I have a number of dell servers and two UPS. I plan to give two different input to servers from two different UPS, my question is:

A short circuit of redundant powersupply unit (PSU1) will trip one UPS(1) section. Will the fault with PSU1's return power supply will affect another psu (2) and will this trip the UPS(2)?


This is exactly why you have redundant power supplies and dual UPS’s. If power supply A fails instead of the load being spread over A/B it is transferred to power supply B.

Also most UPS’s can isolate each output so if there is a problem only that output is affected. Without knowing what UPS you have I couldn’t say yes or no if it has isolated outputs.

As for UPS configuration you have got it spot on. No point in having 2 power supplies going into the same UPS as then the UPS becomes a SPOF (or single point of failure) so many times I have seen people attached 2 power supplies to a single UPS. If you don’t have 2 UPS’s the best thing to do is put one power supply into the UPS and another into the mails via a surge protected power cord.


Not sure if I correctly understood the question.

A short circuit in one power supply should under normal circumstances have no effect on the other power supply.

Of course, your second UPS will need to be powerful enough to hold the increased load. When one UPS fails, the load on the other one will double instantly. So if either of your UPS is already running at >=50%, it will almost certainly overload when the other UPS fails.


Theoretically they protect them selves from power surges . Depending on the manufacturer it should block the power at input , and not distribute it at exit (because they usually regulate external power.


Also, be careful if both UPS feed from the same mains fuse. I have seen offline UPS pass dead shorts straight through and blow upstream fuses. Even worse if ampacity of the mains line is overcommitted (eg two 3000VA capable units from a 230V 16A line ... do not do that!*). Luckily, it seems that server PSU makers have by now gotten wise, it has been a long time since I have seen a server grade PSU short out its source (have seen so in the past esp with 3U/4U servers using commodity ATX PSUs).

*EE basics for server people: Volts times Amperes gives you VA. Which is close to Watts but not exactly on an AC system (pretty close if everything has good PFC). If you have to care about the difference you are operating too close to the limits anyway. Also, loading a UPS to a significant percentage of its rated output gives you short runtime.

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