RE: didn't someone tell me that telco equipment had 40vdc racks once?

The point of all this is that instead of every piece of equipment having it's own switching power supply, with fan, you supply the required voltages to the whole rack from a common pair of failover powersupplies. Each box then gets it's on +12,+5, and -5 (or whatever), and we have one less component per unit to fail.

Converting the power once then distributing it really does beat distributing the AC and converting it at each unit, there are savings in equipment cost, efficiency, and cooling.

Because of this, a 48 volt distribution system doesn't make as much sense, nor does it offer as much advantage over a 120/240V AC distribution system.