Allis-Chalmers MA-150, Instruction Book

Page: 8 / 48

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I.
I GENERAL
PART I. DESCRIPTION
The AI
I
is-Chalmers RUPTAIR movable portion consists of a magnetic circuit
breaker for metal-clad switchgear application, with auxiliary equipment
suitably arranged for best function and easy installation. As part of
standard equipment, each order is furnished with one maintenance closing
device for solenoid operated breakers or a charging crank for stored
energy operated units.
The RUPTA[R magnetic circuit breaker differs essentially from oil breakers
and air-blast breakers in that it does not depend on any stored medium
such as oil or compressed air for arc interruption. The component parts
of the breaker are mounted in a structural steel frame. The operator, the
operating shaft, and connecting links are mounted on the lower section of
the breaker frame and are well shielded. The horizontal terminal studs,
which are insulated with flame retardent tubing, extend through the breaker
bracket and support the other parts of the electrical circuit. I nterruption
occurs within the arc chute assemblies which are mounted at the top, over
the contact structures.
1.2
METHOD OF ARC INTERRUPTION
Interruption is accomplished in air at atmospheric pressure, with the aid
of a self-induced magnetic blowout field and air draft. At the time the
trip coi
I
is energized, current is being carried through the main contacts.
As the movable contact assembly separates from the stationary contact
assembly, the current transfers very quickly from the main contacts to the
arcing contacts, thus keeping the main contact erosion to a minimum.
(
For
breakers equipped with tertiary contacts, the current transfers from the
mains, to the tertiary and then to the arcing contacts.
)
As the movable
contact assembly continues its stroke, the arcing contacts part, drawing
a
power arc, which is transferred first to the stationary end arc runner
then to the moving end arc runner. The transfer of the arc to the arc
runners establishes the full flow of current through the blowout coils,
setting up the magnetic field which, in accompaniment with natural thermal
effects of the heated arc, the configurat�on of the current carrying
circuit, etc., tend to force the arc upward into the barrier stack. The
cool surfaces of the barrier stack cool and deionize the arc, while the
"Vee" slots in the stack reduce its cross section and elongate it.
The arc runners are made of wide, heavy material for maximum heat dissi­
pation and to minimize metal vaporization. To facilitate interruption of
low currents, a puffer assembly provides a movement of air through the
contact area to aid the magnetic field in moving the arc into the barrier
stack. All of the above effects work together to increase the resistance
of the arc and enable it to be extinguished at an early current zero.
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