How to select the short-circuit breaking capacity of a circuit breaker?

The selection of the short-circuit breaking capacity (Ics) of vacuum circuit breaker needs to comprehensively consider the line short-circuit current, the type of circuit breaker, and protection requirements. The specific principles are as follows:

Meeting Ultimate Short-Circuit Breaking Capacity Requirements

The circuit breaker's Ics must be greater than or equal to the expected short-circuit current of the line (usually the ultimate short-circuit breaking capacity Icu). This is a fundamental requirement to ensure that the circuit breaker can safely interrupt current during a fault.

Distinguishing Between Ics and Icu

Icu: The maximum short-circuit current that the circuit breaker can break; after breaking, it cannot continue to carry the rated current.

Ics: The ability to continue carrying the rated current after breaking; used to measure the circuit breaker's continuous power supply reliability.

For example, the main line requires a higher Ics (50% Icu), while branch circuits can accept a lower value ( 25% Icu).

Requirements Based on Circuit Breaker Type:

Frame Circuit Breaker ( three-stage protection system): Requires a higher Ics (typically ≥50% Icu) to ensure it can withstand multiple opening and closing operations after a fault.

Molded Case Circuit Breaker (Branch Circuit): Allows a lower Ics (≥25% Icu) because the impact range after a fault is smaller.

Reference Standards and Calculation Methods:Select circuit breakers that meet the Icu ratio requirements according to GB14048.2 standard.

Short-Circuit Current Calculation:Transformer capacity, short-circuit point location, and system impedance must be considered. A commonly used formula is:Isc=SbaseZlineIsc​=Zline​Sbase​​

where SbaseSbase​ is the transformer capacity and ZlineZline​ is the line impedance.

Balancing safety and economy: Selecting circuit breakers with excessively large Ics will increase costs; a reasonable value must be determined based on actual needs (such as power supply reliability requirements).

For critical circuits (such as main lines), it is recommended to use circuit breakers with three-stage protection to ensure rapid isolation after a fault.

Avoid using Ics=100% Icu as the sole criterion; judgment must be made in conjunction with the circuit breaker type and application scenario.

Regularly verify the circuit breaker performance to ensure it continues to meet short-circuit current requirements during long-term operation.

By adhering to the above principles, it can be ensured that the circuit breaker guarantees safety while also considering economy and power supply reliability.