Looking at a sub-panel installation the otherday, that had a uni-strut frame for support, bolted to a concrete garage floor.
The panel is grounded to the main properly, but would the metal frame be considered an unintentinal alternate path to ground?
Any comments on this?
Having seen a short clear on equipment bolted to a floor once, through the bolt, made me think of this. (Not properly grounded)
Isn't the case of the sub panel grounded with a ground bar?
Or rather isn't the panel supplied with an EGC through either the metallic raceway or a separate equipment grounding conductor as a sub panel should be? If that's the case what difference would it make if there were additional ground paths? Could only help.
“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).]