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Racks with 3-phase PDU's, connecting power supplies to different phases

chuckk281
Trusted Contributor

Racks with 3-phase PDU's, connecting power supplies to different phases

Richard had a 3-Phase load balance question:

 

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Customer needs to know if he has a ProLiant server with dual redundant power supplies, could he connect the power supplies to  different phases, so PSU1 on PDUA-L1 and PSU2 on PDUB-L2

 

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From Greg:

 

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While the answer is yes they can, The question is “should they.”

 

The BladeSystem planning guide illustrates in that diagram a mechanical method of distributing the Load generated by three (3) enclosure hosted in a single rack.  Given that these enclosures would occupy 30rackunits this would be a reasonable population of c7000s in a single rack.  The 3Ph power cabling equally “stripes” the c7000 power connections across the phases.  Note in the diagram there are A and B side 3PH supply PDUs.  This means that the phase loading for that rack should be more evenly distributed.  When this is then replicated across large numbers of racks within a DataCenter the 3ph power is more evenly consumed.

 

Back to a single proliant with dual power supplies.  Yes there is nothing stopping the customer doing that.  If the customer does this 20 times in a rack then you will start to unbalance the consumption in the phase supplies.

 

Personnally I believe that you should not recommend the Customer’s power  topology.

 

However consider what the value is of application(s) hosted.  Should the 2nd power supply be serviced back a power circuit connected to a different power feed?  Generally DataCenters have two separate power suppliers.  Each may have their own Distribution Board.  Racks should be supplied by circuits from diverse power feeds.   A Rack with single DL/ML/Integrity rack based servers still should have diverse feeds, though the total rack power consumption may allow the DC manager to service that rack with 1ph power circuits. If you have cClass enclosures then the hosting rack should have 2 x 3ph circuits 32amp.  Failure within one supplier will not drag down a well connect application.

 

Note: with the new iPDU management intelligence, the PDU will talk to both connected power supplies and determine that the same server is connected to the same iPDU.  If the DC manager has programmed the iPDU to warn of redundancy violations the iPDU will flag this connection red.  This is not a redundant power solution.

 

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Also from Rob:

 

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One word of caution on the best practices for cabling the c7000’s in 3-phase.

 

If you have a large # of racks (I can’t really quantify large) filled with C7000’s all cabled to these best practices, then you will most likely cause an phase balance issue.

The cause of this is that the same power supplies within each enclosure wind up in standby mode.

So for example, if ps #3 & #6 were always connected to phase A and were always in standby mode, then power supplies 1,2,4 & 5 would be bearing the load of the enclosure, thus causing an imbalance with A.

 

I have 1 customer with a large installation as described above.  HP did Factory Express pre-configuration on all of the racks and cabled them all identically.  They could have juggled the power cords around, but chose to disable the power controls within the OA to evenly distribute the power load between all 6 power supplies.

 

So – all good things aren’t always good

 

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And from Dave:

 

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The other option is to turn off the Dynamic Power feature for three phase.

We have one site that uses only the three phase and they request this to be off for all of their enclosures. It balances all phases with it off.

 

You would be ok after the cable swap if there are three enclosures in the rack and never lost a power supply.

Two or four enclosures could be a challenge to cable.

If you pulled a PS or lost one the balance would be lost.

 

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What have you done in these situations? Let us know please.