Mains power insulation of a motor is tested using a high voltage leak test and the test can’t be performed with a volt-ohm meter. You can unplug the tool turn the switch on and test for the resistance from mains power to frame and it should be a value higher than the meter can read. If it reads less then you have confirmation of an insulation failure, but if it reads infinite ohms that only means that an inadequate low voltage test method doesn’t detect an insulation failure. Many times failing insulation can still withstand and block low voltage and breaks down and leaks higher voltage.
If you have received a (mild) shock from a tool rest attached to the frame of a grinder’s motor, then the obvious answer is that there is a leakage path from the power line to the motor’s frame and the motor frame is not earth grounded.
It’s far from impossible for a grinder that isn’t properly grounded, especially one with a buffing wheel on one end, to build up a static electric charge. If all you felt was a static electricity discharge, earth ground the frame and use a ground fault protected outlet in case you were wrong about it being a static electric discharge.
A mains power leakage path *might* occur where the cord enters the tool, if not then it’s more likely the insulation on the motor windings is defective than something exotic about the wiring at the switch, but check it anyway, switches and wire are relatively cheap fixes. It typically isn’t cost effective to have a fractional horsepower motor re-wound. Replacing the motor or the entire tool is usually more cost effective,
Oliver Street, September 2019