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So I've been tasked to document and review the status of the UPSes for my current employer and identify any changes that need to be made. Generally easy stuff, however I came across a few units that are plugged into both regular power and a UPS outlet. I've done similar before but it reminded me of an electrician I used to work with who said doing this could damage the equipment.

I've been looking online to figure out what the specifics of this warning was about, but I haven't found much. From memory, I think it had something to do with using circuits from different sources provided by Three-Phase power. I was working on industrial sites at the time, and some power was provided to my servers by three-phase transformers. Not all was, however.

My question is: Are there any instances where running a system with redundant power supplies could be damaged by running them on different power circuits, assuming both circuits are clean (eg, running both PSU on either circuit alone would cause no damage).


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Wait a second, are you saying that you came across redundant power supplies where, in the same system, one or more PSU is connected through the UPS and one or more other PSU is connected in a way that bypasses the UPS? – Michael Kjörling Jan 10 at 14:10
At a minimum, you must have Ground that is both same and good, and should show a stronger respect to which wire is Line and which is Neutral. These requirements are not as "relaxed" as you can get away with if you were running off the same phase. – Eugene Ryabtsev Jan 11 at 4:57
@MichaelKjörling I used to work with a place that had one power supply of each server plugged into a UPS and the other power supply of each server plugged directly into power. The guy who did it told me "horror stories" of having a UPS die and take all the servers down, so he didn't want to have all power come through a UPS. – Moshe Katz Jan 14 at 20:54
@EugeneRyabtsev Thanks, I think that's exactly what he was talking about. As mentioned by EEAA's answer below, is this an issue with PSUs as well, or just AC systems? BTW if you want to add your comment as an answer I'll mark it as a solution. Thanks, – Insomnia Jan 23 at 2:47
I'm not quite sure what my reasoning was above. When putting it that way, @kasperd, yes I agree with you. Of course you'd probably want all PSU inputs to at least be surge-protected, but that's another matter. – Michael Kjörling Jan 29 at 13:52
up vote 31 down vote accepted

With multi-phase AC systems (motors, etc.), you're right, bad things can and will happen if one of the phases drops out. However, with computer PSUs, each of them operates completely separately, converting its AC input voltage to a variety of DC voltages for the computer system.

You can safely run redundant PSUs on different circuits, different phases, etc. Doing so is actually a really great idea to reduce the number of components that are fate-shared.

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You definitely should be running multiple PSUs on different circuits for redundancy. – Keltari Jan 10 at 17:58

The job of the power supply is to take power that is not 100% clean and make is stable.

People working in data centers have been fired for not having the power supplies run on different circuits. If one circuit breaker goes off, or one UPS dies, the whole server goes down which is not the intended design.

Ideally you would want them on separate circuits on with a different UPS on each circuit so that if anything goes down the server is still good to go.

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Each of your PSU's should use only one phase, it doesn't matter much from which circuit each one comes. Both will provide DC current to your hardware.

From experience I have seen some PSU's fail shortly after exchanging cables on them while the servers ran, one cable after another (after all, why not when you have redundant PSU's???), each on a different fused circuit.

But I suspect this had to do with the age of the hardware, and not an edge case this question searches after. It were like less than 10 PSU's after exchanging cables on several hundred servers, so it should not be significant.

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At a minimum, you must have Ground that is both same and good, and should show a stronger respect to which wire is Line and which is Neutral. These requirements are not as "relaxed" as you can get away with if you were running off the same phase.

This is still an issue with PSUs, because PSUs often contain small capacitors that link Line, Neutral and Ground together (like C118 here or CY11 there). These have large finite impedance 50 Hz and will pass much more of short pulses, so the isolation is not complete. This is most dangerous when you hot-plug stuff, but even if you don't, having highest-frequency components flow across your GND is not too nice to have.

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