c7000 single phase -vs- three phase power?

[Benjamin Boone]

We are having some internal debate over whether to order a single phase or three phase c7000 enclosure. We already have 3 phase power at the rack with a PDU that breaks each phase into single phase sets of plugs. Page 43 of this design document: http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01038153/c01038153.pdf mentions a PDU with 3 phase inputs, and 3 sets of "single phase" output plugs. It suggests the load should be balanced across the phases. If we already have an equivalent PDU in place, is there any efficiency to be gained by ordering a c7000 with 3 phase inputs and having our rack rewired? Since both single phase and 3 phase versions of the enclosure use the same power supplies, doesn't the 3 phase version of the enclosure essentially do what the PDU does? Thanks, Benjamin

[darrenjmurphy]

Pretty much the same situation we have, we chose to go down the single phase chassis route using 3 phase PDU's to split out the phases to the chassis as in page 43 of the doc you mentioned. One of the main reasons we like this idea was that in our two UK based data centre's the newer one uses 3 phase feeds to the enclosures but our older one is the more traditional single phase so by doing the conversion at the PDU we can use the same chassis model in either data centre. One thing we haven't had any confirmation on is whether the phase rotation needs to match across the PSU's (mentioned on page 34 for the 3 phase model) so does PSU 1 need to be on the same phase as PSU4 and so on. Another consideration is that by using 3 phase feeds on to a 3 phase chassis you tend to get through circuit breakers at an alarming rate with each 3 phase feed taking the necessary 3 MCB's so if you use 2 Chassis per rack you've already used 12 MCB slots on you distribution point. If you go down the single phase route you can install 2 x 32AMP 3 phase sockets using 6 MCB's and giving you a 96 AMP redundant feed which should happily feed a several c7000 enclosures (as in page 43)

[Tony Harvey_1]

Benjamin You are correct the 3-Phase enclosure essentially does what the 3 to Single phase PDU does. As far as phase rotation goes the power supplies do not care about phase rotation they will work from any collection of phase angles. From a purely personal standpoint I would tend to keep the rotation consistent, the c7000 doesn't care but he something else in the rack might and I like documented consistent things, it makes fault finding so much easier. Tony

[Benjamin Boone]

Thanks everyone. Does anyone have a schematic, or know how the three-phase c7000 enclosure connects the PSU's to the phases of a three-phase circuit? It has been suggested here - (http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=1292141&admit=109447626+1228059483054+28353475) - that there could be an advantage to the single phase c7000 with 3-phase to single PDU. This due to the ability to manage the distributions of PSUs and load across the individuals phases of a 3 phase circuit. How does the 3-phase c7000 distribute the PSU's across the 3-phase circuit? Benjamin

[Tony Harvey_1]

In the 3-phase Input Module you have 2x 3-Phase power cords, 1 Power Cord connects to Power Supplies 1,2,3 the other to 4,5,6. if you lose the to power cord 1 you lose power supplies 1,2,3 the enclosure will continue to run on 4,5,6 assuming you configured for N+N AC feed redundancy. In the forum what they are refering to is the DPS function that optimizes power supply efficiency by placing power supplies that are not currently required in standby. It works by putting PSUs in standby in pairs so 3 & 6 go first, then 2 & 4 while 1 & 3 always remain on. In the 3-Phase enclosure this can unbalance the phases. In the single phase enclosure attached to a 3-Phase PDU you can compensate with multiple enclosures by rotating the phases across power supplies. The diagram on page 43 of the c-Class Site Planning Guide which you linked to in an earlier post shows how to do this.