The IEC 62368-2 Rationale document (under 5.4.9) states::
"The dc voltage test with a test voltage equal to the peak value of the ac
voltage is not fully equivalent to the ac voltage test due to different
withstand characteristics of solid insulation for these types of voltages.
However in case of a pure dc voltage stress, the dc voltage test is
appropriate. To address this situation the dc test is made with both
polarities."
This might make sense for type testing where insulation is being qualified
especially when the test is conducted as verification that no damage to
insulation occurred as a result of a another test.
For routine testing, my opinion has always been that this is done to catch
damage to insulation that could occur during the manufacture/assembly of the
product. Wouldn't such damage likely be caught by a dc test voltage at either
polarity?
Also note, that reversing the tester leads (for a tester that cannot produce
negative dc voltage) on a Class 1 product with a metal enclosure will result in
increased risk of shock to the test operator especially if the product
enclosure is large. I think in most cases, the tester's return lead is bonded
to the tester's earth connection making the EUT's enclosure safe to touch even
during testing.
If routine testing is done to satisfy safety certification requirements, it
might be worth having the discussion with the certifier to see how they
interpret this requirement.
I haven't heard of any cases where a product was shipped with damaged
insulation because the dc hipot was only tested at positive polarity.
William Barry
(yes - the content contained in this email is of my own opinion and does not
represent in any way the opinion of any former, current, or future employer)
On Wednesday, August 5, 2015 7:24 AM, Charlie Blackham
<[email protected]> wrote:
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div.yiv7309338822WordSection1 {}#yiv7309338822 Rich >The standard is
incorrect in requiring tests of both polarity d.c. There is no physical
rationale for both polarities. Go on, I’ll biteJ What’s this requirement
doing in the standard then? Regards Charlie From: Richard Nute
[mailto:[email protected]]
Sent: 04 August 2015 19:09
To: [email protected]
Subject: Re: [PSES] Dual polarity DC electric strength test Hi Greg:
The standard is incorrect in requiring tests of both polarity d.c. There
is no physical rationale for both polarities. But, at this time, if you opt
for d.c. testing, you must test with both polarities. I would advise testing
with a.c. to avoid capital expense of a new d.c. tester. Best regards,
Rich -
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