Powering dual DC power supplies on switches

[acesea]

I've seen some Foundry and Cisco switches with dual DC power supplies. What benefits are there to using such DC devices as opposed to 120 and 240v AC in racks? How are such DC devices best powered in a rack?

 

[k1pp3r]

Depends on the switch, some may just be there for redundant Power supplies, some may be to get POE on all ports (which requires extra power)

The main benifit of dual PS's is you can plug them into A and B circuits so you have a failover if one circuit fails for some reason. Your also protected from a PS failure.

 

[cymon]

-48VDC is an old telco standard. That's what they use to power the talk battery on analogue phones (the bias for the mic has to come from somewhere). One very nice thing about 48VDC is that it's super easy to set up a UPS for. Take 4 lead acid batteries, at 12V each. Connect in series, which will yield you 48 volts.

The disadvantage is that lower voltages need more current to produce the same power. Therefore, a 6509 loaded to the gills with PoE line cards that is going to pull 5000 to 7000 watts is going to need some very fat power cables (which are very expensive).

5000 watts at 48 VDC requires 104 amps. 5000 watts at 208VAC requires 24 amps.

 

[epimetheus]

DC power is common in the telco and power industries. I design high voltage substations for a living. Our standard setup is a 125VDC battery bank sized to account for an 8 hour charger failure followed by the worst case single failure scenario. Essentially we have to maintain full protective operations at the end of the 8 hour period. Industrial UPS systems also use 125VDC or 250VDC battery banks on the DC portion of the system. Just as cymon said, the economics force the voltage to go up to keep the current reasonable.

 

[acesea]

Gracias for the good info but I should have been more specific.

I saw this foundry dual dc switch on ebay and noticed that it was being scooped up quickly (i can't find spec anywhere, does this FESX424 input 48VDC?):
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270629153967

Hypothetically, how would a typical power conversion topology be setup for several of these switches? And then how would a more loaded environment appear?

If only 5 of these draw from ~500-1000watts, plus some other devices with less or more draw should VDC industrial UPS systems be installed or are ad hoc custom battery banks (deep cycle lead batteries?) a good alternative? What VAC to VDC interface would be good for such a setup?

edit: seller notified me that the switch DC power supply: input 37-72VDC, 9A.