“The panel is grounded to the main properly, but would the metal frame be considered an unintentional alternate path to ground?”
Absolutely, that is why the NEC requires an EGC, or a planned path of sufficiently low impedance and capacity to operate an OCPD quickly and safely. There are several names for this path depending on what industry you are in such as local grounds, incidental grounds, foreign ground, ground loops, etc. In my industry, telephony, we call it the “integrated ground plane”.
For the most part it is harmless, but not allways. It is a fact of life in almost any industrial application where you have panels, raceways, and equipment fastened to structural steel, concrete, and rebar within the facility. Most of the problems associated with these unplanned paths are allowing noise currents on the ground circuits, and allowing fault and lightning currents to flow through equipment frames.
In the telephony industry, on certain types of data, transmission, and switching equipment, we single point ground the power source like a DC plant or isolation transformer, and completely isolate all the raceways, distribution equipment, and equipment frames from the “integrated ground plane” to form what we call the “isolated ground plane”. This “isolated ground plane” prevents any noise, fault, or lightning currents from entering via the single point ground method. Similar to the isolated ground receptacles, except it takes more planning, expense, installation techniques, and care to accomplish.
[This message has been edited by dereckbc (edited 08-01-2004).]