DC power versus AC power

Owen DeLong owen at delong.com
Thu Jan 2 23:08:18 UTC 2003


Also, some AC circuit breakers are of a design that counts on the magnetic
properties of AC, and, therefore, won't trip due to ANY DC load.  I think
these are mostly not available any more, but I remember encountering them
some time ago and realizing that it would be _REALLY_ bad if someone put
them in a DC plant accidentally.

Owen


--On Monday, December 30, 2002 9:18 -0500 "Robert E. Seastrom" 
<rs at seastrom.com> wrote:

>
>
> "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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