What Does Metal Clad Really Mean?

check this

http://ecmweb.com/construction/electric_working_metalclad_cable/

Although MC resembles its close relative, armored cable, there are some distinct differences. Whereas armored cable has a grounding strip in intimate contact with the inside of the metallic sheath, which can then serve as the equipment grounding conductor, MC cable has no such strip. Therefore, the outer sheath of MC cable does not qualify as an equipment grounding conductor. MC cable contains an insulated grounding conductor which, when properly terminated at both ends, makes a reliable return path for fault current, enabling the overcurrent device to perform its protective role. The metal jacket serves to supplement the equipment grounding conductor since it is firmly clamped to grounded metal enclosures at both ends — so that the unit cannot become energized without instantly causing the overcurrent device to trip out.

When you get into high powered systems, you have a few choices of how you want the switchgear configured. One is what is called "Metal Enclosed" and the top of the line is "Metal Clad". "Arc Resistant" is a newer term that is kind of a middle ground, but can also be used to describe modified versions of the other two.

Metal Clad means that each individual circuit breaker or switch is in its own dedicated cubicle, in a "draw-out" unit made of sheet metal on all sides. The draw-out unit then slides into a sheet metal frame and has more sheet metal on all sides. The unit has stabs on the back that will engage the bus bars in the frame. But you can place the unit in place, close the door without the stabs engaging yet, then using a crank that goes through a hole in the door, you engage a racking mechanism that slowly pulls the unit into the bus bars to energize the line side of the breaker. Only after it is fully engaged can you turn the breaker on. You can also remove any unit while the rest are energized, take it to a workbench to service it, and replace it without interrupting service on any other part of the plant. The reason for selecting Metal Clad is because they are usually the most critical distribution system in a plant and if something goes wrong in one part of the plant, you don't want it to take out the entire plant. So if a catastrophic fault happens inside of one unit, the double walled metal "cage" that forms around it by virtue of the frame around the unit and the frame in the cubicle it plugged in to, will keep the explosion contained to that one unit, not involving the adjacent ones.

By contrast, Metal Enclosed switchgear usually has everything sharing the same general air space within the gear and if one unit explodes or needs routing maintenance, everything must be shut down.

Metal Enclosed is a lot cheaper, Metal Clad is a lot more fault tolerant.