DC power versus AC power

Robert E. Seastrom rs at seastrom.com
Mon Dec 30 14:18:40 UTC 2002


"Barton F Bruce" <barton at gnaps.com> writes:

> Typical 120/208V small branch circuit breakers in small buildings and homes
> have an interrupting capacity rated at 10,000 amps, and should not be
> deployed where that can be exceeded. It will be on the label.

It's worth noting that the interrupting capacity of the aforementioned
breakers is 10,000 amps *AC*, and that said circuit breakers should
not be used in *DC* applications despite the fact that the voltage is
less than half as much and the fact that they're downstream from a
600A fuse (and have smaller wire in the circuit that will naturally
limit how many amps can go into a short anyway).

I'm hazy on the theory (perhaps someone more knowledgeable can post
it), but my understanding is that with AC the arc has a chance to
quench 120 times per second (ie, every time there's a zero crossing),
and with DC that opportunity (obviously) does not exist.

Bottom line is that one should buy breakers and fuses that are
designed for use in DC powerplants, rather than trying to cheap out
with something you picked up at Home Depot or Pep Boys.  I'm sure I'm
wasting my breath since _nobody_ who reads NANOG would ever try to cut
corners to save a few bucks...  :)

                                        ---Rob





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