Bill has it. DC requires much greater/faster contact separation than the equivalent AC circuit. By connecting the circuit breaker contacts in series you achieve this.
Bill is correct. Since the DC has no zero crossing voltage there must be a greater gap to extinguish the arc. Be advised, however, a vacuum interrupter may not be suitable for this application depending on the voltage and current.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
It is also a matter of how the breakrers are listed. Many 3-pole AC breakers are Listed (after testing by UL or such) for DC use. Many 1P and 2P MCCBs are also listed for DC use, typically in smaller ratings.
In higher ratings, the 3rd pole comes handy for the reasons mentioned above. More importantly no special breaker has to be manufactured, it is basically an AC breaker tested and listed for DC use. Most of 3P breakers are listed in certain wiring configuraiton showing which poles will be seriesed.
Thanks all for yr valuable coments..
but i m still confused that why dc require much greater contact separtion?how can u achive by connecting CB contacts in series?
AC nicely provides a current zero 100 or 120 times per second, DC never does. If one contact opens 1/4 inch (made up number) then three contacts in series provide 3/4 inch of air gap, three times the dielectric strength to interrupt the current that doesn't want to be interrupted.
