Does the additional load on UPS in case of grid failure amount to 100%?

The scenario is as follows: Servers are equipped with two PSU's each. One of these is connected to the UPS, the other one directly to the power grid (bypassing the UPS).

I would assume that since the PSU's are redundant, load is more or less evenly distributed among these two PSU's 50/50 under normal conditions.

In the case of failure of the power grid (or UPS maintenance), one would think that the load now has to be beared by one PSU alone, amounting to an current load increase of 100% on this line (at least on the according bus).

Is that correct, or have you made different observations? Since I'm no electrical engineer, I don't know if perhaps manufacturers don't run 2 PSU's in full active-active mode but instead have some funky switching going on internally.

Solution 1:

Your understanding is correct: In modern servers power supplies are what is referred to as "Parallel Redundant" - Both power supplies are active, and both are supplying power to the system bus.

In the event of a power supply failure (internal fault, loss of input power, etc.) the remaining power supply (or power supplies) will need to supply additional current to the system bus to support the load (proportionally increasing their current draw from whatever their input source is - usually AC power).

You can see this behavior for yourself if you have a load-metering power strip (or a pair of in-line load meters, like Kill-A-Watt meters:

• Connect both power supplies to separate metered sources
  (This can be the A and B legs of a CDU, or individual Kill-A-Watt Meters)
• Power the system on and let it come up and stabilize. Note the load on each source (meter).
• Unplug one of the power supplies.
  You will notice the input current to the other PSU increases by roughly what the one you just failed was drawing.

This has an important practical implication for power capacity planning that leads to a general rule of thumb: Never load a power circuit above 50% of its capacity.
This allows you to have the companion power leg fail and the load transfer over without risk of blowing a breaker/fuse.

(In reality you often don't want to load a power circuit above 40-45% of its capacity, as operating at exactly 100% capacity can be risky territory. Keeping the maximum potential single-fault load between 80% and 90% gives you some wiggle room.)