In my opinion, I should first check and correct the grounding system of the customer then measure the %Vthd after the correction. I think there was an issue on the customer's equipment where they drove a separate ground rod for their "isolated ground" I presume, this may create a "ground loop" or potential difference between the electrical system and the equipment.
Originally Posted by aelectricalman
"Scientia Et Fides"
"You see, in life, lots of people know what to do, but few people actually do what they know.
Knowing is not enough! You must take action."