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?

  • Make and model of the UPS?
    – winny
    Nov 11, 2019 at 15:02

2 Answers 2


Looks like your problem is frequent electric surges which are of high voltage and low current. Can be avoided by using a surge diverter or an RCCB - residual current circuit breaker.

  • Thanks, an RCCB has been present all along. It's compulsory in France.
    – ondelettes
    Dec 10, 2019 at 20:30

"house in France with three phase electrical power"
"can't tell the whole installation was set properly"

  • multiple power induced failures ???

It appears there is an installation failure with unbalanced voltage surges from I would expect normal transient voltage changes such as < 1 cycle dropouts from transmission line transfers.

I suggest you provide a complete network model and perform power transient quality measurements or get the designer to fix it.

Large surge current loads on dropouts or startups on one phase "may" cause over voltage on the other phases.

A reliable UPS output is ON all the time and charged by AC input.

Transients <1 cycle may occur every day with diurnal loads and multiple transmission line transfers.

  • Thanks. I'm not sure what transient power is. But isn't that supposed to be held by the UPS or any other protection the installation already has?
    – ondelettes
    Dec 10, 2019 at 20:33
  • e.g. if phases are unbalanced and after interruption of say 1 or more cycles, Phase A &B sag 20% and Phase C rises 40% , will that cause a failure? You need to balance the current surges or get better 3 phase protection. Dec 10, 2019 at 20:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.