ET16: Most typically the generator shell of MM and MI carts will be connected to one of the signal ground minus pins (or actually minus pins, as in fact the minus poles only turn into signal grounds in combination with unbalanced phono inputs...). Usually no problem, if the carts sports a plastic mounting frame, that allows for isolated mounting, so that there's no double grounding/ground loop (one path via signal ground, the other via headshell ground, which usually is connected to the separate ground lead together with tonearm and chassis ground...). Sometimes, however, it can happen, that there's douple grounding despite a non-conductive mounting frame - namely, if the screw heads or the nuts happen to touch the generator shell and that doesn't sport a non-conductive laquer layer.
Greetings from Munich!
Manfred / lini