Thermal magnetic circuit breakers utilize
both the instantaneous trip ability of a magnetic breaker
for short circuit currents with the overload tolerance of
thermal circuit breakers for normal motor-starting demands.
When the reset button is activated, the
movable contact completes the electrical circuit between the
stationary contact on the line terminal and the bimetallic
strip. The pawl on the strip keeps the contacts engaged.
Current then flows from the movable contact through the
bimetallic strip to the release arm and into the
electromagnetic coil. The coil connects to the terminal
supplying power to the load.
There are three ways to trip this circuit breaker.
-
Prolonged high current flowing through
the bimetallic strip creates heat. The strip responds by
bending from the heat because of the different
coefficients of expansion between the two metals. When
it bends enough the pawl releases the movable contact
and spring tension snaps the contacts open breaking the
circuit.
-
Trip mechanism uses the electromagnetic
coil as a solenoid. When current through the coil is
high enough the solenoid core is pulled into the coil,
pushing ownwadrd on the metal plate where the bimetallic
strip is fastened.
-
Trip mechanism places force on the
solenoid core by pressing the manual release button.
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