I opened up the space underneath my bathtub for the first time in this house. The tub is a "spa" style and has a pump motor to circulate water through jets. (I'm in NY / USA).
I noticed that the motor's control box is grounded to one of the copper water supply pipes with a bare #10 or #8 copper wire.
As far as I can think there is NOT a continuous path of copper pipe to earth ground because PEX has been used in the house between the tub and our well supply. So I tested this ground wire with a multimeter:
From the hot side of a nearby 120V outlet it does indeed show ~120 volts to ground through that wire.
But it also shows a resistance of ~400 Ω between the ground of the same receptacle and ground through this ground wire. (Seems high?)
I was initially surprised to get any path to ground at all, but then figured it is probably through the water in the pipes. Its well water with plenty of minerals etc. and ought to be conductive.
Here is my quandary:
I am not sure if I need to provide an alternate ground for this pump. Does the resistance reading indicate the ground path is poor? On the other hand I don't understand it why this ground is even needed, since the motor surely is grounded through its power supply. Maybe the intent is to get an equipotential bond at the metal parts of the fixture? In that case I think I can leave it as is? But only one of the supply pipes is bonded (maybe the other is made safe through the fixture).