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神经网络昆斯林的新闻组分类2006

	requires that the white wire be "permanently marked" at the ends
	to indicate that it is a live wire.  Usually done with paint, nail
	polish or sometimes electrical tape.

	Each circuit is attached to the main wires coming into the
	panel through a circuit breaker or fuse.

	There are, in a few locales, circuits that look like (1), (2)
	or (3) except that they have two bare ground wires.  Some places
	require this for hot tubs and the like (one ground is "frame ground",
	the other attaches to the motor).  This may or may not be an
	alternative to GFCI protection.

Subject: "grounding" versus "grounded" versus "neutral".

	According to the terminology in the CEC and NEC, the
	"grounding" conductor is for the safety ground, i.e., the green
	or bare wire.  The word "neutral" is reserved for the white when
	you have a circuit with more than one "hot" wire.  Since the white
	wire is connected to neutral and the grounding conductor inside the
	panel, the proper term is "grounded conductor".  However, the
	potential confusion between "grounded conductor" and "grounding
	conductor" can lead to potentially lethal mistakes - you should
	never use the bare wire as a "grounded conductor" or white wire
	as the "grounding conductor", even though they are connected
	together in the panel.

	[But not in subpanels - subpanels are fed neutral and ground
	separately from the main panel.  Usually.]

	In the trade, and in common usage, the word "neutral" is used
	for "grounded conductor".  This FAQ uses "neutral" simply to
	avoid potential confusion.  We recommend that you use "neutral"
	too.  Thus the white wire is always (except in some light
	switch applications) neutral.  Not ground.

Subject: What does a fuse or breaker do?  What are the differences?

	Fuses and circuit breakers are designed to interrupt the power
	to a circuit when the current flow exceeds safe levels.  For
	example, if your toaster shorts out, a fuse or breaker should
	"trip", protecting the wiring in the walls from melting.  As
	such, fuses and breakers are primarily intended to protect the
	wiring -- UL or CSA approval supposedly indicates that the
	equipment itself won't cause a fire.

	Fuses contain a narrow strip of metal which is designed to melt
	(safely) when the current exceeds the rated value, thereby
	interrupting the power to the circuit.  Fuses trip relatively
	fast.  Which can sometimes be a problem with motors which have
	large startup current surges.  For motor circuits, you can use
	a "time-delay" fuse (one brand is "fusetron") which will avoid
	tripping on momentary overloads.  A fusetron looks like a
	spring-loaded fuse.  A fuse can only trip once, then it must be
	replaced.

	Breakers are fairly complicated mechanical devices.  They
	usually consist of one spring loaded contact which is latched
	into position against another contact.  When the current flow
	through the device exceeds the rated value, a bimetallic strip
	heats up and bends.  By bending it "trips" the latch, and the
	spring pulls the contacts apart.  Circuit breakers behave
	similarly to fusetrons - that is, they tend to take longer to
	trip at moderate overloads than ordinary fuses.  With high
	overloads, they trip quickly.  Breakers can be reset a finite
	number of times - each time they trip, or are thrown
	when the circuit is in use, some arcing takes place, which
	damages the contacts.  Thus, breakers should not be used in
	place of switches unless they are specially listed for the
	purpose.

	Neither fuses nor breakers "limit" the current per se.  A dead
	short on a circuit can cause hundreds or sometimes even
	thousands of amperes to flow for a short period of time, which
	can often cause severe damage.

Subject: Breakers?  Can't I use fuses?

	Statistics show that fuse panels have a significantly higher
	risk of causing a fire than breaker panels.  This is usually
	due to the fuse being loosely screwed in, or the contacts
	corroding and heating up over time, or the wrong size fuse
	being installed, or the proverbial "replace the fuse with a
	penny" trick.

	Since breakers are more permanently installed, and have better
	connection mechanisms, the risk of fire is considerably less.

	Fuses are prone to explode under extremely high overload.  When
	a fuse explodes, the metallic vapor cloud becomes a conducting
	path.  Result? from complete meltdown of the electrical panel,
	melted service wiring, through fires in the electrical
	distribution transformer and having your house burn down.
	Breakers don't do this.

