Hello. The ground plane is sometimes used as current return path. If current is low frequency and amplitude is small then voltage rising due to current flow can be ignorable. However, what is return current is actually very high in both frequency and amplitude? In our lab, gas discharge results in ~240 A discharge current of single cycle for 400 ns, which corresponding frequency is ~2 MHz. You can take a look at the attached image. The points A and B are the electrical wire contact points thus I believe current should choose this straight and shortest path to flow from A to B. If this is true, I guess there is severe local voltage drop just between A and B in such a transient time and after then all points on the ground plane becomes equipotent later. Is my reasoning making sense?