Recently, there was a discussion in the IEEE e-mail safety forum about the relationship between the hi-pot test current, leakage current, and insulation resistance.
In particular, the question was whether the hi-pot test and the insulation resistance test could be combined into a single measurement. Let’s discuss each of these parameters as circuit parameters and as safety parameters.
Insulation resistance is the resistance of an insulation. Insulations do not have an infinite resistance. They appear infinite because ordinary ohmmeters do not have sufficient range to measure values in the gigaohm and teraohm regions, which are typical resistance ranges for insulation.
An ohmmeter is simply a dc voltage source, a precision resistor, and a current meter. Ohmmeters operate by measuring the current through the series circuit of the precision resistor and the resistor under test. They use a small dc voltage, about 1 volt, to provide the current.
Here’s an experiment: Connect the terminals of an ohmmeter and a dc voltmeter together. The ohmmeter will measure the voltmeter input resistance, and the voltmeter will measure the ohmmeter voltage. The voltmeter input resistance will be about 10 megohms, and the ohmmeter voltage will be about 1 volt dc.
To measure insulation resistance, the ohmmeter voltage must be much higher than 1 volt in order to get enough current for an indication. Typical voltage is 500. Some insulation resistance meters have an operator-selectable voltage, from 100 volts to several thousand volts.
Some safety standards require a measurement of insulation resistance. This is usually a type test, not a production-line test. Nevertheless, some manufacturers have an interest in measuring insulation resistance on the production line.
Note that a dc hi-pot tester uses high voltage and may be provided with a dc current meter. If the dc voltage is stable, then the current meter can be calibrated in ohms to read insulation resistance. Easy. Some commercial hi-pot testers include an insulation resistance function.
Leakage current is the sum of all ac currents from mains conductors to ground through these resistances and impedances: the insulation resistance, the capacitive reactance across the insulation resistance, the capacitive reactance (impedance) of the Y capacitors.
Insulation resistance exists in ALL components between the mains circuits and the protective ground circuit. These 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.
For the purposes of this discussion, assume the the power input is 250 volts, 60 Hertz. If we assume the resistance across the insulation in the mains circuit is 1 gigaohm, then the leakage current due to insulation resistance is about 0.25 microamperes.
If we assume the capacitance across the insulation within a mains circuit is 100 pF, then the leakage current due to capacitive reactance across the insulation is about 10 microamperes.
If we assume the Y capacitor is 0.05 microfarads, then the leakage current due to capacitive reactance across the Y capacitor is about 5000 microamperes.
Insulation resistance: 0.25 microamperes
Capacitive reactance: 10.0 microamperes
Y capacitors: 5,000.0 microamperes
This demonstrates that the leakage current due to the insulation resistance is negligible compared to the other sources of leakage current. The insulation resistance cannot be determined from a measurement of leakage current.
Hi-Pot (Dielectric Strength) Test
The dielectric strength (hi-pot) test 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 capacitive reactance of the Y capacitors, the capacitive reactance of the stray insulation, and the insulation resistance. (Indeed, some people use this current to determine that a product is indeed connected to the hi-pot tester; other people use this current to additionally determine that the capacitors are of the approximately correct value.) Because the insulation resistance and reactance of the stray capacitance is so high, the ac test current can be simplified to the leakage current at 250 V times the ratio of hi-pot test voltage to 250 V. If the test voltage is 3000, then the test current would be 3000/250 x 0.5 or 6 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 stray capacitance and the Y capacitors. The dc current is typically in the tens of microamps.
AC cannot be used 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 to combine the two tests, insulation resistance and dielectric strength, into one test is to test with dc. One of my colleagues insists that hi-pot tests should be dc. One of the problems with dc is that if the unit under test is not connected to the hi-pot tester, the tester will nevertheless indicate a pass. My colleague uses a programmable ac/dc hi-pot tester to (1) determine that a unit under test is truly connected to the hi-pot tester and (2) conduct a dc hi-pot test. He programs the first step of the hi-pot tester sequence for 250 V, 60 Hz. The tester measures the “leakage” current. If the current is between two pre-selected values, then the tester moves to the next step which is to apply the prescribed dc hi-pot voltage. (The dc current is proportional to the insulation resistance.) In this way, he is assured that the unit under test is truly connected to the hi-pot tester.