So, there is that house in France with three phase electrical power. I don't know if this is relevant. And I can't tell the whole installation was set properly. But the grounding is recent and strong.

We have the electric panel and its protections (differential circuit breakers).

We also have Uninterruptible Power Supply with battery and their protections.

Then we have some computers —local second hand professional servers, with now very low service load— with their own inner surge protections (in AC-DC block).

Now, from time to time, we noticed some computers got problems. Usually correlated with some general power interruption (at the house level, but AFAIK no thunder implied).

One server would be found off while the BIOS settings said to restart after power interruption; but the BIOS settings had gone. The motherboard battery was changed and same occurred again. That's when the UPS was installed, stopping that problem, and confirming the "electrical shock" hypothesis.

But more recently, again we got some damage. A network area storage got burnt. One server doesn't come fully up any more. Seems some problem with ROM corruption or I can't tell.

Maybe at some point the material plugged into one UPS was consuming (demanding) more power than supported (10A).

So my questions are

— How comes all three protection levels (electric panel, UPS and AC block) get passed through?

— What kind of a shock can that be (technically / physically)?

— How to set proper protections against that?