DC power versus AC power

Kuhtz, Christian christian.kuhtz at BellSouth.com
Sun Dec 29 19:37:19 UTC 2002

From: Wayne Bogan [mailto:wbogan at infoave.net]
> For those AC only powered units, you can also purchase an invertor 
> for DC to AC conversion.  You would then have the advantages of DC
> for your AC equipment.  This does, however, add the potential of 
> another point of failure such as fuses or breakers in the invertor.

Not to mention additional cost of wasted electricity (which does add up
significantly when it is anything but a spot solution) and pitfalls of
inverters (like their imperfect sine waves).  And if you're putting spot
solution UPS units out into the bottom of a particular rack, be ware their
canny ability to catch fire when the price is right.

Inverters or rectifiers should be point solutions in a plant layout.  If you
have a lot of AC & DC, get two protected plants.  Only way to live.
Sometimes that's not just nice to have, but simply required.

If you're primarily a systems shop and have environmentally conditioned
space, get AC.  If you're in a nasty colo spot, with poor environmental
(read: not guaranteed meat locker climate 24x7), get DC & NEBS hardware or
hardware which meets NEBS almost entirely but doesn't carry the sticker for
some silly reason such as "easily flammable plastic used for front grill".
Depending on what your colo requires.  There are many different flavors of

Speaking of flavors, consider the impact of a turf vendor based on whatever
decision you chose when going into colo space.  May or may not be an issue.

See what equipment offers what, do the math on cost of plant and gear, pros
& cons.  And make a business decision based on engineering data. (uh, right)

But, as Stephen already eluded to... Compared with an AC plant design, to
me, one of the biggest drawbacks of a DC plant is safety  (I have had to
kick a fellow worker away from the rack before).  One of the biggest
advantages is simplicity of design when it comes to battery plant.  

There is no universal answer, it all depends.  And one ends up shuffling off
and having to do a bunch of homework.

Of course, this is all IMHO and I've been wrong before.


-----Original Message-----
From: Wayne Bogan [mailto:wbogan at infoave.net]
Sent: Sunday, December 29, 2002 1:06 PM
To: nanog at merit.edu
Subject: Re: DC power versus AC power

----- Original Message -----
From: "Stephen Sprunk" <ssprunk at cisco.com>
To: <ipdude at cattle-today.com>; <nanog at merit.edu>
Sent: Sunday, December 29, 2002 1:58 AM
Subject: Re: DC power versus AC power

> Thus spake ip dude <ipdude at cattle-today.com>:
> > Hello NANOG group. I am trying to make a case for using DC power
> > supplies versus AC power supplies for typical IP networking
> > equipment. Is there any published whitepapers detailing this subject?
> > Do you have any suggestions to aide my argument?
> Most of the argument depends on the facility you're in.  Assuming you're
> asking as an end-customer:
> DC requires clue from your staff when installing/removing equipment, and
> this means safety training at a minimum.  Power choice also affects your
> equipment purchasing: DC versions of gear are often priced higher, and
> not intended for telcos/ISPs may not have a DC option available at all.
> OTOH, many colos -- especially ones run by telcos -- don't provide AC UPS.
> If you want AC UPS in these environments, you'll have to provide your own,
> which is expensive, bulky, and a maintainance burden.
> If you're building your own datacenter, please specify that and I'm sure
> you'll get a whole different discussion :)
> S

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