	Many jurisdictions, particularly in Canada, no longer permit
	fuse panels in new installations.  The NEC does permit new
	fuse panels in some rare circumstances (requiring the special
	inserts to "key" the fuseholder to specific size fuses)

	Some devices, notably certain large air conditioners, require fuse
	protection in addition to the breaker at the panel.  The fuse
	is there to protect the motor windings from overload.  Check the
	labeling on the unit.  This is usually only on large permanently
	installed motors.  The installation instructions will tell you
	if you need one.

Subject: What size wire should I use?

	For a 20 amp circuit, use 12 gauge wire.  For a 15 amp circuit,
	you can use 14 gauge wire (in most locales).  For a long run,
	though, you should use the next larger size wire, to avoid
	voltage drops.  12 gauge is only slightly more expensive than
	14 gauge, though it's stiffer and harder to work with.

	Here's a quick table for normal situations.  Go up a size for
	more than 100 foot runs, when the cable is in conduit, or
	ganged with other wires in a place where they can't dissipate
	heat easily:

		Gauge		Amps
		14		15
		12		20
		10		30
		8		40
		6		65
	
	We don't list bigger sizes because it starts getting very dependent
	on the application and precise wire type.

Subject: Where do these numbers come from?

	There are two considerations, voltage drop and heat buildup.
	The smaller the wire is, the higher the resistance is.  When
	the resistance is higher, the wire heats up more, and there is
	more voltage drop in the wiring.  The former is why you need
	higher-temperature insulation and/or bigger wires for use in
	conduit; the latter is why you should use larger wire for long
	runs.

	Neither effect is very significant over very short distances.
	There are some very specific exceptions, where use of smaller
	wire is allowed.  The obvious one is the line cord on most
	lamps.  Don't try this unless you're certain that your use fits
	one of those exceptions; you can never go wrong by using larger
	wire.

Subject: What does "14-2" mean?

	This is used to describe the size and quantity of conductors
	in a cable.  The first number specifies the gauge.  The second
	the number of current carrying conductors in the wire - but
	remember there's usually an extra ground wire.  "14-2" means
	14 gauge, two insulated current carrying wires, plus bare ground.

	-2 wire usually has a black, white and bare ground wire.  Sometimes
	the white is red instead for 220V circuits without neutral.  In
	the latter case, the sheath is usually red too.

	-3 wire usually has a black, red, white and bare ground wire.
	Usually carrying 220V with neutral.

Subject: What is a "wirenut"/"marrette"/"marr connector"?  How are they
	used?

	A wire nut is a cone shaped threaded plastic thingummy that's used
	to connect wires together.  "Marrette" or "Marr connector"
	are trade names.  You'll usually use a lot of them in DIY wiring.

	In essence, you strip the end of the wires about an inch, twist them
	together, then twist the wirenut on.

	Though some wirenuts advertise that you don't need to twist the
	wire, do it anyways - it's more mechanically and electrically
	secure.

	There are many different sizes of wire nut.  You should check
	that the wire nut you're using is the correct size for the
	quantity and sizes of wire you're connecting together.

	Don't just gimble the wires together with a pair of pliers or
	your fingers.  Use a pair of blunt nose ("linesman") pliers,
	and carefully twist the wires tightly and neatly.  Sometimes
	it's a good idea to trim the resulting end to make sure it
	goes in the wirenut properly.

	Some people wrap the "open" end of the wirenut with electrical
	tape.  This is probably not a good idea - the inspector may
	tear it off during an inspection.  It's usually done because
	a bit of bare wire is exposed outside the wire nut - instead
	of taping it, the connection should be redone.

Subject: What is a GFI/GFCI?

	A GFCI is a ``ground-fault circuit interrupter''.  It measures
	the current current flowing through the hot wire and the
	neutral wire.  If they differ by more than a few milliamps, the
	presumption is that current is leaking to ground via some other
	path.  This may be because of a short circuit to the chassis of
	an appliance, or to the ground lead, or through a person.  Any
	of these situations is hazardous, so the GFCI trips, breaking
	the circuit.

