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Talking About Powerage

Members of the ISP-DSL list dissect the history and current state of AC and DC power in data centers and telephone company central office phone exchanges.

[October 7, 2002]

On the ISP-DSL list in September, PK inquired,

"For a small data center for switching/telco equipment, what is the advantage or disadvantage of using AC versus DC power? Why is it that all data centers and telco COs seem to use DC and not AC?"

A number of respondents explained that it's simply a matter of historical precedent:

[RC observed] "The telephone network is designed to work off of 48 V DC. This may have been determined by the power requirements of the ringer (the line voltage is much lower of course) at the time, 80+ years ago. All of the CO equipment works off of 48 V DC; the generator/rectifier recharges the batteries. I remember opening the back panel of an AT&T 3B20 (a computer that processed data for the 5ESS switches) and seeing four 12 V truck batteries. Another advantage is that the equipment is completely isolated from the power grid so in the event of a commercial power failure, there is no switchover and consequently no possibility of switches/computers dropping out due to excessive switchover delay. The equipment doesn't even see it. If you ever get the opportunity to take a CO tour, do it. You'll be impressed (check out the size of those DC power cables)."

[BS agreed] "Many data centers use DC because telcos use DC, and telco COs were the first data centers. Telcos use DC because talk battery (i.e., the power on a standard phone line) is 48 volts DC, and has been since forever. Back when phone calls were switched at a plug-board by a human operator, talk battery was 48 V DC. So when they started adding automated call switching equipment, they designed the equipment to use 48 V DC. When they started adding digital switching equipment, their power supplies expected a 48 V DC input, because that is the power available in a CO. In a CO, everything is powered from 48 Volt battery banks. They are continuously trickle-charged from AC line power. If city power fails, the batteries simply stop being charged: there is no switchover to a UPS. So even modern datacomm equipment, if it is designed to be installed in a CO, will expect a 48 VDC input."

Others discussed the comparative costs of DC and AC:

[EK noted] "Since all telco stuff is battery backed up even if there's a generator, it's cheaper to set up the equipment to simply draw off the batteries, rather than to convert it to alternating current."

[MR countered] "I'd love to see a real cost comparison between a DC plant and an AC plant given the same loads and heat rise (countered with cooling, battery pile ventilation, etc.). My money says DC is more expensive by far."

[BS disagreed] "I'm not so sure about that. Consider that the equipment all needs to run off DC anyway (phones are DC, digital equipment is DC). If all CO power was AC, each piece of equipment would needits own AC to DC power converter. Furthermore, it would all need to be on a UPS, so you would still have your batteries, along with inverters and everything to convert the DC back to AC (only to have the equipment convert it back to DC again). Doing things the telco way, they convert from AC to DC once, at the battery charger. Power supplies in the equipment are smaller and cheaper. Power conditioning is much simpler and cheaper. And no inverter or other DC to AC conversion is needed, as with an AC UPS."

Still others suggested that most data centers now acknowledge the value of both AC and DC:

[CJ offered] "Most telco equipment is DC based. Routers do have options for DC and AC. Most data centers will supply both AC and DC power to their customers."

[RP advised] "DC is very easy to store (i.e. batteries). AC has to be converted to DC, stored in batteries, then inverted back to AC, which is obviously a more inefficient process. And DC will not induce noise: AC can induce noise on data/signal lines. With DC, you do not have to consider harmonics when designing your power plant. With AC, you have to consider harmonics in addition to the usual problems with power, etc. On the other hand, AC is more commonly available for consumer-grade equipment: it's easier to find off-the-shelf PCs that run 120 VAC than -48 VDC. And it's more convenient to convert one AC voltage to another voltage than it is to do the same thing in DC."

—End