Dual CPU nodes?

Robert G. Brown rgb at phy.duke.edu
Tue Oct 22 11:52:12 EDT 2002


On Tue, 22 Oct 2002, Ken Chase wrote:

> On Mon, Oct 21, 2002 at 11:28:09AM -0400, Robert G. Brown's all...
>  
> > Most standard circuits have enough capacity to run two duals or four
> > singles - you can estimate that they'll draw somewhere in the vicinity
> > of 400-600 Watts, which is roughly a third the capacity of a twenty amp
> > circuit for a switching power supply load, more like half the capacity
> > of a 15 amp circuit.  Then you've got a monitor and network switch,
> > which can be another 100-200 Watts.  A beefy UPS is not a bad thing to
> 
> wow. thats huge. I had 8 dual 2466s with 1.3tbirds on a 15Amp circuit (no
> disk). When I moved it to a new building, I am guessing that the voltage was a
> bit different, and the power bars they were wired into started blowing.
> (perhaps they were drawing more amps at a lower voltage than my install
> area?) At any rate, I put them in 8 per 20 amp circuit (instead of my
> original intall 8/15) and nothing has blown (oh, and only 4 per power bar
> too now :). Are you spec'ing for some seriously heavy disk power loads?

My recent experiences with 2466's and building wiring are in the recent
list archives and they are a sorry mess, although they are in the
process of running the additional neutral wires required to get us
perhaps semistable.

We are finding that we can run at most 8 per 20 amp circuit here as
well.  One reason for the constriction in number of systems relative to
their average power draw is that the switching power supplies draw a
much higher PEAK current to provide the average power than one expects
-- they have a relatively poor power factor.  Apparently the circuit
breakers used are sensitive to peak current more than rms current.
Perhaps your 15 amp circuit used slow-blow breakers that were more
sensitive to rms current than peak?  The danger of doing this is that
one can dangerously overload primary transformers and cause them to
prematurely burn out.  There are URL's in the stuff I posted last week
(and some of the extended discussion that has occurred on this subject)
that explain all of this.

I now just assume that any system I plug in has a power factor no better
than about 0.8 (and will thus draw a peak current that is 25% or better
high) and, because of the harmonic distortion of the supply voltage
sinusoid, prefer to keep circuits relatively underloaded.  We've seen a
LOT of power related problems on circuits loaded close to capacity,
although the non-shared neutrals may help this (I hope).

> [A friend told me about blowing a circuit once at his lab and he had a large
>  metal bracelet on. When the circuit blew, he was standing under the power
>  run (about 5 feet over his head in the ceiling) and his bracelet shot up his
>  arm and yanked on his wrist, quite hard! He had a bruise to prove it.  It
>  was only a 20A circuit - what would cause that? Dangerous wiring situation?]

I have no idea.  The current in the overhead line should have been
balanced -- hot and neutral currents opposing in the conduit, the
conduit itself grounded to boot to shield whatever field the resulting
inductive surge would have caused.  I don't see how an inductive EMP
could have been generated strong enough to create a current in a
bracelet that would magnetically attract it to the dropping line
current.

Unless, perhaps, the overhead power run contained a single, unshielded
high current line that was grounded at the panel, or there was a
transformer nearby, and the pulse from this was responsible.  Even then
I'd be surprised that such a strong impulse force was generated -- 20
amps isn't all that strong a current, so somewhere a much larger current
must have been induced.

But I'm not a wiring expert.  Maybe this is normal.

> > For that reason you might well want to select systems based on their low
> > power requirements rather than their speed...
> 
> Has anyone here actually done this based on a full TCO analysis where
> operations cost were as large (or larger) a factor than initial install?
> Are TMTA's worth it in this situ?

Sorry, TCO I get, but TMTA's?

   rgb

Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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