i would like to ask if my schematics is ok.

I have one (or more) device (here symbolic as "R") which are supplied by a main power supply. I would like to support them also with an usv for the emergency case when the main supply is down then the the devices should be supplied from the usv.

I suppose that there should be some management device which would switch between the two power supplies but this schematics is much simpler.

But i have concerns:

  • For reasons i have not under control i can´t provide a common ground for the two supplies so i suppose that the (not existing) common ground could cause problems.
  • Is it possible that there will flow something like an offset or "balancing" current between the separated systems?
  • while there are diodes with different characteristics is would be difficult to define which power supply will supply the devices. It could be that the usv will supply the device (R) the whole time, then the first power supply would be useless

Is there a other way, some schematics or devices to manage the switching better than with this primitive schema?

enter image description here

  • 2
    \$\begingroup\$ Are any of these lines connected to earth ground, either directly or through the power supplies? \$\endgroup\$ – Elliot Alderson Feb 27 '19 at 15:45
  • \$\begingroup\$ Power supplies are connected to the earth ground. But i don´t know where. The first one maybe threw 3-phase-current, the second one threw normal 230V AC plug, so i don´t know, what is between them. \$\endgroup\$ – Coliban Feb 28 '19 at 4:56
  • \$\begingroup\$ It is crucial to understand how (output) lines of power supplies are connected internally/indirectly, or that they aren't connected at all (isolated). For example, suppose '-' of left PSU is connected (internally) to earth ground and '+' of right PSU (USV) connected (internally) to earth ground also. In this case both PSUs will appear connected in series and the load (R) will receive 48V. \$\endgroup\$ – Eugene K Feb 28 '19 at 7:25
  • \$\begingroup\$ @EugeneK, yes, you are right. There should be a common ground for the two 230V supplies but i can´t say where it is and if it is as supposed, as it should be. Maybe i should go and take a measurement at my test installation. \$\endgroup\$ – Coliban Feb 28 '19 at 9:35

Ok, i decided to

  • measure the resistance/current between the two grounds
  • create a connection between the grounds of the two devices like outlined in the following draft to be shure that there is no cross current or something unexpected

enter image description here

I will report later

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