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What is the dc component for medium voltage circuit breakers? How does it affect the breaking capability?

According to IEC 62271-100, the rated short-circuit breaking current is the highest value of current that the circuit breaker must be capable of breaking at its rated voltage.
It is characterised by two values:
- the r ms value of its periodic component, given by the term: “rated short-circuit breaking current”, and
- the percentage of the aperiodic component.

According to IEC, the circuit breaker must break the rms value of the periodic component of the short-circuit (= its rated breaking current) with the percentage of asymmetry defined by the graph attached.
The relatoin between symmetrical breaking current, asymmetrical breaking current and the percentage of d.c. component is given in attached file. 
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