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I.B. 3A74792H04
!
THE PLATE BOLTED TO THE REAR RECTANGU-
LAR OPENING ON ALL 2000 AMPERE AND ABOVE
CIRCUIT BREAKERS AND IDENTIFIED IN FIGURE 5-
2 MUST BE BOLTED IN POSITION BEFORE PLAC-
ING A CIRCUIT BREAKER IN SERVICE TO INSURE
PROPER OPERATION WITHIN TEST PROVEN
PARAMETERS.
Vacuum interrupters are mounted vertically and support-
ed from the fixed stem which is clamped to the top con-
ductor. The exclusive current transfer system consists of
a series of plated, high-conductivity copper leaf conduc-
tors that are pressed on the movable interrupter stem.
This design provides a multipoint contact resulting in low
electrical and thermal resistance. Utilizing this non-slid-
ing current transfer system between the movable stem
and the circuit breaker main conductor eliminates main-
tenance of this joint.
Direct acting insulated operating rods in conjunction with
the circuit breaker's mechanism provide a fixed amount
of interrupter movable stem motion. This motion is
directly related to the interrupter's "Wipe" and "Stroke,"
each of which is discussed in detail later in this section.
5-2.1 VACUUM INTERRUPTER
Type VCP-W 38kV Vacuum Circuit Breakers utilize vac-
uum interrupters for interruption and switching functions.
The vacuum interrupters use axial-magnetic, copper
chrome contacts for superior dielectric strength, better
performance characteristics, and lower chop current.
Vacuum interruption provides the advantages of
enclosed interrupters, reduced size and weight, short
interrupting time, long life, reduced maintenance, and
environmental compatibility.
Arc interruption is simple and fast (Figure 5-3). In the
closed position, current flows through the interrupter.
When the contacts are opened, the arc is drawn
between the contact surfaces. It is moved rapidly
around the slotted contact surfaces by a self-induced
magnetic force which prevents gross contact erosion as
well as the formation of hot spots on contact surfaces.
The arc burns in an ionized metal vapor which continu-
ally leaves the contact area and condenses on the sur-
rounding metal shield.
At current zero, the arc is extinguished and vapor produc-
tion ceases. Very rapid dispersion, cooling, recombina-
tion, and deionization of the metal vapor plasma, together
with the fast condensation of metal vapor products, cause
Effective 7/97
CAUTION
Shield Involvement
Arc
High Current
Initiation
Arc Mode
Fault
Current
Figure 5-3 Graphic Representation of Arc Interruption
the vacuum to be quickly restored. Hence, the opened
contacts withstand the transient recovery voltage.
5-2.2 CONTACT EROSION INDICATION
The purpose of a contact erosion indicator is to monitor
the erosion of the vacuum interrupter contacts, which is
very minimal over time with vacuum interrupters utilizing
copper-chrome contact material. If contact erosion
reaches 1/8 inch, the interrupter must be replaced. Two
different contact erosion indicators are provided for
maximum user convenience.
NOTICE
Each phase of the circuit breaker is monitored inde-
pendently by either of the two types of contact ero-
sion indicators.
The first contact erosion indicator is monitored from the
rear of the circuit breaker. It provides a very accurate
indication of contact erosion, and is the primary determi-
nant of contact condition. A contact erosion indicator
mark is located on the insulated operating rod of each
interrupter (Figures 6-2 and 6-3).
Page 17
Interruption
Current
Zero
Recovery
Voltage
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