On 11/30/2010 9:45 AM, Jack Coats wrote:On 11/30/2010 1:29 AM, Matt Simmons wrote: > Hi Doug, > > I want to announce the first meeting soon, and the date that I'm > aiming for is Tuesday, December 14th (2nd Tuesday of the month). Would > your space be available for that time, say 6:30pm to 8:30pm (or, if > that's too early, I think 7pm-9pm would work fine). Any thoughts on > the time, and whether it's good for the space? > > Thanks again! > > --Matt I'll check, but it should be fine. > Back 'in the day', I remember Sun selling servers being available with > 48VDC power supplies > especially for the telcom industry. > > Most CO's (central office exchanges) had to provide at least 8 hours > of service even with power > grid not available, so to do this they really ran off of large banks > of batteries... 48V DC was the > preferred. And it was used for all the switching and related > equipment at the CO. They charged > the battery bank from commercial power, and normally have generators > for when the commercial > power fails, but still everything ran off of 48VDC. If you needed AC, > you had to run an inverter, > but again, it had to take in 48VDC. > > Why 48VDC? I am sure it is in the history books for the Bell System > and Western Electric somewhere. > If someone knows the original reason, I would be interested. > > In any case, legacy 48VDC system are there, and some vendors take > advantage of it, since telcom > is a pretty big market and the infrastructure for distributing the > power is already well developed. > > If they need some other voltages, they invert it and use appropriate > switching power supplies to get > the desired voltage.
While 48V DC is safer than 110V AC (or 208, or other higher voltages), the copper costs for this infrastructure are prodigious! I've been in some of these telcom rooms and they have huge copper bus bars for distributing the current around. Why 48VDC? Because it's perfect for running the whole site off of industrial batteries, which is what they do, and then charge the batteries. No switchover time. (plus you don't need fancy wave-form matchers to merge disparate sine waves togehter, you can get by with relatively simple diodes). 48VDC is interesting, but not "where it's at" right now. High voltage distribution is much better since you can eliminate some of the overhead of transformation. There are companies, like Validus, which distribute at HVDC (~600V) to the floor and then have inline cabinets that convert from 600VDC down to 48VDC for local distribution, but you need relatively huge car-battery type cables and can only get 2-3 racks from the distribution point before running out of power for a reasonable density. Liebert has some interesting options here too. Better would be if companies would start selling 480VAC power supplies with their machines. Why? Because this is the standard distribution service for almost every building out there, it's well tested, well understood, doesn't require complex, esoteric custom distribution systems, and is compatible with European and Asian systems (+/- some voltage and frequency differences that are relatively minor). You could get rid of several cycles of transformation and get back about 4-8% depending upon your infrastructure. Also, simplifying the motherboard so it didn't require so many different voltages would be a great thing to do. Some vendors have started this, but it's not widespread, yet. (gratis factual tidbits: * you can get 1.73 X more current over the same piece of copper wire using 3 phase distribution vs single phase. * you can get about 5-10% more current over the same piece of copper with DC vs AC, if you try hard (but you might lose it back in your conversion processes) ) _______________________________________________ Tech mailing list [email protected] https://lists.lopsa.org/cgi-bin/mailman/listinfo/tech This list provided by the League of Professional System Administrators http://lopsa.org/
