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:j...@woodjohn.uk]
*Sent:* Monday 19 February 2018 10:46
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*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 <j...@woodjohn.uk> <mailto:j...@woodjohn.uk>
    *Date: *Monday, 19 February 2018 at 4:22 PM
    *To: *Scott Xe <scott...@gmail.com> <mailto:scott...@gmail.com>,
    <EMC-PSTC@LISTSERV.IEEE.ORG> <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
    *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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