Many DC high potential machines are
halfwave rectifiers. This type of hipot tester
must not be used to test vacuum interrupters.
The capacitance of the Power/Vac bottles is
very low and the leakage in the rectifier and
its DC voltage measuring equipment is such
that the pulse from the halfwave rectifier
may be in the neighborhood of 120kV when
the meter is actually reading 40kV. In this
case, some perfectly good bottles can show
a relatively high leakage current since it is
the peak voltage of 120kV that is producing
erroneous bottle leakage current. In addition,
abnormal X-radiation will be produced.
!
CAUTION
Although an AC high potential test is
recommended for checking the vacuum
integrity, a DC high potential test can also
be conducted on the vacuum interrupter.
If a DC hipot test is to be used, test voltage
should not exceed 50kVDC, and should
be applied for 10 seconds. Interrupters
must be cleaned as previously discussed.
If failure occurs, reverse the polarity and
retest. If the interrupter passes the second
test, STOP. The interrupter is acceptable.
If the interrupter fails the second test, it is
recommended a final test with an AC hipot
before replacement.
No attempt should be made to try to
compare the condition of one vacuum
interrupter with another nor to correlate
the condition of any interrupter to low
values of DC leakage current. There is
no significant correlation. After the high
potential voltage is removed, discharge any
electrical charge that may be retained.
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24
Electrical Checks
Power/Vac® Electrical Ground and Test Devices
Equipped with ML-17 Mechansim
01.4IB.50100
