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
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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]
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