Advantages and Disadvantages of use DC Voltage for
Hipot Test
One of the advantages of using a dc test voltage is that the leakage current trip can be set
to a much lower value than that of an ac test voltage. This would allow a manufacturer to
filter those products that have marginal insulation, which would have been passed by an ac
tester.
When using a dc hipot tester, the capacitors in the circuit could be highly charged and,
therefore, a safe-discharge device or setup is needed. However, it is a good practice to
always ensure that a product is discharged, regardless of the test voltage or its nature,
before it is handled.
It applies the voltage gradually. By monitoring the current flow as voltages increase, an
operator can detect a potential insulation breakdown before it occurs. A minor disadvantage
of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost
of a dc tester may be slightly higher than that of an ac tester.
The main advantage of the dc test is DC Voltage does not produce harmful discharge as
readily occur in AC.
It can be applied at higher levels without risk or injuring good insulation. This higher
potential can literally “sweep-out” far more local defects.
The simple series circuit path of a local defect is more easily carbonized or reduced in
resistance by the dc leakage current than by ac, and the lower the fault path resistance
becomes, the more the leakage current increased, thus producing a “snow balling” effect
which leads to the small visible dielectric puncture usually observed. Since the dc is free of
capacitive division, it is more effective in picking out mechanical damage as well as
inclusions or areas in the dielectric which have lower resistance.
Advantages and Disadvantages of use AC Voltage for
Hipot Test
One of the advantages of an ac hipot test is that it can check both voltage polarities,
whereas a dc test charges the insulation in only one polarity. This may become a concern
for products that actually use ac voltage for their normal operation. The test setup and
procedures are identical for both ac and dc hipot tests.
A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of
Y capacitors, then, depending on the current trip setting of the hipot tester, the ac tester
could indicate a failure. Most safety standards allow the user to disconnect the Y capacitors
prior to testing or, alternatively, to use a dc hipot tester.
The dc hipot tester would not indicate the failure of a unit even with high Y capacitors
because the Y capacitors see the voltage but don’t pass any current.