Thermal-Magnetic Trip Unit

Thermal-Magnetic Trip Unit

In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent condition is sensed.

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

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

Trip Mechanism

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

Trip Unit with Trip Mechanism

Trip Unit with Trip Mechanism

The operating mechanism is held in the “ON” position by the trip 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 DC source could also be used.
The operating mechanism is held in the “ON” position by the trip mechanism.

The operating mechanism is held in the “ON” position by the trip mechanism.

Manual Trip

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

The operating mechanism opens the contacts.

The “PUSH TO TRIP” button also serves as a safety device by preventing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton causes the button to lift up and the breaker to trip.

Manual trip mechanism

Manual trip mechanism

Overload Trip

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions. When sufficient overcurrent flows through the circuit breaker’s current path, heat build up causes the bi-metalic strip to bend. After bending a predetermined distance, the bi-metalic strip makes contact with the tripper bar activating the trip mechanism.

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.

Circuit breaker contacts

Circuit breaker contacts

A bi-metalic strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bi-metalic strip bends when heated. As current rises, heat also rises.

The hotter the bi-metalic becomes the more it bends. After the source of heat is removed, as when the circuit breaker contacts open, the bi-metalic strip cools and returns to its original condition. This allows a circuit breaker to be manually reset once the overload condition has been corrected.

Short Circuit Trip

As previously described, current flow through a circuit breaker’s blow-apart contacts creates opposing magnetic fields. Under normal operating conditions, these opposing forces are not sufficient to separate the contacts. When a short circuit occurs, however, these opposing forces increase significantly.

The current that flows through the contacts also flows through a conductor that passes close to the circuit breaker’s trip unit. At fault current levels, the magnetic field surrounding this conductor provides sufficient force to unlatch the trip unit and trip the breaker.

Short Circuit Trip

Short Circuit Trip

The combined actions of magnetic fields forcing contacts apart while simultaneously tripping the circuit breaker result in rapid interruption of the fault current. In addition, because the magnetic forces are proportional to the current, the greater the fault current, the shorter the time it takes to interrupt the current.

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on


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