Ted,

You got the ground rod correct but it must be the only source of ground and you must connect it to the main ground terminal ( usually a common copper ground bar where all the grounds are connected) inside the mag panel. Do not have the ground wire connected to the machine when you use the megger. We are looking for the quality of the ground and to eliminate any ground loops or voltage spikes that effect feedback signals. Many people use the incorrect ground rod that corrodes and in a few months the machine does not have a ground or develops a ground loop within the shop. I have seen black pipe, galvanized rod, steel rod and others used for a ground rod and all have oxidized, rusted or corroded and caused many intermittent problems related to ground. You need a separate ground rod for each machine, you can't over ground a machine. To answer your other question, your electrician is correct, you want ground isolation between the shop and the office so that any voltage or ground spikes do not go through the office equipment.

Mike http://cncmachineservices.com