How does the circuit breaker operating mechanism work?

Circuit breakers are the "safety guards" of power systems, and the operating mechanism is the "muscles and joints" of this guard, responsible for executing the commands of opening (disconnecting the circuit) and closing (connecting the circuit). Its core mission is to complete the opening and closing operations quickly and reliably, and maintain the state when necessary.

Core Components

A typical circuit breaker operating mechanism typically consists of the following key components:

Energy storage unit: Provides the energy required for opening or closing the circuit breaker. Common forms include:

Spring: Compressed and stored by a motor or manually.

Solenoid coil: Generates a strong electromagnetic force when energized.

Compressed air or hydraulic oil: Stored in a gas or liquid tank and actuated by releasing pressure.

Transmission linkage: A set of mechanical linkages that transmits and amplifies the stored energy, ultimately actuating the circuit breaker's moving contacts.

Lock and trip mechanism: This is the "brains" and "switch" of the mechanism.

Closing lock: Locks the mechanism in the closed position after the closing operation is completed.

Opening release: Receives an opening signal (an electrical pulse) from a protective relay or manual operation, releases the closing lock, and triggers the opening of the circuit breaker.

Opening lock: Locks the mechanism in the open position after the opening operation is completed.

Closing release: Receives the closing signal, releases the opening latch, and triggers closing.

Control and auxiliary unit: Includes local/remote control switches, auxiliary contacts (for signal feedback), and an anti-trip device (to prevent the circuit breaker from repeatedly re-opening if the fault condition persists).

 

Main Types and Operating Principles

Depending on the energy storage method, operating mechanisms are primarily categorized as follows:

 

1. Spring-operated mechanism

 

This is the most common type, especially in the medium-voltage range.

Energy Storage Process:

A small motor starts, and through a gear reduction mechanism, pulls or twists a powerful closing spring, compressing or tightening it, storing a significant amount of elastic potential energy.

After the spring is compressed to a predetermined position, it is locked by a closing latch mechanism, completing energy storage and stopping the motor.

Closing Process:

A closing signal (electrical or manual) is applied, and the closing release activates, releasing the closing latch.

The compressed closing spring instantly releases energy, pushing the transmission link, causing the circuit breaker's moving contacts to rapidly move and close with the stationary contacts, completing the circuit.

At the end of the closing process, the transmission link also compresses the opening spring, storing energy for the next opening. Simultaneously, the mechanism is locked in the closed position by the opening latch.

Opening Process:

When a trip signal is given (from a protective relay or manually), the trip release activates, releasing the trip latch.

The compressed trip spring releases energy, rapidly separating the moving contact from the stationary contact. The arc is extinguished in the interrupter, disconnecting the circuit.

The mechanism is locked in the open position.

Advantages: High reliability, low dependence on power supply (requires low power consumption for motor energy storage), and fast operation.
Disadvantages: Complex structure, numerous mechanical parts, and demanding manufacturing process.

 

2. Electromagnetic Operating Mechanism

 

This mechanism uses the suction force generated by an electromagnet as its power source.

Closing Process:

A strong closing current (DC or AC) is applied to the closing coil.

The coil generates a strong magnetic field, driving the iron core (moving iron core) upward like a cannonball.

The moving iron core directly pushes the moving contact of the circuit breaker through a connecting rod, closing the circuit breaker, which is then held in place by a mechanical latch.

Opening Process:

A small current is applied to the opening coil.

The opening release activates, releasing the latch, and the circuit breaker is opened under the action of the opening spring.

Advantages: Relatively simple structure.

Disadvantages: Closing requires a very high instantaneous current (up to hundreds of amperes), placing high demands on the operating power supply (battery); closing speed is significantly affected by the power supply voltage. This mechanism is now gradually being replaced by spring-operated mechanisms.

 

3. Pneumatic/Hydraulic Operating Mechanism

 

This mechanism uses compressed air or hydraulic oil as the motive force and is commonly used in high- and ultra-high-pressure applications.

Energy Storage Process: A compressor or oil pump pressurizes high-pressure gas (such as air or SF6) or hydraulic oil into a gas/pressure storage tank.

Closing and Opening Process:

A complex system of valves (closing valves and opening valves) controls the flow of the high-pressure medium.

When a closing signal is given, the closing valve opens, allowing the high-pressure medium to enter one side of the cylinder, pushing the piston and closing the contacts.

When an opening signal is given, the opening valve opens, allowing the high-pressure medium to enter the other side of the cylinder (or releasing the pressure on the other side), pushing/driving the piston in the opposite direction, thus opening the circuit.

Advantages: High operating power, smooth operation, and fast speed, making it ideal for large-capacity circuit breakers.

Disadvantages: Requires an additional compression/hydraulic system, resulting in a complex structure, leakage risk, and high maintenance workload.

 

Summary and Key Points

Features	Spring mechanism	Electromagnetic mechanism	Pneumatic/Hydraulic Mechanisms
Power Source	Spring mechanical energy	Electricity of the electromagnetic coil	Compressed Gas/Liquid Pressure Energy
Closing Speed	Fast and stable	Affected by power supply voltage	Very Fast and Smooth
Operating Power	Low power (energy storage motor)	Extreme power (closing coil)	Low Power (Control Valve)
Applications	Mainstream in the medium voltage field	Gradually phased out	High and Ultra-High Pressure Applications
Maintenance	Many mechanical components, requiring lubrication	Relatively simple	Complex Systems, Leak Prevention

The core operational process can be summarized as follows:

Store energy → Wait for a command → Trip and unlock → Release energy to actuate the contacts → Complete the operation and relock/recharge.

The operating mechanism is key to a circuit breaker's reliable interruption of fault currents. Its primary design objective is to ensure that, under all circumstances, the trip command is executed unconditionally and extremely quickly, thereby protecting the safety of the entire power system.

Are you looking to improve the reliability of your power distribution network? Contact Shaanxi Huadian Electric Co., Ltd. for expert advice and high-quality operator solutions. Please email pannie@hdswitchgear.com to discuss how our products can enhance your city's power infrastructure.