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.