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2.27 HPAC - AC HI-POT
Where Used
Hi-Pot or EHT testing (to check for insulation breakdown between windings
or between windings and the screen or core) is often specified for power
line transformers or for switched mode power transformers in applications
where safety is important.
It is typically performed between all primary windings connected
together, and all secondary windings plus the screen connected together.
Specifying the Measurement
During the test, an ac voltage is applied across two groups of windings
with the windings in each group being shorted together.
The voltage and current are monitored throughout the dwell time; if
either the test voltage cannot be maintained, or the current is too large,
then a failure will be recorded.
In programming the AT you may select the voltage (from 100V to 5kVrms),
the frequency (50Hz / 60Hz at the full voltage, or up to 1kHz at reduced
voltage), the current trip level (300μA to 30mApeak), and the ramp up and
dwell times, all to suit the specification of the transformer under test.
Many transformer specifications require Hi-Pot testing to be carried out
with a dwell time of 60 seconds. Although the transformer must be designed
and constructed to meet this, it is common practice to reduce the dwell
time for production testing.
During a Hi-Pot test, the Wattage is continuously monitored and adjusted
to provide the correct output. If the programmed output voltage cannot be
maintained, the tester will automatically display FAIL, illuminate the red
indicator, return a FAIL to the server (if used) and sound a warning
buzzer (if enabled). T