A "48V" rectifier will output 54VDC to the power bus (feeding batteries and systems) since it's battery float voltage. If the rectifier goes offline (loss of AC power) the batteries will power the bus keeping systems online.
I'm curious what the actual difference in practice is between Facebook's 54VDC and Google's 48VDC.
And why -54 VDC? Well typical lead acid chemistry actually yields 2.25 V cells -- 24 of those is 54, so it's another legacy artifact. Modern battery chemistry is different, but so much of the other tech is too. Here's a modern power supply I just found (http://www.tdipower.com/PDF/archive/10A_dcdc_conv.pdf) that supplies between -47 and -54 ... hmm, wonder why?
Thankfully, telecom is fortunately one of the most backwards-compatible industries in the world, which is why the whole rickety edifice continues to work well (and support tons of innovation) even when people use it in ways never intended by its designers.
BTW I'd be disappointed to learn if any reasonable 21st century EE education didn't still include this stuff.
The 19 inch rack is one of the oldest standards in computing. ENIAC used 19 inch racks. Open Compute, though, uses wider, and metric, racks. 19 inch rack gear can be mounted in Open Rack with an adapter.
The Open Compute spec says that shelves of IT gear are provided with 12 VDC power. There's power conversion in the base of the rack. Facebook has standards for distribution to the racks at 54VDC, 277VAC 3Ø, and 230 VAC (Eurovolt). Apparently Google wants to add 48VDC, which was the old standard for telephone central offices.
Facebook's choice of 54VDC distribution is strange. Anyone know why they picked that number?