Perhaps the problem I presented was to big and general to get an answer. I have a very simple problem at the moment which I dont understand. I created a rectangular steel bar. On their ends I applied 10V respectively 0V. As expected their is a constant current flow between the two planes in y-direction as to be seen on the image. The magnitude is due to the fact that the steel bar provides only a low resistance. What I don't understand is the current flow in the other 2 directions, x- and z-direction (z-direction on the image). The magnitude is very small in comparison to the one of the y-direction, still it should be 0. One possible reason could be that it's an error due to the numerical calculation. Can someone confirm this?

I also tried to apply a boundary condition of 0 A on the 4 planes I don't expect current to flow through. This caused the following error:

Can someone explain to why there is a different condition to the adjacent entities? 2 planes with applied voltages and 4 planes perpendicular to them with applied current conditions seem fine to me.

Thank you for any help!!