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# Problems with dual Athlons (power; popping breakers)

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Michael Stein mas at ucla.edu
Wed Jul 31 17:11:57 PDT 2002

```> One thing that they did that had me a bit concerned from the beginning
> was wire the power poles by pulling all three phases to a power pole and
> coming back with a single common ground.

Is that 'ground' (grounding conductor?) or 'neutral' (grounded conductor?)?

> Of course if you have three equal, purely resistive loads on all three
> hot wires you could in principle grab the common cold wire in your hand
> as it would carry no current (Kids! Do Not try this at home!)

Power factor correction (PFC) power supplies should be very close to
looking like a resistive loads and the currents in the neutral should
cancel out (assuming balanced loads on the three phases).

> but in practice it is not at all clear that the load currents of a
> cluster will all be in phase with the current (or even shifted by a
> common amount).

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.

> 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.

> Are there published standards for wiring clusters (or similar
> environments)?  We will soon be having them redo the power pole wiring
> so each line has its own ground.  Is there anything (relatively
> inexpensive) we can add or should insist on at that point to condition
> the power and isolate the harmonics so that they don't bleed through
> from system to system?  I think dual isolation transformers are beyond
> our means, but could a high-quality surge strip (perhaps one with
> limited UPS capacity) serve the same purpose?

One possiblity I've thought of but haven't tried would be to switch the
PC power supplies to the 240 setting and run them off of 208 (between
two phases of the three phases).

You'll need some special power strips (probably a powerstrip with IEC
320 C-13 outlets) and some C-13 to C-14 power cords for the machines.

The pulses will still exist but won't be going into the neutral (since
it's not connected).

I haven't tried this, but I think that some large rack mount systems
do this...

```