How does the circuit breaker trip unit work?

In addition to providing a means of opening and closing its contacts manually, a circuit breaker should automatically open its contacts when an overcurrent condition is detected.

The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.

In a thermomagnetic circuit breaker, the trip unit includes elements designed to sense the heat resulting from an overload and the high current resulting from a short circuit. In addition, some magneto-thermal circuit breakers have a “PUSH TO TRIP” button.

trip uni
How trip unit works

Travel Mechanism

The trigger includes a trigger mechanism that is held in place by the trigger bar. As long as the trigger bar holds the trigger mechanism, the mechanism remains firmly locked in place.

The operating mechanism is held in the “ON” position by the trigger mechanism. When a trip is activated, the trip mechanism releases the operating mechanism which opens the contacts.

Note: The drawings in this section show an AC power source; however, a direct current source could also be used.

Manual travel

Some molded case circuit breakers, especially larger circuit breakers, can be tripped manually by pressing the “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed, the trip bar swings up and to the right. This allows the trigger mechanism to “unlock” the release of the actuator.

The operating mechanism opens the contacts.

The “PUSH TO TRIP” button also serves as a safety device by preventing access to the interior of the “ON” position. If you attempt to remove the circuit breaker cover while the contacts are in the “ON” position, a spring located under the push button will lift it and trip the circuit breaker.

Overload travel

Thermal-magnetic circuit breakers use a bi-metallic band to sense overload conditions. When sufficient overcurrent passes through the circuit breaker current path, the accumulated heat causes bending of the bimetal strip. After bending a predetermined distance, the bimetal strip contacts the trigger bar activating the trigger mechanism.

A bimetallic strip is made up of two dissimilar metals bonded together. Since the two metals have different thermal expansion characteristics, the bimetallic strip bends when heated. As the current increases, so does the heat.

The hotter the bi-metal, the more it leans. Once the heat source is removed, for example when the circuit breaker contacts are open, the bimetal strip cools down and returns to its initial state. This allows a circuit breaker to be manually reset once the overload condition has been corrected.

Short circuit trip

As previously described, current flows through a breaker detachable contacts create opposing magnetic fields. Under normal operating conditions, these opposing forces are not sufficient to separate the contacts. However, when a short circuit occurs, these opposing forces increase dramatically.

The current which passes through the contacts also passes through a conductor which passes near the trip unit of the circuit breaker. At-fault current levels, the magnetic field surrounding this conductor provides sufficient force to unlock the trip unit and trip the circuit breaker.

The combined actions of forcing magnetic fields to Separate contacts while simultaneously activating the circuit breaker result in a rapid interruption of the fault current. In addition, since the magnetic forces are proportional to the current, the greater the fault current, the shorter the time required to interrupt it.

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