What limits
I think the physics is quite simple. Just like rubbing a balloon on your jumper, really. If it is simple, then........ (if not simple then someone who knows what they are talking about will be along shortly!)
The rotating blades impact molecules (like balloon and jumper) and the impact knocks electrons off the outer orbit of an atom. That molecule becomes + charged and the roter takes on a - charge. The one molecule is left behind, but the rotor hits more molecules and so builds a bigger - charge. The - charge then affects the whole airframe.
When the potential difference between the machine and earth is greater than the resistance of a path between the two, a spark runs up from the ground to the machine along the path of least resistance, earthing the charge on the machine. The longer between earthing, the more molecules to hit (eg rain) etc the bigger the - charge built on the airframe before earthing.
The spark has very high voltage but no amps - it is static electricity. Hence a BIG impact felt if it earths through the loader with little damage. The charge can get big enough, however, to create spark erosion type damage at the point where the charge earths from the airframe. I would guess that damage to electric components is theoretically possible if the charge is big enough but the size of the charge would be getting towards "Lightning" (same process) value.