The Hipot testers used on our ATE have internal GFI detection, which is implemented by measuring the source HV current and the return current on the ground lead. When the UUT is also connected to another ground during testing, some of the return current will flow through system ground and return to the Hipot unit via AC ground and not return ground. This causes the unit to display a GFI fault. Isolating the device being tested from any other grounds besides the Hipot return ground corrects this problem and allows the GFI protection to work properly.

This is designed to protect any human operators that may have come in contact with the HV test voltage and are connect to a ground. This is similar to an AC GFI protection device where the AC hot current and the AC neutral current must be balanced (equal) or the device will trip. 

Note that GFI protection will never protect someone getting across hot and neutral or the Hipot source and return. 

Blair

On Tue, 11 Jun 2013 11:23:19 -0700, Ahne Oosterhof <[log in to unmask]> wrote:

>Sounds like you did not eliminate the problem but made it so you did not detect it anymore.
>Where did that current come from (or go to) to make the GFI activate?
>I would expect that a good Hipot tester has a well isolated and shielded line input to secondary circuitry, so the tester does not contribute to measured leakage current.
>
>Ahne.
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg
>Sent: 08 June, 2013 04:51
>To: [log in to unmask]
>Subject: Re: [TN] Hipot Testing
>
>Does the Hipot tester give a GFI fault? If so, I think what that means is that some of the current flowing out from the test lead goes somewhere else, and doesn't come back through the return lead. The device you are testing needs to be isolated from earth / building / system ground during the test. I had a similar problem on an automated tester that performed a hipot test, current would leak through system ground and a GFI fault on the tester would trip. Once I isolated the product under test from system ground during hipot testing it went away.
>
>Blair
>
>On Fri, 7 Jun 2013 11:16:40 -0400, Michael Strong <[log in to unmask]> wrote:
>
>>Wondering if I can pick the brain of somebody out there.  When 
>>performing Dielectric strength testing most of my failures are the 
>>result of a GFI.  I can turn this feature off on my instrument and was 
>>wondering if most people who do this testing do the same.  I know this 
>>is a safety feature but I'm trying to find a more standardized way of 
>>doing this test.  What are the likely consequences of turning this 
>>feature off?  Any advice would be appreciated.
>>
>>
>>
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