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Recently visiting a small data room, I noticed that redundant power supplies of a server are connected through a C14 splitter which, in turn, is connected to a single socket of a UPS.

I told the person who was showing me the data room that in my humble opinion, this makes no sense: if one has only one UPS, then only one of the two PSU should be connected. He replied that with redundant PSUs, it is always better to connect both PSUs, even through a splitter; doing otherwise, i.e. connecting only one, would create unnecessary stress on the connected PSU.

What is this stress thing? Isn't the first PSU doing all the work anyway in an ordinary configuration where both PSUs are connected, the other one being used only when the first one fails or is disconnected?

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Connecting both PSU to the same UPS can keep the machine running in case one of the PSU fails. It does however depend on the exact circumstances of the failure. It is possible for a PSU to fail in a way that takes the machine down even if the other PSU is still in operation.

It is also possible for a failing PSU to take down everything connected to the UPS, though in principle that can be prevented if the UPS has separate fuses on each output.

The purpose of having two PSU is not really to protect against one of the PSU failing. It is to protect against one of the two power sources failing. And you lose all of that if both inputs are connected to the same power source.

So connecting both inputs to the same power source will lose you almost all of the benefits of having two PSU.

If you only have access to a single UPS it will be slightly better for reliability to connect one PSU to the UPS and connect the other PSU directly to mains bypassing the UPS. If you can get a surge protector for the PSU connected directly to mains, that will be better.

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