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Re: DC power versus AC power

  • From: David Lesher
  • Date: Mon Dec 30 10:39:19 2002

Unnamed Administration sources reported that Robert E. Seastrom said:
> 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...  :)

Gawd yes. 

We all know those little 3AG glass fuses, right? They also come
in ceramic. A regular PITA -- you can not look and see they are

But they are there for reason. They are typ. full of power to
quench the resulting arc. A glass 3AG can and will open, yet the
arc just keeps going....slagging the fuseholder, and whatever was
errr... protected.

There is as much diversity and engineering in fusing as router
design. Voltage to be broken, AC vs DC, time curve to open, other
factors all enter into it. I see two fuses in series on "pole pigs"
primaries around here. Finally had a chance to ask the foreman.
One is {say} 10A and that is sized for overloads. The second is
15A, but its sole function is to open FAST if the primary takes a
lightning hit and the spark-gap on the transformer conducts. Seems
they can't get both qualities right in one fuse so they use two.

And RES is correct; DC circuit breakers are different animals than
AC by a long shot. Fuses have different ratings as well. RTFM.

Please be careful; while you might [now] be easily replaced, the
suffering VC's will not appreciate losing all to a dropped tool.

ps: I've not even mentioned the acid and H2 gas [kaaBOOM] issues...

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