Problems with dual Athlons (power; popping breakers)

Bari Ari bari at onelabs.com
Thu Aug 1 03:23:28 EDT 2002


Michael Stein wrote:

>>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?)?
>  
>
This is a good idea for the neutrals in a wye supplied power circuit. 
Wye supplied 120/208 circuits should have the neutral conductors sized 
to twice the capacity of the phase conductors. Having 3 separate 
neutrals going back to the panel meets the rating.

>  
>
>>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).
>
Power factor corrected power supplies are the way to go.

>
>  
>
>>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...
>>    
>>
Could be. I've had electrical inspectors only take a quick scan of the 
work from the outside of the conduit  with junction box covers closed 
and maybe a glance at the load centers. It doesn't take much for an 
electrician to combine branch circuits together to create an imbalance, 
esp. in a delta 120/240 service.  The overcurrent protection devices 
(circuit breaker in your case) rating should be 125% of the maximum 
continuous load. 75% load on a circuit breaker may cause them to trip 
esp. when you have nonlinear loads. Circuit breakers found in common 
load centers are thermally activated. They aren't exactly the most 
highly calibrated devices either.

>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?  
>>
The separate neutrals for each 120VAC circuit is a good idea.

>>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?
>>    
>>
The surge strip protectors won't help you with harmonics.

>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).
>
It depends on the power supply design. There may not be enough margin 
built into the power supply to handle an under voltage of 208 VAC when 
it's designed to run off of 240VAC. Most PC power supplies are built 
pretty weenie with much emphasis placed on BOM cost reductions and 
profit margins. Sure for $2 - $5 more you could boost the size of the 
transformers and filter caps but that effects the bottom line too much 
in the cut throat world of PC sales.

Bari Ari


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