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Basic Knowledge of Switchgear: Detailed Explanation of the Roles of Busbars, Circuit Breakers, and Isolating Switches

Release time:

2025-11-04

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Abstract

As the core equipment of power systems, switchgear undertakes critical functions such as power distribution, protection, and control. Although its internal structure is complex, the core components can be categorized into three main types: busbars, circuit breakers, and disconnect switches. This article will systematically analyze the pivotal roles of these three component types from three perspectives: functional positioning, operational principles, and practical applications.

1. Busbar: The "Aorta" of the Power System

The busbar is a conductor used in switchgear to collect, distribute, and transmit electrical energy—it functions much like the human aorta, delivering power from the source to various branch circuits. In high-voltage switchgear, the busbar must withstand the thermal effects and electromagnetic forces generated by short-circuit currents; thus, it demands extremely rigorous material selection and structural design.

1.1 The Functional Positioning of the Busbar

The main functions of the busbar include:

Electrical energy aggregation: Concentrating and transmitting electrical energy from transformers or generators into the switchgear interior;

Branch distribution: Distribute electrical energy to devices such as circuit breakers and disconnect switches via branch busbars;

System Connection: In dual-bus or single-bus sectionalized systems, the bus tie cabinet facilitates switching between different operating modes via the busbar.

For example, in a 10kV distribution system, the main busbar is made of rectangular copper bars, with the cross-sectional area determined by calculations based on the rated current and short-circuit capacity. If the system's short-circuit current reaches 31.5kA, a copper busbar with a cross-sectional area of at least 80×10mm² must be selected to ensure thermal stability during a short-circuit event lasting no more than 0.2 seconds.

1.2 Busbar Structure and Material

Busbars can be classified into three types according to their cross-sectional shapes: rectangular, circular, and槽-shaped. Rectangular busbars are the mainstream choice for switchgear due to their large heat dissipation area and minimal skin effect. Primarily made of copper, they offer a conductivity rate 1.8 times higher than aluminum, resulting in a current-carrying capacity that is 40% greater than aluminum busbars of the same cross-sectional area. In special environments—such as coastal areas with high corrosion levels—the surface of copper busbars typically requires tin plating or protection using heat-shrinkable sleeves to extend their service life.

1.3 Application Scenarios of Busbars

Incoming switchgear: The main busbar connects to the transformer's output terminal, and electrical energy is distributed to circuit breakers via branch busbars.

Bus tie cabinet: In a double-bus system, the bus tie switch enables the two sections of the bus to operate either in parallel or in split mode via the busbar.

Low-voltage drawer cabinet: The vertical busbar features a standardized design, with a rated current capacity of up to 1200A, and supports modular expansion.

II. Circuit Breaker: The "Safety Guardian" of Power Systems

The circuit breaker is the only device in the switchgear equipped with arc-extinguishing capability, enabling it to swiftly disconnect circuits under both normal and fault conditions, thereby preventing accidents from escalating. Its functions encompass power distribution, overload protection, and short-circuit tripping, making it a critical safeguard for the safe operation of power systems.

2.1 The Functional Positioning of Circuit Breakers

The main functions of a circuit breaker include:

Electrical power distribution: Energize the circuit via closing operations to enable power transmission;

Overload protection: When the current exceeds 1.2 to 1.5 times the rated value, the time-delay trip device activates to disconnect the circuit;

Short-circuit protection: When the current exceeds 10 times the rated value, the instantaneous trip device disconnects the circuit within 0.02 seconds.

For example, the DW15 universal circuit breaker can handle a rated current of up to 4000A and boasts a short-circuit breaking capacity of 80kA, making it widely used in industrial power distribution systems. Its equipped electronic trip unit allows for precise setting of protection curves, effectively preventing false tripping.

2.2 Classification and Selection of Circuit Breakers

Depending on the arc-extinguishing medium, circuit breakers can be classified as:

Vacuum circuit breaker: Equipped with a vacuum arc-extinguishing chamber, it boasts strong breaking capacity and a service life of up to 20,000 operations, making it ideal for applications involving frequent switching.

SF6 circuit breaker: Uses sulfur hexafluoride gas as the insulating medium, making it suitable for high-altitude or polluted environments;

Molded-case circuit breakers: Compact in size, with rated currents up to 630A, and commonly used in low-voltage distribution boxes.

When selecting equipment, consider the load type, short-circuit capacity, and environmental conditions. For example, motor circuits should use circuit breakers equipped with overload long-time and short-circuit short-time protection to prevent false tripping caused by inrush current.

III. Isolating Switches: The "Safety Isolation Barrier" of Power Systems

The isolating switch is a no-load operating device used in switchgear to isolate the power supply. Its core function is to provide operators with a visible break, ensuring operational safety during maintenance work.

3.1 Functional Positioning of Isolating Switches

The main functions of an isolating switch include:

Power isolation: When inspecting or servicing circuit breakers or cables, ensure a clear disconnection point is established through the trip operation to prevent accidental re-energization.

Switching operations: In a double-bus system, switching the operating mode is achieved by using disconnect switches;

Small-current switching: Capable of switching the excitation currents of equipment such as voltage transformers and surge arresters.

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Basic Knowledge of Switchgear: Detailed Explanation of the Roles of Busbars, Circuit Breakers, and Isolating Switches