Problems with dual Athlons (power; popping breakers)
mas at ucla.edu
Thu Aug 1 11:15:02 EDT 2002
> > Typical (?) PC power supplies are not PFC and draw a big pulse of current
> > on each rising peak (120 times a second for 60 Hz power). If you have
> > a bunch (cluster) of PCs spread over three phases, the pulse from each
> > phase doesn't occur at the same time since the phases are 120 degrees
> > apart so these pulses can't cancel out in the neutral.
> Define "big pulse". If a supply draws order of 1 A rms average, is the
> peak draw 2 A? 4 A? Depending on the response time of the circuit
> breakers, this could explain why they pop well below the rms power
> capacity presumed from their current rating.
I once looked at it with a current transformer and a scope. I seem to
remember something like 3 times. So try thinking of 3A. Since it's a
pulse it's not on full, more like 1/2 or less.
Resistive losses (wire losses) are I squared R. So 3 times the current
gives 9 times the losses but for something like 1/4 to 1/2 the time.
So perhaps (very much "perhaps" and arm waving) 2 to 4.5 times the losses.
> > > In practice, our racks were popping breakers before they reached 75%
> > > of nominal load, making me wonder if they actually had three DIFFERENT
> > > phases sharing a ground...
> > Breakers should only be in the "hot" lines not in the neutral. So this
> > overload of the neutral shouldn't pop the breaker. Rather the neutral
> > should melt and start a fire...
> > I'd worry why you are popping breakers and keep looking for some other
> > problem... And worry about the neutral too.
> I'm doing exactly those two things. They have four circuits per power
> pole, for example, not three, but only three phases. I'm very curious
> as to how they are sharing the neutral, as just a silly little mistake
> would have two circuits with the same phase sharing the neutral. Code
> is conservative enough, and the run short enough, and the room cold
> enough, that this might not start a fire, but we'll all feel better when
> they rewire it with a neutral per circuit.
Watch out. The stories I once heard (didn't happen here) was that you
really need a fatter neutral all the way back to the transformer which
supplies you power. Or else you will just overheat some larger neutral
which feeds your local power panel.
> but first I'm just going to give in and a) put an oscilloscope on the
> lines; b) put a poor man's GFCI (one I wire out of parts downstairs:-)
> inline with a strip and plug in a segment of the cluster to test for
> current imbalance. If the supplies are all drawing a really big pulse
> of current in phase and dumping it on neutral in phase, it could be (as
> Don suggested offline) deforming the waveforms and dropping the de facto
> line voltages at peak to where the push the supplies out of spec.
A very good idea.
I'd suggest a current transformer and scope the current too. The one
I used I pulled out of an old GFCI. Easy enough to calibrate with
a 100 W light bulb.
Or perhaps a clamp on meter attachment (only use a scope). Then you
could scope each hot and the neutral (and ground too while your at it).
Beowulf mailing list, Beowulf at beowulf.org
To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
More information about the Beowulf