53569

神经网络昆斯林的新闻组分类2006

	When you install a GFCI, it's a good idea to use the little
	"ground fault protected" stickers that come with it and mark
	the outlets downstream of the GFCI.  You can figure out which
	outlets are "downstream", simply by tripping the GFCI with the
	test button and see which outlets are dead.

Subject: What's the purpose of the ground prong on an outlet, then?

	Apart from their use in electronics, which we won't comment on,
	and for certain fluorescent lights (they won't turn on without
	a good ground connection), they're intended to guard against
	insulation failures within the device.  Generally, the case of
	the appliance is connected to the ground lead.  If there's an
	insulation failure that shorts the hot lead to the case, the
	ground lead conducts the electricity away safely (and possibly
	trips the circuit breaker in the process).  If the case is not
	grounded and such a short occurs, the case is live -- and if
	you touch it while you're grounded, you'll get zapped.  Of
	course, if the circuit is GFCI-protected, it will be a very
	tiny zap -- which is why you can use GFCIs to replace
	ungrounded outlets (both NEC and CEC).

	There are some appliances that should *never* be grounded.  In
	particular, that applies to toasters and anything else with
	exposed conductors.  Consider:  if you touch the heating
	electrode in a toaster, and you're not grounded, nothing will
	happen.  If you're slightly grounded, you'll get a small shock;
	the resistance will be too high.  But if the case were
	grounded, and you were holding it, you'd be the perfect path to
	ground...

Subject: Why is one prong wider than the other?  Polarization

	Nowadays, many two-prong devices have one prong wider than the
	other.  This is so that the device could rely (not guaranteed!)
	on one specific wire being neutral, and the other hot.
	This is particularly advantageous in light fixtures, where the
	the shell should neutral (safety), or other devices which want to
	have an approximate ground reference (ie: some radios).

	Most 2-prong extension cords have wide prongs too.

	This requires that you wire your outlets and plugs the right
	way around.  You want the wide prong to be neutral, and the
	narrow one hot.  Most outlets have a darker metal for the
	hot screw, and lighter coloured screw for the neutral.
	If not, you can usually figure out which is which by which
	prong the terminating screw connects to.

Subject: What kind of outlets do I need in a kitchen?

	The NEC requires at least two 20 amp ``small appliance
	circuits'' for kitchens.  The CEC requires split-duplex
	receptacles.  Outlets must be installed such that no point is more
	than 24" (NEC) (900 mm CEC) from an outlet.  Every counter wider
	than 12" (NEC) or 300 mm (CEC) must have at least one outlet.
	The circuit these outlets are on may not feed any outlets except
	in the kitchen, pantry, or dining room.  Furthermore, these circuits
	are in addition to any required for refrigerators, stoves, microwaves,
	lighting, etc.  Non-dedicated outlets within 6' of a sink *must* be
	protected by a GFCI (NEC only).

	Split duplex receptacles are fed with a 220V circuit.  The tab
	is broken on the hot side of the outlet, and one hot goes to
	the upper outlet, and the other hot goes to the lower outlet.
	The neutral connects to both outlets through one screw.  When
	"carrying through" to another outlet, the neutral must be
	pigtailed, such that removing the outlet, or having the neutral
	connection fall off doesn't cause the neutral to disconnect
	from downstream outlets.

Subject: Where must outlets and switches be in bathrooms?

	There must be at least one outlet in each bathroom, adjacent to
	the sink, in addition to any outlet that may be incorporated in
	the light fixture.  All such outlets *must* be GFCI-protected.

Subject: What is Romex/NM/NMD?  What is BX?  When should I use each?

	Romex is a brand name for a type of plastic insulated wire.
	Sometimes called non-metallic sheath.  The formal name is NM.
	This is suitable for use in dry, protected areas (ie: inside
	stud walls, on the sides of joists etc.), that are not subject
	to mechanical damage or excessive heat.  Most newer homes are
	wired almost exclusively with NM wire.  There are several
	different categories of NM cable.

	BX cable -- technically known as armored cable or "AC" has a
	flexible aluminum or steel sheath over the conductors and is
	fairly resistant to damage.

	TECK cable is AC with an additional external thermoplastic
	sheath.
	
	Protection for cable in concealed locations: where NM or AC cable
	is run through studs, joists or similar wooden members, the outer
	surface of the cable must be kept at least 32mm/1.25" (CEC & NEC)
	from the edges of the wooden members, or the cable should be protected
	from mechanical injury.  This latter protection can take the form of
	metal plates (such as spare outlet box ends) or conduit.

	[Note: inspector-permitted practise in Canada suggests that armored
	cable, or flexible conduit can be used as the mechanical protection,
	but this is technically illegal.]

	Additional protection recommendations (these are rules in the
	Canadian codes - they are reasonable answers to the vague
	references to "exposed to mechanical damage" in both the NEC
	and CEC):

	    - NM cable should be protected against mechanical damage
	      where it passes through floors or on the surface of walls
	      in exposed locations under 5 feet from the floor.
	      Ie: use AC instead, flexible conduit, wooden guards etc.
	    - Where cable is suspended, as in, connections to furnaces
	      or water heaters, the wire should be protected.  Canadian
	      practise is usually to install a junction or outlet
	      box on the wall, and use a short length of AC cable
	      or NM cable in flexible conduit to "jump" to the appliance.
	      Stapling NM to a piece of lumber is also sometimes used.
	    - Where NM cable is run in close proximity to heating
	      ducts or pipe, heat transfer should be minimized by
	      means of a 25mm/1" air space, or suitable insulation
	      material (a wad of fiberglass).
	    - NM cable shall be supported within 300mm/1' of every box
	      or fitting, and at intervals of no more than 1.5m/5'.
	      Holes in joists or studs are considered "supports".
	      Some slack in the cable should be provided adjacent to
	      each box.  [while fishing cable is technically in violation,
	      it is permitted where "proper" support is impractical]
	    - 2 conductor NM cable should never be stapled on edge.
	      [Knight also insists on only one cable per staple, referring
	      to the "workmanship" clause, but this seems more honoured
	      in the breach...]
	    - cable should never be buried in plaster, cement or
	      similar finish.
	    - cable should be protected where it runs behind baseboards.
	    - Cable may not be run on the upper edge of ceiling joists
	      or the lower edges of rafters where the headroom is more
	      than 1m (39").

