The NEC permits use of plastic boxes with non-metallic cable
	only.  The reasoning is simple -- with armored cable, the box
	itself provides ground conductor continuity.  U.S. plastic
	boxes don't use metal cable clamps.

	The CEC is slightly different.  The CEC never permits cable
	armor as a grounding conductor.  However, you must still
	provide ground continuity for metallic sheath.  The CEC also
	requires grounding of any metal cable clamps on plastic boxes.

	The advantage of plastic boxes is comparatively minor even for
	non-metallic sheathed cable -- you can avoid making one ground
	connection and they sometimes cost a little less.  On the other
	hand, plastic boxes are more vulnerable to impacts.  For
	exposed or shop wiring, metal boxes are probably better.

	Metal receptacle covers must be grounded, even on plastic
	boxes.  This may be achieved by use of a switch with ground
	connection.