Metal Clad Vs. Metal Enclosed/Shielded Vs. Non-Shielded 1

[birddogger]

I already know what the difference is between metal clad and metal enclosed. MC has barriers everywhere to prevent accessing one compartment from another, and ME does not. My question is, how do you determine which to use for a given application? It would seem that you would always want to go with MC purely for safety reasons. Is it purely a cost issue?

I've often thought the same thing for shielded vs. non-shielded MV cable. When exactly would you use non-shielded cable? I suppose voltage stress might not be an issue on very short runs, for instance between a primary switch and an adjacent transformer, but really, what contractor would go out and get a roll of non-shielded cable for such applications when they can just cut a few more lengths off the many rolls of shielded cable they already have on site? Where's the cost savings?

 

[jraef]

Metal Enclosed is often smaller than Metal Clad as well. Metal Clad is designed to have "draw-out" cells for additional safety isolation. If your process is critical and requires regular maintenance, a spare cell can be plugged in while the normal one is being serviced. The racking mechanism for those cells and the distance required to maintain safety from flashover to the stab assemblies ends up making the Metal clad gear up to 8 feet deep!

Some Metal Enclosed designs are as shallow as 30 inches, but the controllers or switches cannot be drawn out so routine maintenance requires an interruption in operation.

Hope that helps.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

Oh, and to the cable issue, non-shileded cable is typically only used by the switchgear manufacturers. I don't beleive that contractors would ever use non-shielded in field installations unless they are repairing gear. There is no reason why you cannot use shielded in gear, its just that it is very difficult to manipulate and terminalte in tight spaces so we tend not to.

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

IEEE Std 141-1993 (red book) §10.3 “Switchgear” and §12.2 “Cable construction” give descriptions.

 

[jbartos]

Suggestion: References:
1. Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition (or current 14th Edition), McGraw-Hill, Inc., 1993,
Section "Switchgear Assemblies" on page 10-94

2. Electrical Power Research Institute "Wire and Cable", Power Plant Electrical Reference Series, Volume 4.