Dual Power Supply Server with one on a UPS one not

[Anonymous user]

I recently had a conversation with a colleague on the issues of providing power to a server with dual power supplies. He argued that providing power to a server by plugging one of its power supplies directly into a plug strip and plugging the other power supply into a UPS that was then plugged into that same power strip would cause a shift in phase of the power being provided to the server which would in turn cause a decrease in the life expectancy of those server power supplies. I don't doubt that the UPS could change the phase of the power being sent to the server. However, I believe that modern power supplies convert the incoming AC signal to a DC voltage which they then use to construct new pulsed AC waveforms to generate the multiple internal voltages needed which are then heavily filtered before being sent on to the server circuitry. If this is true, then the original incoming waveform phase shift between sources is not an issue and would not be detrimental to the longevity of the power supplies. Can anyone give us a definitive authoritative answer?

[BillP]

"Definitive and authoritative" probably don't apply to an answer to this question.  Depending on the type of UPS you have, the UPS may "reconstruct" the incoming waveform when on commercial power or simply pass the input to the output through an internal switching (relay) arrangement.  In the case of passing the input through to the output there wouldn't be any appreciable phase shift at all unless you are concerned about the additional length of the wiring inside the UPS itself.

In any event and although I could be wrong, I don't think today's modern power supplies configured redundantly within a server "care" one bit about the phase of the incoming AC waveform.

This could end up in a discussion much like happens when the advocates of single rail vs. multiple rail power supplies get together over a brewski.

[BillP]

Hi,

We have Kbase FA158852 "Using surge strips with APC's Back-UPS and Smart-UPS products". 

There would not be a phase shift however there could be a short, disruption of ground. If you are going to use a plug strip you should plug 1 power supply into it, the other into the UPS, and the UPS directly into the wall outlet. It would be even better if the wall outlets the plug strip and UPS are connected to are on different circuit breakers to mitigate a single point of failure. 

[Anonymous user]

Thanks Joe: I agree that it is most likely that the system doesn't care but I was hoping to get some kind of documented opinion.

[Anonymous user]

Thanks Bill, good information but not quite what I'm after. Our situation is we are building a new server room. It will have three racks with three independent hard wired power connections. These power multiple outlet power strips inside each rack. My plan is to plug a UPS into this power strip and plug one of the server redundant power supplies into the UPS and the other directly into the power strip. No surge suppression at this point. While trying to determine the size and number of UPS devices we got into the discussion mentioned above. We didn't want to create a single point of failure by connecting both power supplies to the same UPS.

[Terry_Kennedy_apc]

It is quite likely that the power coming into your building is either 3-phase or two phase legs of the three. Normal circuit breaker panels alternate between supply feeders in either an ABCABCABC OR ABABABAB sequence, so if the two power supplies in your device are fed from different circuits, it is quite likely they're on different phases already.

[BillP · Accepted solution]

HI,

I asked one of our engineers that work on the Smart-UPS product line. He said it would not make any difference because the power supplies work independently of one another. There should be no increase of mean time between failures by plugging one supply into a PDU and the other into a UPS. 

[Anonymous user]

Thanks Bill. I appreciate your taking the time and trouble to find an answer for us.

Ron