Problems with dual Athlons (power; popping breakers)

Michael Stein 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).
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