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?
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. Also, some hardware only is
available in DC form such as the ons15454's I believe.
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.
Thus spake ip dude <ipdude@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 gear
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
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.
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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?
We've built a number of "joiner" boxes in-house at KTHNOC; basically an
aluminium box with rectifier bridge and heatsink, and screw terminals.
We use them for 2511 terminal servers and similar. So, yes.
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.
YMMV, but 4 times 2x10mm� + 16mm� PE (The DC connects for a 12n16 GSR) I
find bulkier than 4 10A power cords ;-). You are right on spot about the
tidy/sturdy part, though.
- --
M�ns Nilsson Systems Specialist
+46 70 681 7204 KTHNOC MN1334-RIPE
We're sysadmins. To us, data is a protocol-overhe
Thus spake "Wayne Bogan" <wbogan@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.
Well, that's how AC UPS's work -- just beware what percentage of the input
current is lost in the conversion from DC to AC, plus what is lost
converting from AC back to DC inside the equipment. Much simpler to specify
DC equipment to begin with if you really care about availability.
S