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Siemens 38-3AH3 38 kV Instruction Manual page 15

Vacuum circuit breaker
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Introduction
The type 38-3AH3 vacuum circuit breaker
is of drawout construction designed for
use in medium-voltage, metal-clad
switchgear. The 38-3AH3 circuit breaker
conforms to the requirements of ANSI and
IEEE standards, including C37.20.2,
C37.04, C37.06, C37.09 and C37.010.
A type 38-3AH3 vacuum circuit breaker
consists of three vacuum interrupters, a
stored-energy operating mechanism,
necessary electrical controls and interlock
devices, disconnect devices to connect the
circuit breaker to both primary and control
power and an operator housing. Insulating
barriers are located along the outer sides
and between phases as shown in
Figure 11: Type 38-3AH3 vacuum circuit
breaker with inter-phase and outer-phase
barriers installed on page 17.
This section describes the operation of
each major sub-assembly as an aid in the
operation, installation, maintenance and
repair of the type 38-3AH3 vacuum circuit
breaker.
Vacuum interrupters
The operating principle of the vacuum
interrupter is simple. Figure 9: Vacuum
interrupter cutaway view is a cutaway view
of a typical vacuum interrupter. The entire
assembly is sealed after a vacuum is
established. The vacuum-interrupter
stationary contact is connected to the
upper-disconnect stud of the circuit
breaker. The vacuum-interrupter movable
contact is connected to the lower-
disconnect stud and driving mechanism of
the circuit breaker. The metal bellows
provides a secure seal around the movable
contact, preventing loss of vacuum while
permitting vertical motion of the movable
contact.
Vacuum interrupter/
operator
When the two contacts separate, an arc is
initiated that continues conduction up to
the following current zero. At current zero,
the arc extinguishes and any conductive
metal vapor that has been created by and
supported by the arc condenses on the
contacts and on the surrounding arc
shield. Contact materials and configuration
are optimized to achieve arc motion, resist
welding and to minimize switching
disturbances.
Fixed contact-current connection
Ceramic insulator
Arc shield
Fixed contact
Moving contact
Ceramic insulator
Metal bellows
Guide
Moving contact-current connection
Figure 9: Vacuum interrupter cutaway view
15

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