Grounding..Re: [Beowulf] Re: UPS & power supply instability

Jim Lux James.P.Lux at jpl.nasa.gov
Fri Sep 30 12:36:20 EDT 2005


Folks.. all this discussion about grounding should reference the "tome" on 
these things..

IEEE Std 1100-1999 ( I assume that's the latest rev..)
IEEE Recommended Practice for Powering and Grounding Electronic Equipment

aka "the Emerald book"

This extremely handy 400+ page book talks all about grounding strategies 
(including how it is impossible to build something with "one true ground".. 
unless you give up shielded cables), harmonic content, induction, etc.etc.etc.

Anyone who has responsibility for putting together anything bigger than a 
couple boxes sitting on a shelf, even a single rack, should own a copy, if 
only to be able to point out relevant sections to vendors, bidders, 
facilities folk, etc.

There's quite a nice discussion about "Conflicting design philosophies for 
performance and safety", which is good reading because, inevitably, what 
YOU want to do for grounding will conflict with someone else's 
understanding and interpretation of the NEC.  Tactful and clever 
interpretation of such regulatory requirements is essential.

As you've discovered, wiring up a hundred anythings in a room is 
substantially different than the run of the mill residential, industrial, 
or commercial electrical wiring practices.

Another book, slightly less useful, but good for context, is IEEE 142-1991 
IEEE Recommended Practice for Grounding of Industrial and Commercial Power 
Systems (the "Green" book).  This one is more aimed at factories and such, 
but is useful, because it describes all the schemes your electrician is 
likely familiar with.  (this one covers all those 
neutral/ground/delta-midtap connection issues, for instance, and why you 
might want a high impedance grounding system, etc.)

At 08:38 AM 9/30/2005, Robert G. Brown wrote:
>David Kewley writes:
>
>>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.
>
>James Lux, P.E.
>Spacecraft Radio Frequency Subsystems Group
>Flight Communications Systems Section
>Jet Propulsion Laboratory, Mail Stop 161-213
>4800 Oak Grove Drive
>Pasadena CA 91109
>tel: (818)354-2075
>fax: (818)393-6875


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