I.B. 3A74792H04
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6-4 VACUUM INTERRUPTER INTEGRITY TEST
Vacuum interrupters used in Type VCP-W 38kV Vac-
uum Circuit Breakers are highly reliable interrupting ele-
ments. Satisfactory performance of these devices is
dependent upon the integrity of the vacuum in the inter-
rupter and the internal dielectric strength. Both of these
parameters can be readily checked by a one minute ac
high potential test. Refer to Table 6.2 for the appropriate
test voltage. During this test, the following warning must
be observed:
APPLYING ABNORMALLY HIGH VOLTAGE ACROSS
A PAIR OF CONTACTS IN VACUUM MAY PRODUCE
X-RADIATION. THE RADIATION MAY INCREASE
WITH THE INCREASE IN VOLTAGE AND/OR DE-
CREASE IN CONTACT SPACING. X-RADIATION
PRODUCED DURING THIS TEST WITH RECOM-
MENDED VOLTAGE AND NORMAL CONTACT
SPACING IS EXTREMELY LOW AND WELL BELOW
MAXIMUM PERMITTED BY STANDARDS. HOWEV-
ER, AS A PRECAUTIONARY MEASURE AGAINST
POSSIBILITY OF APPLICATION OF HIGHER THAN
RECOMMENDED VOLTAGE AND/OR BELOW NOR-
MAL CONTACT SPACING, IT IS RECOMMENDED
THAT ALL OPERATING PERSONNEL STAND AT
LEAST FOUR METERS AWAY IN FRONT OF THE
CIRCUIT BREAKER.
With the circuit breaker open and sitting solidly on the
floor or secured in a fixed position, connect all top pri-
mary studs (bars) together and to the high potential
machine lead. Connect all bottom studs together and
ground them along with the circuit breaker frame. Start
the machine at zero potential, increase to appropriate
test voltage and maintain for one minute.
A successful withstand indicates that all interrupters
have a satisfactory vacuum level. If there is a break-
down, the defective interrupter or interrupters should be
identified by an individual test and replaced before plac-
ing the circuit breaker in service.
After the high potential is removed, discharge any elec-
trical charge that may be retained. To avoid any ambi-
guity in the ac high potential test due to leakage or dis-
placement (capacitive) current, the test unit should have
sufficient volt-ampere capacity. It is recommended that
the equipment be capable of delivering 25 milliamperes
for one minute.
Although an ac high potential test is recommended, a dc
test may be performed if only a dc test unit is available.
In this case the equipment must be capable of delivering
5 milliamperes for one minute to avoid ambiguity due to
field emission or leakage currents and the test voltage
shall be as shown in Table 6.2.
The current delivery capability of 25 ma ac and 5 ma dc
apply when all three VIs are tested in parallel. If individ-
ual VIs are tested, current capability may be one third of
these values.
SOME DC HIGH POTENTIAL UNITS, OPERATING AS
UNFILTERED HALF-WAVE RECTIFIERS, ARE NOT
SUITABLE FOR USE TO TEST VACUUM INTER-
RUPTERS BECAUSE THE PEAK VOLTAGE AP-
PEARING ACROSS THE INTERRUPTERS CAN BE
SUBSTANTIALLY GREATER THAN THE VALUE
READ ON THE METER.
6-5 CONTACT EROSION
In order to determine if the contacts have eroded to the
extent that the interrupter must be replaced, close the
circuit breaker and observe the erosion mark on each
operating rod from the rear of the circuit breaker. If the
mark on the interrupter stem is measured to be more
than 12 inches from the base of the interrupter, the
interrupter is satisfactory. If the mark is 12 inches or
less, the interrupter assembly must be replaced (Figures
6-2 and 6-3).
6-6 CONTACT WIPE
The adequacy of contact wipe can be determined by a
simple measurement of each loading spring indicator
with the circuit breaker closed. Refer to Figures 6-4 and
6-5 and measure the indicator distance shown for each
interrupter. If the measurement taken is in the range of
0.91 to 1.22 inches, the contact wipe is adequate.
Vacuum Interrupter Integrity Test Voltage
ac 60 Hz
dc
38 kV rms
60 kV rms
85 kV
Circuit Breaker Rated
Maximum Voltage
Table 6.2 Test Voltage
!
WARNING
!
CAUTION
Summary of Contents for Cutler-Hammer VCP-W
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