Mike,
How does the maximum leakage or touch current for the product fit
into all of this? ITE has a 3.5 mA maximum requirement which is measured
during the evaluation. This measurement should be looking at the system
parasitic as well as designed capacitance in the system. The hi-pot tester
shouldn't be able to generate more leakage current than the system has
designed into it. It is only a gross check of the primary to ground
insulation system and pretty much of a digital thing, it either holds or
doesn't
Gary
-----Original Message-----
From: mike harris [mailto:[email protected]]
Sent: Tuesday, September 26, 2000 10:21 PM
To: Ron Pickard; [email protected]
Cc: [email protected]
Subject: Re: Verifying functionality of the equipment for Production
Safety Testing
Hi All,
This exchange prompts me to mention a recurring theme that began over 30
years ago, when I transitioned from MIL-SPEC hipot testing, where hipot
leakage was specified, to UL hipot where leakage was not specified. At that
time I was incredulous that UL did not specify what seemed to me critical
factor, but it was explained that with a 500VA hipot, breakdown would be
obvious.
As hipots got smaller, UL said that less than 500 VA could be used if there
was a voltmeter at the output to assure voltage was applied for the full
duration, and a reliable means to indicate breakdown. In the early days that
means was often a current-sensing relay to detect some level of excessive
leakage.
Hipots got progressively smarter, using microprocessors and circuits to
distinguish non-linear changes in leakage, but also circuits to distinguish
between capacitive leakage (typically from mains capacitors in switching
supplies) and arc-over currents. Some units have capacitive leakage of over
20mA, so a hipot with lower current capabilities will indicate "failure".
Testing with a more powerful AC hipot, or with a DC hipot, might allow the
same unit to pass.
My concern is that UL (and the similar agencies) still does not specify a
maximum leakage, so any effort to use resistors for calibrating a hipot
appears to me to be setting a leakage standard that may have more to do with
the hipot current capability than its arc-over breakdown detection
capabilities. Resistors are convenient, no doubt, but I suggest that the
true measure of the hipot's ability to detect breakdown is either
a) a controlled spark gap
or
b) an avalance current detector which looks for an exponential rate of rise
of current rather than just the level of current.
This topic continues to be a source of debate within UL as recently as last
week.
Comments?
Mike Harris/Teccom
-----Original Message-----
From: Ron Pickard <[email protected]>
To: [email protected] <[email protected]>
Cc: [email protected] <[email protected]>
List-Post: [email protected]
Date: Tuesday, September 26, 2000 12:11 PM
Subject: Re: Verifying functionality of the equipment for Production Safety
Testing
>
>
>Hi Paul,
>
>First, I would caution you from holding the hipot test lead while the
tester is in operation. It
>could certainly be hazardous to your health, and all that. I wonder if this
is one of those "don't
>try this at home" cases.
>
>Anyway, in another life while undergoing an initial BABT 340 quality system
audit, the auditor asked
>us "How do we know that the tester is working properly, and how do you know
the tester will give a
>failure indication when expected?" Well, these were questions we had not
anticipated and were
>unprepared with a quick answer. To meet the auditor's challenge, however,
we fashioned a setup that
>verified the complete tester, including cables. This verification setup
included 2 resistors (one
>for hipot and the other for ground continuity). The values for each
resistor were selected so that
>the testers would indicate a failure at just over the current trip point of
each tester. To verify
>the setup each day, the testers were tested using these resistors (a
failure indication was a pass
>and a pass indication was a failure). The resistors and the test equipment
were all placed on a
>routine calibration cycle. This satisfied BABT and other safety agencies
that performed the
>facility/product audits.
>
>I hope this helps.
>
>Best regards,
>
>Ron Pickard
>[email protected]
>
>
>
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