What is the service breaking capacity and what is the advantage of that on the reliability of breaker?
The rated service short-circuit breaking capacity (Ics) In a correctly designed installation, a circuit-breaker is never required to operate at its maximum breaking current Icu. For this reason a new characteristic Ics has been introduced. It is expressed in IEC 60947-2 as a percentage of Icu (25, 50, 75, 100%)
The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuit-breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity (Icu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for re-closure, after the faulty circuit has been repaired. It is for these reasons that a new characteristic (Ics) has been created, expressed as a percentage of Icu, viz: 25, 50, 75, 100% for industrial circuit-breakers. The standard test sequence is as follows:
O - CO - CO (at Ics) Tests carried out following this sequence are intended to verify that the CB is in a good state and available for normal service For domestic CBs, Ics = k Icn. The factor k values are given in IEC 60898 table XIV. In Europe it is the industrial practice to use a k factor of 100% so that Ics = Icu.