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 Post subject: DC Arc Flash, Effects of Circuit Time Constant
PostPosted: Tue Sep 04, 2012 1:44 pm 

Joined: Thu Aug 02, 2012 6:12 am
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I'm trying to get comfortable with the effects of circuit time constant on arcing time in DC analysis. I'm familiar with the Cynthia Cline paper but it only mentions fuses specifically. Does anyone have any good info on circuit breakers? Can the same method be applied? The manufacturer's I've contacted all say the time current curves can be used without modification if the breaker is rated for DC (which seems dubious to me).


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PostPosted: Wed Sep 05, 2012 7:50 am 
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I'd say the effect described in Cynthia Cline's paper is not as pronounced in breakers as it is noticeable in fuses. Only the upper-left (short and long time-delay, typically 1sec+ ) portion of circuit breaker trip curve displays the breakerÔÇÖs thermal response. The lower right portion of the curve displays magnetic response of the circuit breaker. The magnetic trip is activated not by heat related to RMS current value but by the instantaneous current approaching steady state in approximately 5 time constants (compare to 20X time constants for RMS or heating effect of the current to achieve its steady state value). Also, one should take into account the breaker rated time constant and compare it to the circuit time constant. Same observation also applies to DC rated fuses where the fuse rated DC voltage is associated with some time constant value, and for time constants exceeding that (normally published, typically 10msec) value the fuse DC voltage capabilities vs. time constant chart should be consulted.

Michael Furtak, C.E.T.

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