20
Instructions for Installation, Operation and
Maintenance of Magnum SB Insulated Case
Low Voltage Power Circuit Breakers
EATON CORPORATION
www.eaton.com
Instructional Leaflet
IB2C12063H03
Effective March 2012
Basic Circuit Breaker Assembly
Magnum SB circuit breakers use a rigid frame housing
construction of engineered thermoset composite resins . This
construction provides high strength structural properties,
excellent dielectric characteristics and resistance to arc
tracking .
The 3-piece construction approach provides support while
isolating and insulating power conductors (
Figure 21
):
1 . A 2-piece engineered thermoset composite resin case
encloses current paths and arc chambers . The chambers
act to channel arc gases up and out of the circuit breaker
during interruption .
2 . The operating mechanism sits on the front of the case
and is electrically isolated and insulated from current
contact structures . It is covered by an insulating front
cover .
Pole Units
A current carrying pole unit is individually enclosed and rigidly
supported by the case . The individual chambers provide for pole
unit isolation and insulation from one another . Each pole unit
has one primary contact assembly, which consists of a moving
portion and a fixed portion . The exact design configuration
depends upon the breaker’s frame size . Circuit breakers
with frame sizes of 4000 amperes and higher use two pole
units and arc chute assemblies connected mechanically and
electrically in parallel to form one phase .
(Front Cover)
(Case)
Figure 21. Typical Magnum Construction (Right Side View)
Primary Moving Contacts
Depending upon the frame size, each primary moving contact
assembly is comprised of multiple individual copper contact
fingers connected to the load conductor through flexible
braided connectors (
Figure 22
) . Two flexible connectors are
used to connect each finger to the load conductor . The number
of fingers used depends upon the circuit breaker’s continuous
and short-circuit current ratings (
Figures 23 and 25
) . On some
ratings fingers are removed and replaced with spacers .
The single contact finger performs both the main and arcing
contact functions on different parts of the same finger (
Figure
22
) . A highly conductive alloy pad is part of the contact finger
and functions as the moving main contact, and is called the
“Heel .” The tip of the same contact finger functions as the
moving arcing contact, and is called the “Toe .”
In addition to the contact finger information given above,
SBSE utililizes an inner and outer carriage design to facilitate
a fast opening blow open contact structure (
Figure 24
) . The
contact fingers mounted in the inner contact carrier can move
independently from both the outer carrier and the opening
mechanism in the breaker . This independence is the core design
feature of its fast opening blow open contact structure .
Pivot Point
Single Contact
Finger
Moving
Arcing
Contact Area
(Toe)
Moving Main
Contact Conductive
Pad (Heel)
Dual Flexible
Connections
Figure 22. Features of Magnum Moving Conductor Assembly