	GFCIs do not protect against all kinds of electric shocks.  If,
	for example, you simultaneously touched the hot and neutral
	leads of a circuit, and no part of you was grounded, a GFCI
	wouldn't help.  All of the current that passed from the hot
	lead into you would return via the neutral lead, keeping the
	GFCI happy.

	The two pairs of connections on a GFCI outlet are not symmetric.
	One is labeled LOAD; the other, LINE.  The incoming power feed
	*must* be connected to the LINE side, or the outlet will not be
	protected.  The LOAD side can be used to protect all devices
	downstream from it.  Thus, a whole string of outlets can be
	covered by a single GFCI outlet.

Subject: Where should GFCIs be used?

	The NEC mandates GFCIs for 110V, 15A or 20A single phase
	outlets, in bathrooms, kitchens within 6' of the sink, garages,
	unfinished basements or crawl spaces, outdoors, near a pool, or
	just about anywhere else where you're likely to encounter water
	or dampness.  There are exceptions for inaccessible outlets,
	those dedicated to appliances ``occupying fixed space'',
	typically refrigerators and freezers, and for sump pumps and
	laundry appliances.

	The CEC does not mandate as many GFCIs.  In particular, there
	is no requirement to protect kitchen outlets, or most garage or
	basement outlets.  Basement outlets must be protected if you
	have a dirt floor, garage outlets if they're near the door to
	outside.  Bathrooms and most exterior outlets must have GFCIs.

	Even if you are not required to have GFCI protection, you may
	want to consider installing it anyway.  Unless you need a GFCI
	breaker (see below), the cost is low.  In the U.S., GFCI
	outlets can cost as little as US$8.  (Costs are a bit higher in
	Canada:  C$12.)  Evaluate your own risk factors.  Does your
	finished basement ever get wet?  Do you have small children?
	Do you use your garage outlets to power outdoor tools?  Does
	water or melted snow ever puddle inside your garage?

Subject: Where shouldn't I use a GFCI?

	GFCIs are generally not used on circuits that (a) don't pose a
	safety risk, and (b) are used to power equipment that must run
	unattended for long periods of time.  Refrigerators, freezers,
	and sump pumps are good examples.  The rationale is that GFCIs
	are sometimes prone to nuisance trips.  Some people claim that
	the inductive delay in motor windings can cause a momentary
	current imbalance, tripping the GFCI.  Note, though, that most
	GFCI trips are real; if you're getting a lot of trips for no
	apparent reason, you'd be well-advised to check your wiring
	before deciding that the GFCI is broken or useless.

Subject: What is the difference between a GFCI outlet and a GFCI breaker?

	For most situations, you can use either a GFCI outlet as the
	first device on the circuit, or you can install a breaker with
	a built-in GFCI.  The former is generally preferred, since GFCI
	breakers are quite expensive.  For example, an ordinary GE
	breaker costs ~US$5; the GFCI model costs ~US$35.  There is one
	major exception:  if you need to protect a ``multi-wire branch
	circuit'' (two or more circuits sharing a common neutral wire),
	such as a Canadian-style kitchen circuit, you'll need a
	multi-pole GFCI breaker.  Unfortunately, these are expensive;
	the cost can range into the hundreds of dollars, depending on
	what brand of panel box you have.  But if you must protect such
	a circuit (say, for a pool heater), you have no choice.

	One more caveat -- GFCI outlets are bulky.  You may want to use
	an oversize box when installing them.  On second thought, use
	large (actually deep) boxes everywhere.  You'll thank yourself for it.

	Incidentally, if you're installing a GFCI to ensure that one
	specific outlet is protected (such as a bathroom), you don't
	really have to go to all of the trouble to find the first
	outlet in the circuit, you could simply find the first outlet
	in the bathroom, and not GFCI anything upstream of it.  But
	protecting the whole circuit is preferred.