I dont know where the 600 TeV bit came from. I'll get to that.

But my two cents:

200 GeV, with a breakdown upon landing on surface. It cant happen before that because there is no route between the two objects for the electrons to flow through.

Do bear in mind though, that 212 GeV is unification point for the nuclear weak force and electromagnetism. So the voltage cant be much higher on the comet, otherwise it would have as much gravity as average planetary cores, and there would be no electrical discharge at all due to the movement of all electrical charges to the central point of energetic singularity. Unless the Orbiter has amassed a substantial positive electric charge.

It'll be interesting to see what happens. It is gonna change the world for sure though, no matter how it turns out. I for one strongly stand by the E.U. Theory.