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 Associates 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>
*Date: *Monday, 19 February 2018 at 4:22 PM
*To: *Scott Xe <scott...@gmail.com>, <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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