Circuit breakers in switchgear are the core components used to connect and disconnect circuits, automatically cutting current and protecting electrical equipment and lines in the event of overload, short circuit or undervoltage. It is an indispensable control and protection device in power system. Here's a closer look:
I. Basic Functions functions of a Circuit Breaker
Normal on/ off controls
Manual operation: To manually close or open a circuit breaker by means of an operating mechanism (such as a handle or button) on the switch panel to control the connection or disconnection of the circuit.
Automatic control: interact with relays, phase-lock rings and other automation devices to achieve remote or timed switching, suitable for unmanned operation.
Fault Protection Functions
Overload Protection: When the current exceeds the rated value (e.g. 1.2 times the rated current within 1 hour), the circuit breaker's thermal release is activated, disconnecting the circuit.
Short-circuit protection: When a short circuit causes a sharp increase in current (up to several dozen times the rated current), the electromagnetic release activates instantaneously, quickly disconnecting the circuit (usually within 0.1 seconds). Undervoltage protection: When the voltage drops below a certain percentage of the rated value (e.g. 70% -35%), inadequate pressure release is activated to prevent equipment from operating at low pressure and causing damage.
Leakage protection (select model): Detect leakage current in the circuit and disconnect the circuit if the leakage current exceeds the set value (e.g., 30mA) to prevent electrocution.
ii. The role of circuit breakers in switchgear
Circuit Control Center
Switching equipment is the ``hub"of the power system. circuit breakers as its core component is responsible for controlling the state of the main circuit on and off, distributing and switching power.
For example, in a low-voltage distribution board, circuit breakers can control the power supply of different branches to meet the power needs of multiple devices.
Equipment protection barrier
When downstream equipment (such as motors, transformers, etc.) is overloaded or short-circuited, circuit breakers quickly interrupt the fault current to prevent equipment damage or fire.
When the motor starts, for example, the current can reach 5 to7 times its rated value. Circuit breakers can withstand short-term overload and avoid false tripping. System Safety isolation
During inspection or maintenance, the circuit breaker can be triggered manually to completely isolate the faulty part from the power supply, ensuring the safety of personnel.
For example, in high-voltage switchgear, circuit breakers works with the circuit breaker to implement the "five protections" (preventing the circuit breaker from running unexpectedly and opening and closing under load).
III. Types and Classification of Circuit Breakers
By Voltage
Low voltage circuit breakers: rated voltage ≤ 1kV (e.g., AC 400V, DC 800V) for building distribution, industrial control, etc.
High voltage circuit breakers: rated voltage greater than 1kV (e.g. 10kV, 35kV, 110kV) for high voltage transmission systems such as power plants, substations, etc.
By Structure Type
Framed circuit breaker:
Suitable for largecurrent applications such as 630A to 6300A, such as industrial motor control and data center distribution.
Modular design allows for the integration of multiple protection functions (e.g., overload, short circuit and ground fault). Molded Case Circuit Breaker (MCCB):
Suitable for moderate current applications (e.g. 10A-1600A) such as building distribution and commercial lighting.
It is compact, low-cost cost, but relatively simple to protect.
Miniature Circuit Breaker (MCB):
Suitable for low current applications such as 1A-125A, such as home distribution and terminal equipment protection.
It is compact and can include leakage protection (e.g., RCD).
By Arc Extinguishing Medium:
Air circuit breaker: using air as arc extinguishing medium, simple structure, low cost, but limited crushing capacity.
Vacuum Circuit Breaker: use the high insulation strength of vacuum to extinguish arc, suitable for medium to high voltage (e.g., 10kV-35kV), high breakability, long life.
SF6 Circuit Breaker: use of sulfur hexafluoride (SF6) gas to extinguish arc for ultra-high pressure applications (e.g. above 110kV). SF6, however, is a potent greenhouse gas that requires special treatment. IV. INTRODUCTION Key Technical Parameters of Circuit Breakers
Rated current: The maximum current allowed to flow through a circuit breaker over a long period of time. This parameter should be selected according to the load current (e.g., 1.2-1.5 times the rated motor current of the rated motor).
Rated voltage: The voltage level at which a circuit breaker normally operates. This parameter should match the system voltage (e.g., AC 400V, DC 110V).
Crushing capacity (Icu/Ics):
Ultimate Breaking Capacity (Icu: the maximum short-circuit current (e.g. 50kA) that the circuit breaker can safely interrupt.
Service outage Capacity: The ability of a circuit breaker to continue working after interrupting a short-circuit circuit current (usually 50% -100% Icu).
Service life: Mechanical life (e.g. 10,000 closing and opening cycles) and electrical life (e.g., 2,000 short-circuit interruptions) must meet application requirements.
V. Circuit Breaker Selection and Application Examples
Selection Principles
Load Matching: select the rated current of the circuit breaker based on the rated current of the device and starting current. Protection Coordination: Up-and-down level circuit breakers must operate in a coordinated manner to avoid over-tripping (for example, primary line circuit breaker should operate longer than branch circuit breaker).
Environmental Adaptability: circuit breakers with a high protection rating (e.g. IP65) should be used in hot, humid and corrosive environments.
Application Cases
Data Center Power Distribution: use circuit breakers as main switch, set overload, short circuit, ground fault protection in one, ensure the continuity of power supply.
Home distribution box: Miniature circuit breakers (MCB) combined with residual current device (RCD) provides branch circuit overload protection and personal shock protection.
Renewable energy generation: highvoltage vacuum circuit breaker is used for grid control of photovoltaic plants and wind farms to quickly interrupt fault currents and protect inverters and other equipment.
