[Beowulf] Re:UPS & power supply instability

Maurice Hilarius maurice at harddata.com
Wed Sep 28 23:04:21 PDT 2005

David Kewley wrote:

> ..
>One quick update: We finally had a high-level engineering conference 
>call with Liebert this morning, at their instigation.  It was a very 
>good call, and they're being very helpful.  I hope there'll be a 
>workaround soon, but we may have to live with this problem for a while 
That sounds like positive progress..
Did they name a reason this is happening, or are they taking steps to
send someone down with a scope to see what is happening?

>P.S. I suppose you can guarantee that Hard Data power supplies would not 
>induce current oscillations in the room? :)  Didn't think so.
Actually, if the room is wired correctly we can.
We have been building servers and installations in server rooms since
1992, about 2 times as long as Dell have.
And, we DO claim PFC compliance on our power supplies, and can produce
test results and compliancy test reports to back that up.

However, instead of trying to deflect from the real issue, let's look at
what I replied to, word for word:

>Mind you, the "blame" may be shared by the 
>Liebert UPS and the Dell power supplies, but I'm relying on Liebert to 
>figure out why things go unstable *when their UPS is online, supplying 
>a load that should be quite normal*, and so far they have no solution 
>for me.  We can't just wait on Liebert; this problem is hamstringing 
>our use of our new 1024-node cluster.  So now I turn to this list.

You have obviously decided, in advance, that the problem is with the
Liebert equipment.
You mention absolutely nothing about testing the power supplies.
That step should be the first, and fortunately is the easiest.
Almost any modern scope will do the job.
As it is low frequency it does not have to be an expensive or
specialized scope.
Instead of trusting "Kill-a-Watt clones" why not check the actual power
supply response, on a standard 115V single phase power input circuit?

I have seen power regulation equipment fail in a similar fashion before,
where the power supplies are pulling down too much current to the
neutral phase,
and making the power feed overload on one phase, driving it into
This is a classic symptom of cheap, poorly designed and made power supplies.
Or bad room wiring, with undersized neutral lines.

Liebert make big UPS and power units, and those are their "bread & butter"

Frankly I am surprised they have not yet dispatched a tech down to your
site with test equipment by now..

When you say "Liebert has been on this case for something like 4 weeks
now." what does that mean?

>Can anyone here offer ideas, or better yet, experience?
I was trying to.
Apparently you do not appreciate suggestions, except ones that support
your distrust of Liebert.
Why not test the power supplies?
If doing it yourself is not something you are comfortable with, there
are many electrical inspection labs in your region that provide this
service, usually for under $150.
Look in the yellow pages under "testing" or similar.

Many will allow you to stand there and watch and ask questions as they
do it.


With our best regards,

Maurice W. Hilarius        Telephone: 01-780-456-9771
Hard Data Ltd.  FAX:       01-780-456-9772
11060 - 166 Avenue         email:maurice at harddata.com
Edmonton, AB, Canada       http://www.harddata.com/
   T5X 1Y3

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