DC power versus AC power

jlewis at lewis.org jlewis at lewis.org
Sun Dec 29 05:46:56 UTC 2002

On Fri, 27 Dec 2002, Scott Granados wrote:

> It is likely that in many settings during power failures transition from ac
> street power to ac generator power will have some lag and during that time
> your hardware could loose power.  This of course depends on ups systems in
> use and many factors.  Dc usually however is clean in its transition and
> goes with out saying is battery backed up.

I'll add to that, that since DC removes the need for your own UPS's, by
going with DC, you save rack space, deploy less gear (UPS's are HEAVY),
and don't have to worry about which POPs have how many UPS's with dead
batteries at any given time.  OTOH, since with DC you're unlikely to have
any backup power of your own, it is important to wire up both an A side
and B side.  Some places (like certain telcos) like to briefly turn off
parts of their DC power grids somewhat regularly.  This makes gear with
only one set of DC inputs rather annoying.  Does anyone actually wire up 
both the A side and B side to a single DC power supply and use diodes to 
keep the two supply grids separate?

DC also avoids bulky AC power cords...and not only are the wires less
bulky, but you'll likely cut them to the actual length needed.  Since DC
wiring is usually screwed down, they don't get bumped or accidentally
pulled out of the outlets as often.

 Jon Lewis *jlewis at lewis.org*|  I route
 System Administrator        |  therefore you are
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