I probably should have been clearer about my intentions. The end goal is to pick up EMI from this motor. Considering the low frequency range, I'm expecting the ground wire to radiate the coupled signal pretty effectively. I'm really interested in the amount of noise that couples to this ground wire as I can't really calculate the parasitic capacitance inside, and measuring it would be simpler and more accurate than trying to estimate it.
Like I stated, the end goal is non intrusive in nature, I've thought about current probes and vibration / sound side channels but they don't fit my bill (for other reasons). I just want to know for now the kinds of signals that leak to the ground (which harmonics couple more than others). Also, the amount of signal leakage is dependent on the parasitic capacitance (constant, as intrinsic to the design of the machine) but also the inductance / impedance of the ground wire. As the wire gets longer, so does its inductance. Ideally a sort of LISN (line impedance stabilisation) device would be used to simulate how the signal responds to different lengths of wire, all this by means of a tunable inductor.
I've added a schematic to show what I have in mind. What I would really like is your opinion on whether this is the best design for my need and if my equipment is protected from any damage.