The problem:
Some standards or companies rely on AC hipot testing over DC hipot
testing. This can cause a problem if there are motors, capacitors, or
other components with a connection between live conductors and ground.
Depending on the capacitance of the connection, leakage current can be
developed during the hipot test that can possibly overcome the
capabilities of the hipot tester, causing a false failure indication.
This is due to the AC waveform, which goes from a positive peak
voltage to a negative peak voltage and back again 60 times a second.
This change in voltage causes the capacitance to charge, discharge and
charge again for each peak value. This charging takes current, and it is
called leakage current. The current is developed by the hipot tester,
and if the leakage current is too large, the hipot tester may be asked
to develop current which is more than it is set to deliver. The hipot
tester interprets this leakage current as a failure, stops the test, and
lights the FAIL light. However, there may not be anything wrong with
the EUT.
Determining the leakage current of the EUT:
You can determine the expected leakage current of the EUT by
measuring the capacitance and applying a formula as noted below. This
will give you a rough idea of whether the hipot tester you are using is
capable of performing the test.
Measure the capacitance: You are only concerned with the primary
capacitance to ground, so you may be able to make this determination
from the AC power plug of the EUT. Make sure all primary switches are
closed, and short the hot and neutral wires together. Using a DMM,
measure the capacitance between the hot and neutral wires shorted
together, and the ground of the EUT. (After you obtain the measurement,
be sure to remove the short.)
Using the formula I = 377VC, find the leakage current I (in amps) by
multiplying the voltage that your hipot test is conducted at (V) by the
capacitance you measured between line and ground (C), and multiplying
that product by 377. This will give you the anticipated leakage current I
(in amps).
Check the specifications of the hipot tester you are using to see if
it can deliver this current. If not, you will probably need to find a
higher capacity tester. If so, the suggestions in the next section may
help.
The solution:
The leakage current limit of the tester may not be set to the maximum. You may be able to increase the leakage limit trip point.
The ramp time may be set too fast. The problem is magnified during
the ramping portion of the test, where the hipot tester is raising the
voltage from 0 to the testing voltage. Try slowing down this ramp time
to see if that reduces or eliminates the false failures.