Instruction Book
Page 36
Effective: March 2007
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I.B
6513C80G
6-4 VACUUM INTERRUPTER INTEGRITY TEST
Vacuum interrupters used in Type DHP-VR Vacuum
Circuit Breaker Elements are highly reliable interrupting
elements. Satisfactory performance of these devices is
dependent upon the integrity of the vacuum in the
interrupter and the internal dielectric strength. Both of
these parameters can be readily checked by a one
minute ac high potential test. Refer to Table
6.1
for the
appropriate test voltage. During this test, the following
warning must be observed:
WARNING
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
DECREASE IN CONTACT SPACING. X-RADIATION
PRODUCED DURING THIS TEST WITH
RECOMMENDED VOLTAGE AND NORMAL CONTACT
SPACING IS EXTREMELY LOW AND WELL BELOW
MAXIMUM PERMITTED BY STANDARDS. HOWEVER,
AS A PRECAUTIONARY MEASURE AGAINST
POSSIBILITY OF APPLICATION OF HIGHER THAN
RECOMMENDED VOLTAGE AND/OR BELOW
NORMAL CONTACT SPACING, IT IS
RECOMMENDED THAT ALL OPERATING
PERSONNEL STAND AT LEAST FOUR METERS
AWAY IN FRONT OF THE BREAKER ELEMENT.
With the breaker element open, connect all top primary
studs (bars) together and the high potential machine
lead. Connect all bottom studs together and ground
them along with the 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 satisfactory vacuum level. If there is a breakdown,
the defective interrupter or interrupters should be
identified by an individual test and replaced before
placing the breaker in service.
WARNING
AFTER THE HIGH POTENTIAL IS REMOVED, AN
ELECTRICAL CHARGE MAY BE RETAINED BY THE
VACUUM INTERRUPTERS. FAILURE TO DISCHARGE
THIS RESIDUAL ELECTROSTATIC CHARGE COULD
RESULT IN AN ELECTRICAL SHOCK. ALL SIX
PRIMARY TERMINALS AND THE CENTER RING OF
EACH VACUUM INTERRUPTER OF THE CIRCUIT
BREAKER SHOULD BE GROUNDED TO REDUCE
THIS ELECTRICAL CHARGE BEFORE COMING IN
CONTACT WITH THE PRIMARY CIRCUIT.
To avoid any ambiguity in the ac high potential test due
to leakage or displacement (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. When a DC test voltage is
used, a high field emission current from a microscopic
sharp spot on one contact can be misinterpreted as a
sign of a vacuum interrupter filled with air. To avoid a
misinterpreted test result, the open interrupter should
always be subjected to both voltage polarities. That is,
apply the DC voltage:
-first, so that one contact of the interrupter is
the cathode, and
-second, so that the other contact of the
interrupter is the cathode.
A bad interrupter filled with air will have a similarly high
leakage current in both polarities. A good interrupter with
a good vacuum level may still have a high leakage
current, but this will generally be in only one polarity.
Such an interrupter usually has a tiny sharp spot on one
contact that produces a high field emission current when
the sharp spot is a cathode, but not on an anode. In
addition, such an interrupter would also normally with-
stand the required AC voltage which is the definitive test
of its vacuum integrity.
T a b l e 6 . 1
Vaccum Interrupter Integrity Test Voltage
Breaker Rated
Maximum Voltage
ac 60 HZ
dc
Up to and
including 15.0KV
27 KV
40KV
6-5 CONTACT EROSION AND WIPE
Since the contacts are contained inside the interrupter,
they remain clean and require no maintenance. However,
during high current interruptions there may be a minimal
amount of erosion from the contact surfaces. Maximum
permitted erosion is 1/8 inch. To determine contact
erosion, close the breaker and observe the vacuum
interrupter moving stem from the rear of the breaker. If
the mark on each stem is visible, erosion has not
reached maximum value thus indicating satisfactory
contact surface of the interrupter. If the mark is not
visible, the vacuum interrupter assembly must be
replaced
(Figures 6-2 and 6-3).
The adequacy of contact wipe can be determined by
simply observing the vacuum interrupter side of the
operating rod assembly on a closed breaker. Figures
6-4
and
6-5
show the procedure for determining the contact
wipe. If the wipe is not adequate, the vacuum interrupter
assembly (Pole Unit) must be replaced. Field adjustment
is not possible.
