The server should not be supplied from different phases on 1 UPS

[DCIM_Support]

Hi all

I have been getting this recommendation from DCO 7.5.0 when a device with 6 power inlets is connected to phases L1, L2 & L3 on both AP8886 rPDU's or AP7557 rPDU's in the rack. I have noticed that the devices, typically HP C7000 blade chassis, are connected this way in the belief of balancing the load over each phase.

Q1. Is this a correct approach to balance phase load at the device level ?

Q2. If not then what is the reason for the recommendation ?

Q3. Would best practice be to connect the same phase on both rPDU's for the 6 power inlets ? (i.e. 6 x L1)

Thanks in advance

Jim Davis - NIE Melbourne. 

(CID:105470914)

[DCIM_Support]

Hi Jim

Normally the recommendation we give is to connect load on just one phase. I understand that there can be circumstances where you have to do something else.

There are a couple of reasons for this the advice to put load on a single phase:

1. For some blade chassis it is difficult to know how much load is put on each of the power supplies, since sometimes a power supply powers only some blades or only the fans. If you have the power connections to different phases it is difficult (also for StruxtureWare Operation) to predict the load on each phase.
2. If the load is connected to different phases in a setup with redundancy at the distribution level (all connections are at the top of the power path protected by the same UPS), then the UPS must reserve capacity for the full load of the blade chassis on both of the connected phases. Otherwise it cannot guarantee that the equipment can fail over to the other phase if there is a failure at the distribution level (for example if a PDU or rack PDU fails). This can in the worst case mean that you can only use half of the UPSs capacity. This is the reason for the recommendation you see in Operation.

If you can balance the load by putting one chassis on L1 and one on L2 and a third on L3 and so, I would recommend that instead.

I hope that helps.

Best,

Niels

(CID:105470925)

[DCIM_Support]

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