A circuit breaker is a switching device used to close, carry, and interrupt abnormal circuit currents within a specified time. It is operated manually or electrically and comes in single-pole, two-pole, three-pole, and four-pole configurations. Its main structural components include main contacts, auxiliary contacts, alarm contacts, shunt trip units, and undervoltage trip units. Below, we will provide a detailed introduction to these components.
I. Circuit Breaker Working Principle
1. The main contacts of the low-voltage circuit breaker are closed manually or electrically. After the main contacts are closed, the free-tripping mechanism locks the main contacts in the closed position.
2. The coil of the overcurrent trip unit and the thermal element of the thermal trip unit are connected in series with the main circuit, while the coil of the undervoltage trip unit is connected in parallel with the power supply. When a short circuit or severe overload occurs in the circuit, the armature of the overcurrent trip unit is attracted, causing the free-trip mechanism to operate and the main contacts to disconnect the main circuit. When the circuit is overloaded, the thermal element of the thermal trip unit heats up, causing the bimetallic strip to bend and pushing the free-trip mechanism to operate.
3. When the circuit is undervoltage, the armature of the undervoltage release device is released, which also causes the free trip mechanism to operate. The shunt trip device is used for remote control. During normal operation, its coil is de-energized. When remote control is required, the start button is pressed to energize the coil.
II. Main Structure of Circuit Breakers
1. Main contacts
It carries load current for extended periods, connecting last when closing and disconnecting first when opening. Main contacts are present in all switches.
2. Auxiliary contact
The contacts that are mechanically linked to the opening and closing mechanism of the circuit breaker's main circuit are mainly used to indicate the opening and closing status of the circuit breaker. They are connected in the control circuit of the circuit breaker and control or interlock related electrical appliances through the opening and closing of the circuit breaker.
3. Alarm contacts
This alarm contact is used for circuit breaker accidents. It only operates after the circuit breaker has tripped. It is mainly used when the circuit breaker trips freely due to overload, short circuit, or undervoltage faults. The alarm contact changes from its normally open position to the closed position, connecting indicator lights or bells in the auxiliary circuit to display or remind users of the circuit breaker's fault tripping status.
4. Shunt trip unit
A shunt trip unit is a trip unit excited by a voltage source, and its voltage is independent of the main circuit voltage. It is an accessory for remotely controlling circuit breaking. It can reliably disconnect the circuit breaker when the power supply voltage is equal to any voltage between 70% and 110% of the rated control power supply voltage. The shunt trip unit operates on a short-time duty cycle; the coil energization time generally cannot exceed 1 second, otherwise the wire will burn out.
5. Undervoltage trip unit
An undervoltage release is a type of release that causes a circuit breaker to trip with or without a delay when its terminal voltage drops to a specified range. When the power supply voltage drops (even slowly) to within 35% of the rated operating voltage, the undervoltage release should operate. When the power supply voltage is equal to 35% of the release's rated operating voltage, the undervoltage release should be able to prevent the circuit breaker from closing. When the power supply voltage is equal to or greater than 85% of the undervoltage release's rated operating voltage, under hot conditions, it should be able to ensure reliable circuit breaker closure.
