15
ground of the test machine or by applying a grounded cable to
the phase bus bars.
If a failure of a vacuum bottle should incur during the integrity
test, the test procedure should be reviewed and the pole
piece cleaned. Failure rates for vacuum bottles is 0.0007 per
field unit.
Note the voltage level at failure on the first test; then perform
a retest on the phase pole piece. If the pole piece passes the
retest, the vacuum bottle is acceptable - STOP
STOPSTOP
STOP. If the test fails
again but at a higher voltage level than was observed in the
first test. Clean the pole piece and retest. If a failure of the
integrity test occurs a third time, consider the vacuum bottle
to have lost vacuum and replace the complete pole piece as
described under Repair of Interrupter Assembly.
CAUTION: MANY DC HIGH POTENTIAL MACHINES ARE
HALFWAVE RECTIFIERS. THIS TYPE OF HIPOT TESTER
MUST NOT
NOTNOT
NOT BE USED TO TEST VACUUM INTERRUPTERS.
THE CAPACITANCE OF THE POWERVAC 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 120 kV WHEN THE METER IS
ACTUALLY READING 40 kV. IN THIS CASE, SOME PER-
FECTLY GOOD BOTTLES CAN SHOW A RELATIVELY HIGH
LEAKAGE CURRENT SINCE IT IS THE PEAK VOLTAGE OF 120
kv THAT IS PRODUCING ERRONEOUS BOTTLE LEAKAGE
CURRENT. IN ADDITION, ABNORMAL X-RADIATION WILL
BE PRODUCED.
Although a AC hi-potential test is recommended for checking
the vacuum integrity, a DC hi-potential test can also be
conducted on the vacuum interrupters at 50 kV and held for 10
seconds with the restrictions noted as follows:
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.
Acceptable AC high potential machines are:
Hipotronics Model 7BT 60A
Hipotronics Model 60HVT
James G. Biddle Catalog 222060
Phoenix Model 660-10P
Acceptable DC high potential machines are:
Hipotronics Model 860PL
Hipotronics Model 880PL
9.5—Insulation Tests
The primary circuit insulation on the breaker may be checked
phase to phase and phase to ground using a 2500 Volt or
other suitable megohmeter.
Since definite limits cannot be given for satisfactory insulation
values, a record should be kept of the megohmeter readings
as well as temperature and humidity readings. This record
should be used to detect any weakening of the insulation from
one check period to the next. Generally, readings should equal
or exceed 10,000 megohms.
To measure the breaker secondary circuit insulation
resistance, disconnect the motor leads and thread a wire
connecting together all secondary disconnect pins except
#24, the ground pin. The measurement is made by
connecting a 500 Volt megohmeter from the wire to ground.
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