Black box that allows just an A or B DC feed

Barton F Bruce barton at
Mon Jun 9 18:28:24 UTC 2003

You just need two suitably large diodes. Big ones need heatsinks.

You will find some of the rack-top fuse panel makers that include diode "OR-ing" their main A and B feeds. No switchover time at
all. The power is just there.

Pretty sure Telect has some. Beware of Hendry. Good stuff, but savagely overpriced unless you are buying Bell class quantities and a
huge percent of neat options in the catalog seem to never have been built! They offer every weird combo anyone might want, and then
build when they see the order. You WILL wait if it isn't a popular item on some distributor's shelf.

The Canadian NoranTel folks are VERY responsive and make a nice 1U 200 AMPS per A/B feed panel that has up to 50 amps for each of 4
A/B outputs as well as a pile of grasshopper outputs for smaller up to 15 AMP loads. The unit we get does not have diodes, but if
they don't have something suitable, these folks are very responsive to new products fast.

There may even be some fuse panel vendor with those diodes on the individual outputs., but I don't remember any.

Even worse than that single input device is the sleazy device that LOOKS as though it has dual inputs but is actually single input
with the 2 connections bussed together with no blocking diodes (and no woarnin to customers...) and when wired as 2 seperate inputs
will gleefully backfeed A to B or B to A depending on what died. This is no problem if a fuse/breaker to just this device dies, but
when a main A or B feed dies (either to the rack or worse to a BDFB/CDBD)  everything else on that panel is now being backfead via
the clueless device.

Cisco's CERENT 15454 SONET ADMs will try this for you and the result is typically just losing the 15454 needlessly when a main fuse
feeding it blows because its own fuses can't carry  the rest of the rack/room/<whatever> that it is trying to backfeed and the wrong
one of those 2 fuses to the 15454 may well go first. And a SONET ADM is not what you want to have die - especially needlessly.

We have been seriously considering making a small diode-or device just for the darn 15454s. Main fuses don't go often, but techs
have been known to intentionally kill an A or a B feed for safety during maintenance when they are "sure" everything is dual fed.
Just warn them about 15454s so they can kill the correct backfeed path first.

NB that the fuse panel with diode OR-ed main inputs does fix the backfeed liability, but does not give you the backup of two
individual device fuses for the single input device.

----- Original Message ----- 
From: "james" <hackerwacker at>
To: <nanog at>
Sent: Monday, June 09, 2003 1:38 PM
Subject: Black box that allows just an A or B DC feed

> *This message was transferred with a trial version of CommuniGate(tm) Pro*
> I have a device that only has one DC power input. Is there a box that
> can accept an A & B DC leg, keep both isolated, & and feed my
> device with just A or B ? ie, if the A or B leg fails at the co-lo
> this blackbox will switch to what ever leg is up. Quickly, I hope !
> Or are there other ways to solve this issue ?
> James Edwards
> Routing and Security Administrator
> jamesh at
> At the Santa Fe Office: Internet at Cyber Mesa
> Store hours: 9-6 Monday through Friday
> Phone support 365 days till 10 pm via the Santa Fe office:
> 505-988-9200 or Toll Free: 888-988-2700

More information about the NANOG mailing list