What is the difference between tripping mechanism of DC MCCB and AC MCCB?

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MCCB designed for interrupting an AC load do not require the contacts to separate as fast as for a DC load since AC goes through a zero crossing every half cycle. The arc will be extinguished each time the voltage goes through a zero crossing, provided the load is not highly inductive. The contacts only have to get far enough apart to prevent re-flash. Not so with interrupting a DC load. Not only do the contacts have to separate quickly, there also has to be a means of quenching (cooling) the arc to extinguish it since there is no zero crossing in DC Loads.