[Beowulf] Re: UPS & power supply instability

David Kewley kewley at gps.caltech.edu
Thu Sep 29 17:43:38 PDT 2005

```On Thursday 29 September 2005 15:45, David Mathog wrote:
> > > I liked Jim's idea of trying a
> > > balanced, large resistive load on the lines.  A heater, an
> > > electric range.
> >
> > I want to say (but am not sure at the moment) that the
> > instabilities show up
> > at around 140kW.  I don't have 140kW of resistors available easily,
> > let alone a full-power load of 350kW.
>
> Sure, but if things are really, really, badly screwed up one
> might see a largish neutral current, percentage wise, no
> matter what size the resistive load.  Try three matched
> space heaters (one per phase) and see if the neutral current
> is zero.  If it isn't then you (or your electrician) can
> diagnose at least part of this problem without having to plug

Now that *is* an interesting idea, and a doable one.  I can certainly
try that, although I don't know exactly when. ;)

> For the experts out there: could a huge iron pipe, or a lot
> of iron rebar, produce this much of an inductive load if these were
> located just below the floor, say 6 inches from the power
> lines running to the racks?  The lab in question is on
> the bottom floor of the building so there could conceivably
> be something like that underneath it.  I'm thinking not, but
> then I've never worked with the kinds of currents that are present
> here.

If this magnitude of effect is important, then we probably should also
think further about ground currents in the room, which we have measured
at various points.  We have eliminated the one place where a delta-wye
transformer secondary's neutral was tied to ground at two places.  Now
the PDUs do not indicate any ground current, whereas with the two-tie
topology they did.  All the rest of the equipment (e.g. HVAC) is
3-phase, so should not be a problem.

But as much as 2-3A *is* flowing when we look with clamp-on meters at
various ground cables.  This ground current is likely related to the
fact that there are three ground paths to the room, all referred back
to the same earth electrode point.  These paths are likely tied
together in the room as well as at the remote earth electrode, creating
a loop.

One ground path travels with the computer power lines.  A second is
grounding the raised floor metal components.  And a third ground
travels with the power to the HVAC systems.  To complicate that third
path, the HVAC ground path splits between the HVAC main breaker box and
the HVACs, one path going through a devoted UPS, and one directly
connected to the HVACs (the two branches power different subcomponents
of the HVAC).  Very likely the HVAC and PDU chasses are tied to the
floor grid, thereby connecting all 2-3 paths back to the earth
electrode.

If there's no leakage between neutral and ground anywhere, then the only
other mechanism I can think of that would cause these currents is
induction.  Can induction typically produce currents this large on a
~1000 ft^3 ground loop?  Am I missing something?

David

```