Medium voltage circuit breakers are the main switchgear used in power systems to control and protect medium voltage (1kv ≤ voltage ≤ 72.5kv) grids, with moderate usage levels and breaking capacity. The basic voltage range is between 1KV and 72.5kV, and the segmented capacity range is generally between 20KA and 40KA. It has a complete arc extinguishing device, which is used to connect and disconnect load current during normal operation. In some electrical main wiring, it also serves to change the operation mode of the main wiring. When there is a fault, it is used to disconnect the short-circuit current and cut off the faulty circuit. Its importance lies in being able to cut off the circuit in a timely manner in case of faults such as excessive current or short circuit, to prevent serious consequences such as equipment damage and fire.

How to choose the required medium voltage circuit breaker, roughly based on the following points:

1. Determine the basic parameters:

（1）Rated voltage: Refers to the rated voltage of the system during normal operation of the circuit breaker. The rated voltage must be greater than or equal to the highest operating voltage of the system (for example, a 10kV system requires a 12kV circuit breaker).

（2）‌Rated current: refers to the maximum current that a circuit breaker can pass through for a long time under specified usage and performance conditions. This parameter determines the carrying capacity of the circuit breaker, and generally requires a 20% to 30% margin (such as selecting 630A or 800A for an actual current of 500A).

（3）Rated breaking current: Refers to the effective value of the maximum short-circuit current periodic component that a circuit breaker can reliably interrupt at its rated voltage, reflecting the breaking capacity of the circuit breaker. It must exceed the maximum prospective short-circuit current at the installation point (determined through short-circuit calculations, such as 25kA or 31.5kA). If the short-circuit current increases rapidly (e.g., due to new energy integration), future upgrade potential should be considered.

（4）Dynamic stable current: Refers to the peak current value reached by the rated short-time withstand current of the circuit breaker in the closed position during the first half-wave, reflecting the equipment's capability to withstand the electrodynamic effects caused by short-circuit currents. Electrodynamic withstand capability under short-circuit current (peak withstand current ≥ 2.5 times the short-circuit current).‌

（5）Insulation level: refers to its ability to block and isolate electric current, that is, the capability to withstand arcing and electric shock. It primarily depends on the operating voltage range and design requirements. Generally, it includes power-frequency withstand voltage (e.g., a 12kV circuit breaker requires 42kV/1min) and lightning impulse withstand voltage (75kV);

（6）Mechanical lifespan: refers to the number of operations that a circuit breaker can complete under normal usage conditions. The mechanical lifespan of different models of medium voltage circuit breakers varies. Distribution requirements: ≥ 10000 operations (such as selecting 30000 operations for capacitor cabinets with frequent switching); Power station demand: ≥ 5000 times

2. Determine the required type of medium-voltage circuit breaker:

Medium-voltage circuit breakers can be roughly categorized based on their arc-extinguishing medium and structure into:

(1) Multi oil/low oil circuit breaker: a circuit breaker that uses transformer oil as the arc extinguishing and insulation medium;

Its structure is simple, but it has environmental pollution issues and incurs maintenance costs.

(2) SF6 circuit breaker: a circuit breaker that uses SF6 gas as the arc extinguishing and insulation medium;

It has strong breaking capacity and is suitable for large capacity breaking in substations and power plants. It has good insulation performance and arc extinguishing ability, but requires special storage and handling, and attention should be paid to greenhouse effect issues.

(3) Vacuum circuit breaker: a circuit breaker that extinguishes the arc with a high vacuum medium, vacuum (pressure below 10 ⁻⁴ Pa);

It is small in size, easy to maintain, has a long lifespan, and is environmentally friendly, suitable for frequent operation scenarios such as power distribution networks and industrial electricity. Generally, vacuum circuit breakers are preferred: suitable for over 90% of scenarios.

(4) Compressed air circuit breaker: using high-pressure air to blow the arc and extinguish it;

Fast action, short breaking time, high breaking ability, can adopt building block structure, strong series. But the production cost is high, and an additional air compression device is required during use.

3. Determine application scenarios and requirements:

(1) Application scenarios: power distribution networks, industrial plants and mines, wind/photovoltaic power stations, etc

(2) Indoor: Generally installed inside switchgear, it is necessary to consider whether the cabinet size matches.

(3) Outdoor: Generally, moisture-proof and stain resistant (IP rating ≥ IP65) are required for outdoor use

(4) High humidity environment: Choose circuit breakers with anti condensation design.

(5) High corrosion: stainless steel casing or special coating.

(6) Extreme temperature: -40 ℃~+70 ℃, wide temperature type is required.

(7) Intelligent requirements: whether remote monitoring, fault diagnosis and other functions are needed

4. Common Error Avoidance

(1) Selecting only based on the rated current and ignoring the short-circuit current resulted in disconnection failure.

(2) Choosing indoor circuit breakers in outdoor environments can easily lead to insulation failures.

(3) Frequent operation scenarios using ordinary mechanical life circuit breakers may cause premature damage.