R610 redundant power supply

We are installing Dell Poweredge R610 servers with redundant powersupply's:

High Output Power Supply Redundant, 717W  

So each server has two power supplies.

I'm trying to figure out whether during normal operation, the power draw through each PSU is 50% on the primary and 50% on the secondary.Or whether the primary draws 100% and the secondary draws 0%.

Solution 1:

It will evenly balance the load between the two power supplies except of course under failure. Keep in mind, each PSU can provide all the power needed to run the server. Under extreme load, it will probably still only 75-80% utilize a single PSU. So each one will probably run at 25-40% at any given time. The iDrac can probably give you very good information, and you could track this with an SNMP monitoring system.

Solution 2:

As mentioned in other posts, the redundant power supplies will balance the load between them, and either can take on the full load of the server if the other becomes unavailable (whether by failure, being physically removed, or loss of power on the circuit it's connected to).

The important thing to consider in your electrical load is that EACH circuit that you have a power supply connected to needs to be able to handle the FULL load of the server. For example, if you have electrical circuit A and electrical circuit B and connect supply 1 in the server to circuit A and supply 2 to circuit B, this will provide maximum electrical redundancy. If you lose either electrical circuit then the other may still be available and the server can use that.

A single electrical circuit should not be loaded to more than 80% or so of its rated capacity under normal operation. This allows for occasional spikes and prevents the breakers from being tripped. Since each circuit in this set up would normally only be operating at 50% of its potential, that means that the NORMAL current draw should be at around 40% of the rated capacity under normal operation. If the other circuit is lost then all of the servers connected will suddenly double their use of the remaining circuit, so if you're not careful you could lose one circuit, and then end up over-loading and tripping the remaining circuit if your distribution is wrong.

If you want to get the maximum use of your available current, connect both supplies in a given server to a single circuit. That will ensure that the use of each circuit remains constant even if one of them is lost. The downside is that the servers on the down circuit lose power. If you split the server between circuits, be sure you account for the total potential draw given the loss of the other circuit so that it can handle the load. Which option you choose would be dependent upon how much downtime you can afford and how much extra current you can afford to have available to account for that lower normal operational load you can put on each circuit.