	Whenever BX cable is terminated at a box with a clamp, small
	plastic bushings must be inserted in the end of the cable to
	prevent the clamps forcing the sharp ends of the armor through
	the insulation.

	BX is sometimes a good idea in a work shop unless covered by
	solid wall coverings.

	In places where damage is more likely (like on the back wall of
	a garage ;-), you may be required to use conduit, a
	UL- (or CSA-) approved metal pipe.  You use various types of
	fittings to join the pipe or provide entrance/exit for the
	wire.

	Service entrances frequently use a plastic conduit.

	In damp places (eg: buried wiring to outdoor lighting) you will
	need special wire (eg: CEC NMW90, NEC UF).  NMW90 looks like
	very heavy-duty NMD90.  You will usually need short lengths of
	conduit where the wire enters/exits the ground.  [See underground
	wiring section.]

 	Thermoplastic sheath wire (such as NM, NMW etc.) should not be
 	exposed to direct sunlight unless explicitly approved for that
 	purpose.

	Many electrical codes do not permit the routing of wire through
	furnace ducts, including cold air return plenums constructed
	by metal sheeting enclosing joist spaces.   The reason for this
	is that if there's a fire, the ducting will spread toxic gasses
	from burning insulation very rapidly through the building.
	Teflon insulated wire is permitted in plenums in many areas.
	
 	Canada appears to use similar wire designations to the US,
 	except that Canadian wire designations usually include the
 	temperature rating in Celsius.  Eg: "AC90" versus "AC".
	In the US, NM-B is 90 degrees celcius.

	NOTE: local codes vary.  This is one of the items that changes
	most often.  Eg: Chicago codes require conduit *everywhere*.
	There are very different requirements for mobile homes.
	Check your local codes, *especially* if you're doing anything
	that's the slightest out of the ordinary.

	Wire selection table (incomplete - the real tables are enormous,
	uncommon wire types or applications omitted)

	Condition			Type	CEC	NEC

	Exposed/Concealed dry		plastic	NMD90	NM
					armor	AC90	AC
						TECK90

	Exposed/Concealed damp		plastic	NMD90	NMC
					armor	ACWU90
						TECK90

	Exposed/Concealed wet		plastic	NMWU90
					armor	ACWU90
						TECK90
	
	Exposed to weather		plastic	NMWU
						TW etc.
					armor	TECK90
	
	Direct earth burial/		plastic	NMWU*	UF
	Service entrance			RWU
						TWU
					armor	RA90
						TECK90
						ACWU90
	[* NMWU not for service entrance]

Subject: Should I use plastic or metal boxes?

	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.

Subject: Junction box positioning?

	A junction box is a box used only for connecting wires together.

	Junction boxes must be located in such a way that they're accessible
	later.  Ie: not buried under plaster.  Excessive use of junction
	boxes is often a sign of sloppy installation, and inspectors may
	get nasty.

Subject: Can I install a replacement light fixture?

	In general, one can replace fixtures freely, subject to a few
	caveats.  First, of course, one should check the amperage
	rating of the circuit.  If your heart is set on installing half
	a dozen 500 watt floodlights, you may need to run a new wire
	back to the panel box.  But there are some more subtle
	constraints as well.  For example, older house
	wiring doesn't have high-temperature insulation.  The excess
	heat generated by a ceiling-mounted lamp can and will cause the
	insulation to deteriorate and crack, with obvious bad results.
	Some newer fixtures are specifically marked for high
	temperature wire only.  (You may find, in fact, that your
	ceiling wiring already has this problem, in which case
	replacing any devices is a real adventure.)

	Other concerns include providing a suitable ground for some
	fluorescent fixtures, and making sure that the ceiling box and
	its mounting are strong enough to support the weight of a heavy
	chandelier or ceiling fan.  You may need to install a new box
	specifically listed for this purpose.  A 2x4 across the ceiling
	joists makes a good support.  Metal brackets are also available
	that can be fished into ceilings thru the junction box hole and
	mounted between the joists.

	There are special rules for recessed light fixtures such as
	"pot" lamps or heat lamps.  When these are installed in insulated
	ceilings, they can present a very substantial fire hazard.
	The CEC provides for the installation of pot lamps in insulated
	ceilings, provided that the fixture is boxed in a "coffin" (usually
	8'x16"x12" - made by making a pair of joists 12" high, and covering
	with plywood) that doesn't have any insulation.  (Yes, that's 8 *feet*
	long)

	NEC rules are somewhat less stringent.  They require at least 3"
	clearance between the fixture and any sort of thermal insulation.
	The rules also say that one should not obstruct free air movement,
	which means that a CEC-style ``coffin'' might be worthwhile.
	Presumably, that's up to the local inspector.  [The CEC doesn't
	actually mandate the coffin per-se, this seems to be an inspector
	requirement to make absolutely certain that the fixture can't get