How to understand the parameters of a frame circuit breaker?

Analyzing the parameters of a frame circuit breaker requires focusing on the frame current, rated current, and protection setting parameters, as detailed below:

Frame Current (Iu)

Definition: The maximum current value that the circuit breaker frame can withstand, determining the physical dimensions of the equipment and the specifications of the current-carrying conductor.

Function: Ensures the maximum rated current matching of the installed trip unit, supporting functional expansion.

Example: In model NSX100N-TM80D, the number 80 in the yellow box indicates a frame current of 80A.

Rated Current (In)

Definition: The maximum current for long-term stable operation of the trip unit, based on GB/T14048.2 standard, verified by temperature rise testing.

Function: Limits the line load current, preventing contact overheating and aging.

Example: In the same model, the number 100 in the green box indicates a rated current of 100A.

Protection Setting Parameters

Overload Protection: The setting current range is typically 1.05~1.2 times the rated current (e.g., 105~120A for a 100A load).

Short Circuit Protection: The instantaneous trip current (Ic) must be greater than the peak short-circuit current, and the short-delay current (Is) must be 5~10 times the rated current (e.g., 1000~2000A for a 200A circuit breaker).

Relationship: Frame current ≥ Rated current ≥ Setting current (Iu ≥ In ≥ Is).

Selection: Ensure the frame current ≥ the maximum line current (including short-circuit current), the rated current matches the load requirements, and the protection setting parameters cover fault scenarios.

Avoid referring only to theoretical peak current; actual operation requires consideration of temperature rise and equipment lifespan.

Selection must be combined with system short-circuit current calculations to ensure the ultimate short-circuit breaking capacity (Icu) ≥ the peak short-circuit current.

By analyzing the above parameters, the applicability and protection performance of the frame circuit breaker can be systematically evaluated.