MOTOR PROTECTION CIRCUIT BREAKERS 


What do the two values of breaking capacity Icu and Ics, indicated for motor protection circuit breakers, represent?

The breaking capacity of a motor protection circuit breaker is established by tests conducted according to product standards, IEC/EN 60947-2, and is divided in two categories:
Icu: Rated ultimate short-circuit breaking capacity
Ics: Rated service short-circuit breaking capacity
The tests include the sequence of operations:
O - t - CO for Icu
O - t - CO - t - CO for Ics, whereas:
O =     a breaking operation in short-circuit conditions. In this phase, the circuit breaker will be connected by an external device
t  = time interval between two successive short-circuit operations
CO = a making operation followed by a breaking operation in short-circuit conditions.
After these tests, the breaker undergoes verifications for dielectric withstand and overload releases operation. Only for the Ics capacity, testing also includes over-temperature and operational performance (opening and closing at rated voltage and current values) verifications.

Specifically speaking, the standard prescribes the breaker must maintain its performance characteristics after it has tripped for short-circuit conditions at Ics value while some of these can be reduced after tripping for short-circuit conditions at the Icu value .
  
  
   

     
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