7
3.2 Switching Mechanism
The ON, OFF and TRIPPED conditions are shown in
Fig. 3.2. In passing from ON to OFF, the handle ten-
sion spring passes through alignment with the toggle
link (“dead point” condition). In so doing, a positive,
rapid contact-opening action is produced; the OFF to
ON contact closing acts in a similar way (“quick make”
and “quick break” actions). In both cases the action of
the contacts is always rapid and positive, and inde-
pendent of the human element – i.e., the force or
speed of the handle.
In auto tripping a rotation of the bracket releases
the cradle and operates the toggle link to produce the
contact-opening action described above. In the tripped
condition the handle assumes the center position be-
tween on and off, providing a visual indication of the
tripped condition. Also, auto trip is “trip free,” so that
the handle cannot be used to hold the breaker in the
ON condition. The protective contact-opening func-
tion cannot be defeated.
In multipole breakers the poles are separated by
integral barriers in the molded case. The moving con-
tacts of the poles are attached to the central toggle
link by a common-trip bar, however, so that tripping,
opening and closing of all poles is always simulta-
neous. This is the “common trip” feature, by which
single phasing and similar unbalance malfunctions are
effectively prevented.
Spring tension line
Toggle link
Cradle
Bracket
Spring
a) On
b) Off
c) Tripped
ON to OFF dead-point line
OFF to ON dead-point line
Handle centered; indicates
tripped condition
Fig. 3.2 Switching Mechanism Action
3.3 Automatic Tripping Device
There are three types of device, the thermal-magnetic
type, the hydraulic-magnetic type and the electronic
trip relay type.
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