edited to add some details asked for in comments

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 electrical board 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 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.

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 board, UPS and AC block) get passed through?

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

— How to set proper protections against that?

  • An answer to this question will not be generic or simple, meaning, with all of the best protections in place damage to equipment can still occur, or damage to the protection equipment can occur. The qualities of the UPS is important. Filters on the supply power, or an isolation transformer is also relevant. As well as what else is on the power that shares the same feed to the computer equipment. Also, what are the conditions of the damage, is it in rush current, brownouts, lightning strike, short cycling, etc.
    – noybman
    Nov 9, 2019 at 9:06
  • Where are you on this planet, first off? Nov 9, 2019 at 14:37
  • Has the neutral to ground voltage been verified? Switching power supplies do have problems with a high ground/ neutral voltage, with 3 phase if the system is not well balanced this can cause a high neutral to ground voltage. I saw this when we built a fully automated wafer fab in the mid 90’s, because the high draw of some equipment we could not balance all the time and had to install a massive ground grid , we started to see problems at about 1.75v and many problems at 2.25v. Once we installed the grid we never had above 1.1v and never had the problems again.
    – Ed Beal
    Nov 9, 2019 at 16:35
  • what shock are you talking about?
    – jsotola
    Nov 9, 2019 at 21:58
  • @ThreePhaseEel Thanks for your interest. This is happening in France. You may check my comment to Harper's answer for more technical details.
    – ondelettes
    Nov 10, 2019 at 16:18

1 Answer 1


First you should look at your grounding electrode system.

Then you should look out for lost neutrals, which can cause voltage to rise as high as 400V on 230V-nominal legs on a 400V "wye".

Finding none of those, you should look at the quality of the equipment - an awful lot of PC equipment is cheaply made foreign garbage with frequent failure rates, especially in the developing world. It is possible the failures have nothing to do with power quality. Going out of your way to get well-regarded power supplies can help a lot.

Finally, look at localized surge protection at the equipment.

  • Thanks for your help. — Grounding is new and strong. I trust it. — Does "lost neutral" refer to "tension between ground and neutral"? — Servers used are second hand but good professional servers used in France, with now very low service load. Showed no sign of decline before "collapse". UPS model is home class (10A) bought in Luxembourg, maybe getting old.
    – ondelettes
    Nov 10, 2019 at 16:14
  • @ondelettes No, there's supposed to be (small) tension between ground and neutral, because neutral is not ground. Lost neutral means a broken neutral wire, causing neutral to no longer be "in the center" at the point-of-use. The the 3 phases are in a tug-of-war to decide where neutral is, the "tug" being the conductance of the appliances on each leg. Nov 10, 2019 at 16:23

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