You should always connect the incoming service ground to your machine chassis, and preferably connect it or bond it to the common star Earth ground point in your enclosure.
It would be extremely bad practice if they used S.S. components, the body of which had a potential on them to ground.
The reason for your readings is leakage, inductive or capacitive from the incoming hot leads to the ungrounded chassis.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.