19
User Manual
MN013016EN
Effective July 2022
Magnum PXR and Power Defense SB low
voltage power circuit breakers user manual
EATON
www.eaton.com
Basic circuit breaker assembly
All Magnum PXR and PD-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 three-piece construction approach provides support while
isolating and insulating power conductors (
) .
Case
Front cover
Figure 19. Typical construction (right side view)
a
A two-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.
b
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 . Double frame circuit breakers use two
pole units and arc chute assemblies connected mechanically and
electrically in parallel to form one phase .
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
(
) . 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
(
) . 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 (
) . 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 .”
Pivot point
Single contact
finger
Moving
arcing
contact area
(toe)
Moving main contact
conductive pad (heel)
Dual flexible
connections
Figure 20. Features of moving conductor assembly
