I have also seen a single phase go out (bird poop into a transformer) and having redundant PSUs on different phases saved our bacon

The house I grew up in lost transformers multiple times a year from squirrels.

The center tapped transformer that provides residential split phase 120/240 on its secondary winding connects to just 1 of the 3 grid phases on its primary winding. If that one phase goes out on the grid, both sides of the split phase service go out together. There would need to be a fault on just one side of the split, downstream of that transformer, for your suggestion to hold up. Certainly not impossible, but far less common than a "losing one phase" scenario which would typically originate upstream on the high voltage side.

On a commercial 3 phase service, yes, connect redundant PSUs to separate phases, since each phase on the panel actually corresponds to each phase of the grid.

> The center tapped transformer that provides residential split phase 120/240 on its secondary winding connects to just 1 of the 3 grid phases on its primary winding.

I believe that's a USA peculiarity. Where I live, the usual residential and commercial wiring is from 13.8 kV to 127V/220V through a three-phase delta-wye transformer, in which the primary connects between each pair of phases, and the secondary connects between one phase and the neutral (the high-voltage primary side does not have a neutral). When one phase of the high voltage side is lost (very common, since each high-voltage phase is a separate wire and has an independent fuse upstream of the transformer), what happens is that one phase of the low voltage side stays normal (the one between the two intact high voltage phases), and the other two have a lower voltage which varies depending on their relative load.

> I believe that's a USA peculiarity.

Yep. Sounds like you have true 3-phase service, whereas most places in the US just get split-phase.

Is there a ELIA5 of what this distinction is?

Look up Wye vs Delta; but the basic point is in delta the phases are connected in series with each other, and in wye they are connected in parallel. It's easier to visualize when you can see the tap points on the transformer secondaries though.

You can definitely loose one phase of a split phase transformer. I've seen it 3 times in my life. Sometimes it is completely out, other times I've seen only getting 80v rather than the full 120.

All depends on how it failed.

But if the issue is a local breaker flip, then being on a different phase is very effective!

I don't have a power outage more than once a year, but we manage to blow a breaker more than a few times a decade (vacuum + water boiler was one).

If the issue is a breaker flip, then you only need to be on different breakers, right? Phase is irrelevant in that case

Right, because the main breaker would be 2 pole (or at the very least handle tied).

Generally, yes, but there are some exceptions like different breakers on a shared neutral, which may be tied so that both circuits break.

Agreed, as I said it seems like around a 10% improvement, which considering it's fairly easy to ensure the circuits are on different phases, is worth doing IMHO.