Where we have seen ground shifts were in places where there were separate grounding systems in separated buildings but the buildings were connected with LAN cables. We bonded the machine frames to prove the problem but in the end the right fix was probably bonding the ground electrode systems together.
At that time I was in the data end of the business and bonding machine frames was the fix we often used. If there was a scary amount of current on that wire we got the electricians involved. If the voltage was much more than 35 volts or so we wouldn't touch it until the grounding system was inspected but 20-30 volts was not unusual.
