INTRODUCTION
10
current reading. This means that unless the product is sensitive to a sudden application of
voltage the operator can immediately apply full voltage and read current without any wait
time.
3. 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
1. A key 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.
2. 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
1. 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.
2. The other advantage to DC testing is that since the charging current only needs to be
applied momentarily 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
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.
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