Magnum
20
1812.900A
3.2
Basic circuit breaker assembly
Magnum 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 3-5):
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.
3.3 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.
Double-wide circuit breakers use two pole units and arc
chute assemblies connected mechanically and electrically
in parallel to form each phase.
Figure 3-5: Typical Magnum Construction (Right Side View)
1. Case
2. Front Cover
3.3.1 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 3-6). 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 (Figure 3-7 and Figure 3-8). 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 3-6). 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.”
Figure 3-6: Features of Magnum Moving Conductor
Assembly
1. Pivot Point
2. Single Contact Finger
3. Moving Arcing Contact Area (Toe)
4. Moving Main Contact Conductive Pad (Heel)
5. Dual Flexible Connections
1
2
1
3
2
5
4
