EEC SE 7430, Руководство пользователя, Страница: 17 / 106

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This can make it very difficult to detect products that have excessively high leakage current.
Another disadvantage of AC testing is that the hipot has to have the capability of supplying
reactive and leakage current continuously. This may require a current output that is actually much
higher than is really required to monitor leakage current and in most cases is usually much higher
than would be needed with DC testing. This can present increased safety risks as operators are
exposed to higher currents.
DC testing characteristics
During DC hipot testing the item under test is charged. The same test item capacitance that causes
reactive current in AC testing results in initial charging current which exponentially drops to zero in
DC testing.
DC testing advantages
Once the item under test is fully charged, the only current flowing is true leakage current. This
allows a DC hipot tester to clearly display only the true leakage of the product under test.
Another advantage to DC testing is that the charging current only needs to be applied
momentarily. This means that the output power requirements of the DC hipot tester can typically
be much less than what would be required in an AC tester to test the same product.
DC testing disadvantages
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.
Since a DC hipot does charge the item under test, it becomes necessary to discharge the item after
the test.
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.
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.