Hello Dr. branadic,
that's a great finding!
I have never seen that polarity change effect in my lab, as all instruments and all the references are supplied from the identical phase.
We should ask the colleagues from CERN, PTB tomorrow, how they analyse and manage the mains and power supply (CMMR), illumination (EMC) and temperature (T.C.) characteristics in their labs.
This is another very important aspect, to achieve lower uncertainty and higher stability.
In our EE company, all mains plugs are marked individually, I.e. the red ones are connected to the USV system, and the black ones are marked for the 3 different phases, if I remember right. Have not been in the company recently.
I guess that our engineers don't care, because they are not doing such high precision stuff, usually
Frank