I think that wood was discontinued a very long time ago. But other
materials were certainly hygroscopic. Military-grade ebonite, used for
mechanical applications, was polished with tin foil at one time, which
rather impaired its insulating qualities. Also, creepage and surface
contamination were often not taken into account in design, leading to
surface tracking followed by smoke and flame.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2018-02-19 10:02, ce-test, qualified testing bv, Gert Gremmen wrote:
As far as I understand, this requirement was needed in the time that
insulation materials
might have hygroscopic properties. Insulation materials as wood were
common in the past.
If they were not suitably treated, in humid circumstances insulation
might drop to unacceptable levels.
Gert Gremmen
ce-test qualified testing
((partially) retired in France)
*From:*John Woodgate [mailto:[email protected]]
*Sent:* Monday 19 February 2018 10:46
*To:* [email protected]
*Subject:* Re: [PSES] Insulation resistance test
The flash test is necessary because there are high-voltage pulses on
the electricity supply caused by switching operations and distant
lightning. It is very likely that a product that fails the insulation
resistance test would fail the high-voltage test as well. But it is
not inevitable; a 1.8 megohm insulation resistance passes 1.67 mA at 3
kV.
As I said, it would be most unusual for a non-faulty modern product to
show an insulation resistance as low as 1.8 megohms (if discharge
resistors are disconnected).
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk <http://www.woodjohn.uk>
Rayleigh, Essex UK
On 2018-02-19 09:30, Scott Xe wrote:
John,
I almost forgot the Megger gear that I tried it once in the lab
when I studied radio engineering. Thereafter I am using
battery-operated tester and now more and more multi-purpose
testers including it into one unit.
The main reason why I raised this query is that the flash test
gives harsher test on safety strength than insulation resistance
test. The debate is in safety standards it still requires it but a
lot of young engineers consider waste time if flash test is
included. Why do we focus on flash test? Probably, it is a very
old test and still remain in many safety standards.
Regards,
Scott
*From: *John Woodgate <[email protected]> <mailto:[email protected]>
*Date: *Monday, 19 February 2018 at 4:22 PM
*To: *Scott Xe <[email protected]> <mailto:[email protected]>,
<[email protected]> <mailto:[email protected]>
*Subject: *Re: [PSES] Insulation resistance test
The test is very old, and pre-dates the high-voltage test by very
many years. You mentioned 'Megger' in your other post, yes, that
was how it was done. (A Megger (brand name) was/is a type of
magneto with an ohmmeter attached. The stable output voltage
required for the ohmmeter is achieved by a mechanical governor,
which limits the armature speed however fast you turn the handle.)
The test might not be totally irrelevant for modern electronic
equipment, but the requirements in most standards are certainly in
need of revision. These requirements are for a minimum insulation
resistance of 1 or 2 megohms. A modern piece of electronic
equipment typically has a resistance of hundreds of megohms
(unless condensation occurs), so a measured value of a few
megohms shows that something is in fact seriously wrong. I have
raised this point in TC108 before, but no-one was willing to take
action. Tradition, you know.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk <http://www.woodjohn.uk>
Rayleigh, Essex UK
On 2018-02-19 05:45, Scott Xe wrote:
Insulation resistance test is one of most common safety tests
nowadays: Flash test, earth continuity test, leakage current
test and insulation resistance test. Can someone share the
history of this teat to use DC and 500 V. The products are
working on AC and test voltage is higher than normal operating
voltage but much lower than the flash test. It is a trend to
skip this test on production line. What are the distinct
benefits for this test?
Thanks and regards,
Scott
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