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

[Charles Steinkuehler]

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Billy Crook wrote:
> DC is also great at:
> Corosion
> Explosions
> Arc welding
> Electrocution

:)

Your comments are generally correct, but don't take into account current
technology.  Comments inline.

> I wonder if the cost savings take into account the price of all that
> thick copper needed to transmit DC throughout the datacenter.  I've
> seen firsthand, 2 inch copper cables.  There's a datacenter downtown
> that already has dc infrastructure in place, and a big battery, power
> stepping/switching, and UPS room.  It might be more energy efficient,
> but I wouldn't bother with it, unless you could deliver +-12, =-5, and
> +3.3 to the rack.  DC-to-DC conversion is notoriously inefficient.  If
> you totally ruled that out, you might actually *see* some of the power
> savings.  

Not any more.  DC-to-DC conversion can now be *VERY* efficient, thanks
to low RDs-On MOSFETs, high switching speeds, and a variety of
interesting switching topologies (ie: the multi-phase DC-DC switcher
that's likely sitting on the MoBo of your computer, feeding 50-100 Amps
of low-voltage into your CPU with tight regulation).

> The other problem I'd see is with DC, the voltage
> (noticeably) decreases over distance.  

No more so than AC.  Voltage drop is a function of the resistance of the
wire and the current you're passing through it.  The reason AC works
better for long distances (ie: from your house to the power plant) is
they run the transmission lines at *VERY* high voltages.  High voltage
and low current = lots of power with low resistive losses.  The voltage
is reduced at power substations, and again at the transformer behind
your house.  DC is actually slightly *MORE* efficient than AC, as you
only have resistive losses in the cable.  AC also includes losses due to
the impedance of the transmission line (parasitic inductance &
capacitance making a low-pass filter that increases the effective
resistance of the line).

Back in the day, the technology didn't exist to (easily) convert DC
power from one voltage to another, but transformers did the trick for
AC.  With modern power circuits, however, you can convert pretty much
anything to anything with very high (90+ percent) efficiency.

> If that's noticeable in a
> datacenter, you'd have to have same-length runs to each rack.  Might
> be especially nice if you didn't have to cool the large AC-to-DC
> substation as much as the datacenter, or if you could just put it on
> the roof.  It will also probably be less safe, by the nature of DC not
> pulsing to release two shorted connections.  DC circuit breakers are
> more expensive, and most folks are less experienced with DC.

It sounds like you're referring to arc quenching, which is mostly of
concern for things like relays and mechanical switches.  For active
circuit protection (think GFCI outlets for AC vs. a mechanical breaker)
the costs should be basically identical between AC and DC, although
obviously demand and production volumes play a roll.

As far as safety, generally, AC won't make you any less dead than DC if
you grab onto something you shouldn't.  High frequency AC will create a
'skin-effect' where the bulk of the current will run along the surface
of a conductor (ie: fry your skin which can recover and not your
internal organs which won't), but 60 Hz power is too low a frequency to
exhibit any meaningful additional safety margin vs. DC due to skin
effect.  Even much higher frequencies are still very dangerous:

http://en.wikipedia.org/wiki/Tesla_coil#The_skin_effect_myth

- --
Charles Steinkuehler
charles at steinkuehler.net
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