[54458] in North American Network Operators' Group
Re: DC power versus AC power
daemon@ATHENA.MIT.EDU (Owen DeLong)
Thu Jan 2 18:08:46 2003
Date: Thu, 02 Jan 2003 15:08:18 -0800
From: Owen DeLong <owen@delong.com>
To: "Robert E. Seastrom" <rs@seastrom.com>,
Barton F Bruce <barton@gnaps.com>
Cc: nanog@merit.edu
In-Reply-To: <87hecvoain.fsf@valhalla.seastrom.com>
Errors-To: owner-nanog-outgoing@merit.edu
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@seastrom.com> wrote:
>
>
> "Barton F Bruce" <barton@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
>
>
