is the megger compared to the ac hipot (at the values I have suggested)?

    Why megger when you can hipot?

    FYI; Link here to see the same question posed on an Engineer Forum. This thread has links to a spec sheet on the unit in question from my company and a link to a drawing of the unit; http://www.eng-tips.com/viewthread.cfm?qid=340268
     
    Last edited: Mar 6, 2013
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  9. gerty

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    Aug 30, 2007
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    In my somewhat limited experience, I have seen the megger in the field and hipot in the production arena. I don't think I've ever seen someone use both on the same device.
    I've been on a bunch of construction jobs that test the cabling and they always use a megger.
     
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  10. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Packratking, and also a sort of " Rube Goldberg " builder of this and that... My Hi-Pot device, is handy to judge the dielectric strength - say - a shrinktube boot on the contact end of a switch..
    Once at the aforementioned job, one of our pet teenagers was going to use plumbers' teflon tape to insulate some wiring...:rolleyes:... The Hi-pot blew that idea out of the water in short order... :D
     
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  11. gerty

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    One of my students drove his friends car to school to work on the stereo. All the splices were insulated with Band Aids :eek:
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Black tape is sticky, Band Aids are sticky. They must be the same thing!
     
  13. strantor

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    Oct 3, 2010
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    Good, now you know ;)
    The difference between megger an hipot tests is a lot more complicated than it would seem on the surface. I skimmed the replies to your post on the other site, and they seem to be saying the same thing in more words. I cannot explain the differences because I don't understand all of it, but what I can tell you is that the hipot test is a semi-destructive "one time only" test. A proof test. Like when a knife manufacturer subjects their blades to torture tests before shipping, just to prove that they won't break. You wouldn't ever do those mean things to your own knife. After years of service, your knife would probably break if subjected to those same tests again. Same with the hipot. Once to prove your product isn't crap, and then not again. The megger is a gentler test. Think of the megger as gently applying side pressure to the knife to see how much it deflects, and think of the hipot as hammering the knife against a rock with a hammer, trying to make it break. Megger tests are performed in field, hipot tests aren't (that I've seen).
     
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