What Are Icu Ics Icw and Icm of Circuit Breakers?
What is Icu?
Icu stands for ultimate breaking capacity of a circuit breaker, which is the maximum fault current the circuit breaker can interrupt without damage. For Miniature Circuit Breakers (MCBs), the maximum Icu typically ranges from 6 kA to 10 kA, while for Molded Case Circuit Breakers (MCCBs), it can reach up to 200 kA.
What is Ics?
Ics refers to the rated service breaking capacity, or service short-circuit breaking capacity. It indicates the short-circuit current that a circuit breaker can successfully interrupt under normal service conditions, followed by continued reliable operation. After testing, the circuit breaker's performance is evaluated, and Ics is defined as a percentage of Icu. Common values are 20%, 30%, 40%, 60%, 70%, and 100%, depending on the application.
Icw: Short-Circuit Withstand Capacity
Icw denotes the rated short-time withstand current—the level of short-circuit current a circuit breaker can endure for a specified time (typically 0.1 to 3 seconds) without suffering thermal or mechanical damage. During this period, the integrity of the circuit breaker’s temperature and physical structure must remain intact. Since circuit breakers require a short time to open during a fault—typically 20 to 30 milliseconds for Air Circuit Breakers (ACBs)—the fault current may complete two to three cycles. Therefore, the breaker must be designed and tested to withstand this current. Generally, Icw follows the order: Class A MCCB < Class B MCCB < ACB.
Rated Making Capacity (Icm)
Icm is the maximum instantaneous current that a circuit breaker is capable of safely closing onto at its rated voltage under specified conditions. In AC systems, Icm is related to Icu by a multiplying factor k, which depends on the power factor (cos φ) of the short-circuit current loop.
Example: A Masterpact NW08H2 circuit breaker has a rated ultimate breaking capacity (Icu) of 100 kA. The peak value of its rated making capacity (Icm) will be 100 × 2.2 = 220 kA.
