Extech Instruments 7470 Operation Manual page 15

Another advantage of AC testing is that since AC voltage cannot charge a load there is
no need to discharge the item under test after the test.
AC testing disadvantages
One disadvantage of AC testing surfaces when testing capacitive products. Again,
since AC cannot charge the item under test, reactive current is constantly flowing. In
many cases, the reactive component of the current can be much greater than the real
component due to actual leakage. 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
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