Let's start with the A300, I believe all twin Airbus are the same. Electrically the Green (center system) has a engine pump on both engines, the left engine has a Blue pump and the right engine has a yellow pump. This is normal operation with engines running. The green system is the most critical, it has 2 transfer pumps that will power B&Y (non reversible motor pumps). The green system has two electric pumps that will power B&Y w/o engines running, If you lose an engine the Green will power the others. On the ground of course the Green sustains all.