10
typically be much less than what would be required in an AC tester to test the same product.
DC testing disadvantages
1. Unless the item being tested has virtually no capacitance, it is necessary to raise the voltage
gradually from zero to the full test voltage. The more capacitive the item the more slowly
the voltage must be raised. This is important since most DC hipots have failure shut off
circuitry which will indicate failure almost immediately if the total current reaches the
leakage threshold during the initial charging of the product under test.
2. Since a DC hipot does charge the item under test, it becomes necessary to discharge the
item after the test.
3. DC testing unlike AC testing only charges the insulation in one polarity. This becomes a
concern when testing products that will actually be used at AC voltages. This is an
important reason that some safety agencies do not accept DC testing as an alternative to AC.
4. When performing AC hipot tests the product under test is actually tested with peak voltages
that the hipot meter does not display. This is not the case with DC testing since a sinewave
is not generated when testing with direct current. In order to compensate for this most
safety agencies require that the equivalent DC test be performed at higher voltages than the
AC test. The multiplying factor is somewhat inconsistent between agencies which can
cause confusion concerning exactly what equivalent DC test voltage is appropriate.
1.5.2 Insulation Resistance Test
Some "dielectric analyzers today come with a built in insulation resistance tester. Typically,
the IR function provides test voltages from 500 to 1,000 volts DC and resistance ranges from
kilohms to gigaohms. This function allows manufacturers to comply with special
compliance regulations. BABT, TÜV and VDE are agencies that may under certain
conditions, require an IR test on the product before a Hipot test is performed. This typically
is not a production line test but a performance design test.
The insulation resistance test is very similar to the hipot test. Instead of the go/no go
indication that you get with a hipot test the IR test gives you an insulation value usually in
Megohms. Typically, the higher the insulation resistance value the better the condition of the
insulation. The connections to perform the IR test are the same as the hipot test. The
measured value represents the equivalent resistance of all the insulation which exists between
the two points and any component resistance which might also be connected between the two
points.
Although the IR test can be a predictor of insulation condition it does not replace the need to
perform a dielectric withstand test.