Siemens GMSG Instruction Manual page 15

Introduction
The type GMSG vacuum circuit breaker is
of drawout construction designed for use
in medium-voltage, metal-clad switchgear.
The GMSG 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 GMSG 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
as shown in Figure 11: Type GMSG vacuum
circuit breaker 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 GMSG 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.
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.
Vacuum interrupter/operator
Primary disconnects
Figure 10: Upper and lower primary
disconnects is a side view of the circuit
breaker with the outer-insulating phase
barrier removed to show details of the
primary disconnects. Each circuit breaker
has three upper- and three lower-primary
disconnects. Upper-primary disconnects
are connected to the stationary contacts of
the vacuum interrupters, and the lower-
primary disconnects are connected to the
movable contacts.
Each disconnect arm has a set of multiple
spring-loaded fingers that mate with bus
bars in the metal-clad switchgear. The
number of fingers in the disconnect
assembly varies with the continuous and/
or interrupting rating of the circuit breaker.
There are three insulating push rods. Each
push rod connects the movable contact of
one of the vacuum interrupters to the jack
shaft driven by the closing and tripping
mechanism. Flexible connectors provide
secure electrical connections between the
movable contacts of each vacuum
interrupter and its bottom-primary
disconnect.
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
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