Hi Rich,
Sincerely thanks for your detailed explanation that gives me a good lesson.
Combining dielectric strength test and insulation resistance measurement
seems won't work in reality and gives misleading results. It is because of
the following:-
Firstly, dielectric strength test requires to be conduced in AC, not DC.
Secondly, the "FAIL" indicator on with trip current setting at 0.75mA
does
not mean any failure in dielectric strength or insulation resistance since
the permissible current is 6.25mA or 5.9mA, not 0.75mA derived from
insulation resistance requirement.
Regarding insulation resistance measurement, I notice that some latest
safety standards have been removed this requirement, i.e., EN
60335-1:1994/A11:1995. I belive that matches your explanation.
Best regards,
Raymond
================================================================
At 10:13 AM 15/10/97 -0700, you wrote:
>
>
>Hi Raymond:
>
>
>You ask about the relationship of three parameters:
>
> Insulation resistance
> Leakage current
> Dielectric strength current
>
>1. Insulation resistance is a dc parameter, not an ac
> parameter. (If it were an ac parameter it would be
> "insulation impedance.")
>
> The only difference between an "ordinary" resistance
> and an "insulation" resistance is the range of values
> of resistance. Insulation resistances are in the tens
> of megohms and greater, typically gigaohms. Because
> the resistance is so high, the ohmmeter needs to impress
> a high voltage across the resistance in order to obtain
> a reading. Typically, insulation resistance meters use
> 500 V dc. Some insulation resistance meters have an
> operator-selectable voltage ranging from hundreds of
> volts to thousands of volts.
>
> An ohmmeter (and insulation reistance meter) is simply
> a voltage source, a precision resistor, and a current
> meter. The current meter is calibrated in ohms, since
> the current is proportional to the resistance being
> measured.
>
> For this reason, a dc hi-pot tester can be configured
> to also operate as an insulation resistance meter. Both
> Quadtech and Associated Research make dielectric strength
> testers that include insulation resistance.
>
>2. Leakage current is an ac parameter since the source of
> leakage current is the ac mains circuit.
>
> The principal current path of leakage current is through
> the Y capacitors of the EMC filter.
>
> Leakage current is proportional to:
>
> the capacitive reactance (impedance) of the Y capacitors,
> the value of the mains voltage, and
> the frequency of the mains voltage.
>
> Another current path is the insulation resistance between
> the mains circuits and the protective ground circuit. The
> insulations include the wire insulation of the mains cord,
> the solid insulations of appliance couplers, fuseholders,
> switches, circuit boards, and transformers. Also included
> is the insulation resistance of the Y capacitors.
>
> The capacitive reactance of the Y capacitors is very low
> compared to the insulation resistance. For example, assume
> the leakage current is 0.5 mA rms (0.7 mA peak) at 240 V
> input, 50 Hz. If the insulation resistance was as low as
> 4 megohms, the resulting leakage current would be 0.080 mA.
> This would mean that the leakage current due to the Y
> capacitors would be 0.42 mA, and that the capacitive
> reactance would be 570 kilohms. This is very much less
> than the 4 megohms minimum insulation resistance required
> by IEC 60065.
>
> Modern insulations have insulation resistances on the order
> of 1 gigaohm. For all practical purposes, the leakage
> current through the insulation resistance can be ignored.
>
>3. Dielectric strength is a test of the electric strength of
> one or more insulations. The electric strength of an
> insulation is proportional to the distance through the
> insulating medium, whether solid insulation or gaseous
> insulation (i.e., air).
>
> Electric strength can be tested with either ac or dc.
>
> If the test is ac, then the current during the test is
> a function of the value of the Y capacitors and of the
> insulation resistance. (Indeed, some people use this
> fact to determine that a product is indeed connected to
> the hi-pot tester; some people use this fact to
> additionally determine that the capacitors are of the
> approximately correct value.) The ac current is simply
> the leakage current at 240 V times the ratio of hi-pot
> test voltage to 240 V. If the test voltage is 3000,
> then the test current would be 3000/240 x 0.5 or 6.25 mA.
>
> If the test is dc, then the current during the test is
> a function of the insulation resistance of the system,
> including the insulation resistance of the Y capacitors.
> The dc current is typically in the tens of microamps.
>
>The answer you ask for is: you cannot us ac for an insulation
>resistance test.
>
>Even if the product has no Y capacitors, there is still a lot
>of capacitance that exists across every insulation. The total
>capacitive reactance will be very much less than the insulation
>resistance. Consequently, ac cannot be used for measuring
>insulation resistance.
>
>The only way you can combine the two tests, insulation
>resistance and dielectric strength, into one test is to test
>with dc.
>
>This raises the question why EN 60065 and BEAB would require
>an insulation resistance measurement since the parallel
>capacitive reactance is very much less than the insulation
>resistance. The capacitive reactance is the principal
>contributor to leakage current and therefore to the safety
>of the product.
>
>
>Best regards,
>Rich
>
>
>
>-------------------------------------------------------------
> Richard Nute Quality Department
> Hewlett-Packard Company Product Regulations Group
> San Diego Division (SDD) Tel : 619 655 3329
> 16399 West Bernardo Drive FAX : 619 655 4979
> San Diego, California 92127 e-mail: [email protected]
>-------------------------------------------------------------
>
>
>
>
>
>
------------------------------------------------------
Raymond Li
Dixons Asia Ltd.
Ph: (852)2542 5303 Fax: (852)2541 9067
