Working Principle of Trip Units

A low-voltage circuit breaker generally consists of several components, including the trip unit, contact system, arc extinguishing device, operating mechanism, frame, and housing. When the circuit breaker is put into operation, the operating handle closes the main contacts, and the free trip mechanism locks the main contacts in the closed position. At this point, various trip units enter their operational states. Below, we will focus on the different trip units in a circuit breaker:

• 1. Electromagnetic Trip Unit

The electromagnetic trip unit is connected in series with the protected circuit. When the circuit carries a normal current, the electromagnetic force generated by the electromagnet is less than the pulling force of the reaction spring, and the armature cannot be attracted by the electromagnet, allowing the circuit breaker to operate normally. When a short-circuit fault occurs in the circuit, the current exceeds the normal current by several times, and the electromagnetic force generated by the electromagnet becomes greater than the force of the reaction spring. The armature is then attracted by the electromagnet, and through the transmission mechanism, it pushes the free trip mechanism to release the main contacts. The main contacts separate under the action of the opening spring, cutting off the circuit and providing short-circuit protection.

• 2. Thermal Trip Unit

The thermal trip unit is also connected in series with the protected circuit. When the circuit carries a normal current, the heating element heats up, causing the bimetallic strip to bend to a certain extent (just enough to contact the transmission mechanism) and reach a dynamic equilibrium state, where the bimetallic strip no longer continues to bend. If an overload occurs, the current in the circuit increases, and the bimetallic strip will continue to bend. Through the transmission mechanism, it pushes the free trip mechanism to release the main contacts. The main contacts separate under the action of the opening spring, cutting off the circuit and providing overload protection.

• 3. Undervoltage Trip Unit

The undervoltage trip unit is connected in parallel to the power supply side of the circuit breaker and provides under-voltage and zero-voltage protection. When the power supply voltage is normal, operating the handle closes the normally open auxiliary contacts of the circuit breaker, energizing the electromagnet. The armature is then attracted by the electromagnet, allowing the free trip mechanism to lock the main contacts in the closed position, and the circuit breaker is put into operation. When the power supply is interrupted or the voltage is too low, the electromagnetic force generated by the electromagnet is insufficient to overcome the pulling force of the reaction spring, and the armature is pulled upward. Through the transmission mechanism, it pushes the free trip mechanism to trip the circuit breaker, providing under-voltage and zero-voltage protection.

When the power supply voltage is between 75% and 105% of the rated voltage, the undervoltage trip unit ensures that it remains engaged, allowing the circuit breaker to close smoothly. When the power supply voltage drops below 40% of the rated voltage, the undervoltage trip unit ensures that it disengages, causing the circuit breaker to trip and open the circuit.

Additionally, a normally closed button connected in series with the electromagnetic coil of the undervoltage trip unit can be used to perform a manual trip operation.

• 4. Shunt Trip Unit

The shunt trip unit is used for remote tripping control of the low-voltage circuit breaker. Its electromagnetic coil is connected in parallel to the power supply side of the low-voltage circuit breaker. When a tripping operation is required, pressing the normally open button energizes the electromagnet of the shunt trip unit, attracting the armature. Through the transmission mechanism, it pushes the free trip mechanism, causing the low-voltage circuit breaker to trip.

When a low-voltage circuit breaker is equipped with two or more types of trip units, it is referred to as having a compound trip unit.