Hi John:
> > My last (3) employers have required all repaired or modified units to
be
> > "hi-potted". If a unit has been repaired, then the cover was removed,
and
> > the unit is no longer "controlled" by the oroginal production hi-pot.
> >
>
> I think this is too stringent. Repeated hi-pot tests must be
> *minimised*, because of the possibility of progressive degradation of
> insulation.
Yes, indeed, repeated hi-pot tests must be minimized.
However, significant degradation of modern insulations
at these low voltages and durations is doubtful for the
lifetime of the equipment.
Some years ago, I undertook a test to determine when
an insulation would fail if subjected to a continuous
hi-pot voltage. I connected several units to the hi-pot
voltage for 8 hours/day. The units started failing
after about 10 days. That would be about 48,000 minutes.
So, we can say that we should probably not exceed 1/1000th
of 48,000 minutes, 48 minutes, of hi-pot test time for
the equipment lifetime. That would be 48 1-minute hi-pot
tests. I don't believe any equipment would be so tested.
Now, the transients are only 50 microseconds. 1.2 million
transients would comprise one minute of degradation. As
I recall, an industrial site would incur 5-10 such
transients per day. That means, about 120,000 days for
an accumulation of 1 minute of overvoltage. That's over
300 years.
While repeated hi-pot tests must be minimized, the
degradation due to repeated hi-pot tests is not likely
to have an effect for the lifetime of the equipment.
Best regards,
Rich
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