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神经网络昆斯林的新闻组分类2006

	utility knives fitted with linoleum cutters (hooked blades)
	can be used to strip sheath, but there is still the possibility
	that you'll gouge the conductors.

	For any substantial amount of work with armored cable, it's well
	worth your while to invest in a rotary cable splitter (~US$ 18).
	Hack saws are tricky to use without cutting into the wire
	or the insulation.

	Three-prong outlet testers are a quick check for properly-wired
	outlets.  About $6.  Multimeters tell you more, but are a lot more
	expensive, and probably not worth it for most people.  A simple
	voltage sensor, which can detect potential through an insulated
	wire not supplying any devices, is extremely helpful; they cost
	about US$ 10 at Radio Shack.

	You should have a voltage detector - to check that the wires are
	dead before doing work on them.  Neon-bulb version are cheap ($2-3)
	and work well.  If you get more serious, a "audible alarm" type is
	good for tracing circuits without a helper.  (Though I've been known
	to lock the drill on, and hit breakers until the scream stops ;-)

	For running wires through existing walls, you need fish tape.
	Often, two tapes are needed, though sometimes, a bent hanger or
	a length of thin chain will suffice.  Fish tapes can be rented.

	Electrical tape.  Lots of it ;-)  Seriously, a good and competent
	wiring job will need very little tape.  The tape is useful for
	wrapping dicy insulation in repair work.  Another use is to wrap 
	around the body of outlets and switches to cover the termination
	screws - I don't do this, but drywall contractors prefer it (to
	prevent explosions when the drywall knife collides with a live outlet
	that has no cover plate).

Subject: What is UL listing?

	The UL stands for "Underwriters Laboratory".  It used to be
	an Insurance Industry organization, but now it is independent
	and non-profit.  It tests electrical components and equipment
	for potential hazards.  When something is UL-listed, that means
	that the UL has tested the device, and it meets their requirements
	for safety - ie: fire or shock hazard.  It doesn't necessarily
	mean that the device actually does what it's supposed to, just
	that it probably won't kill you.

	The UL does not have power of law in the U.S. -- you are
	permitted to buy and install non-UL-listed devices.  However,
	insurance policies sometimes have clauses in them that will
	limit their liability in case of a claim made in response to
	the failure of a non-UL-listed device.  Furthermore, in
	many situations the NEC will require that a wiring component
	used for a specific purpose is UL-listed for that purpose.
	Indirectly, this means that certain parts of your wiring
	must be UL-listed before an inspector will approve it and/or
	occupancy permits issued.
	
Subject: What is CSA approval?

	Every electrical device or component must be certified by the
	Canadian Standards Association before it can be sold in
	Canada.  Implicit in this is that all wiring must be done
	with CSA-approved materials.  They perform testing similar to
	the UL (a bit more stringent), except that CSA approval is
	required by law.

	Again, like the UL, if a fire was caused by non-CSA-approved
	equipment, your insurance company may not have to pay the
	claim.

	In Canada, there is a branch organization of the UL, called ULC
	(UL of Canada).  ULC does not have power of law, and seems to
	be more a liason group between the CSA and insurance
	companies.

Subject: Are there any cheaper, easier to read books on wiring?

	USA: The following three books were suggested by our readers

	    Residential Wiring
	    by Jeff Markell,
	    Craftsman Books,
	    Carlsbad CA for $18.25. ISBN 0-934041-19-9.

	    Practical Electrical Wiring
	    Residential, Farm and Industrial,  Based on the National
	    Electrical Code    ANSI/NFPA 70
	    Herbert P. Richter and W. Creighton Schwan
	    McGraw-Hill Book Co.

	    Wiring Simplified
	    H. P. Richter and W. C. Schwan
	    Park Publishing Co.
	
	Try to make sure that the book is based on the latest NEC
	revision.  Which is currently 1990.

	Canada: P.S. Knight authors and publishes a book called
	"Electrical Code Simplified".  There appears to be a version
	published specific to each province, and is very tied into the
	appropriate provincial code.  It focuses on residential wiring,
	and is indispensible for Canadian DIY'ers.  It is better to get
	this book than the CEC unless you do a lot of wiring (or answer
	questions on the net ;-).

	It is updated each time the provincial codes are.  This book is
	available at all DIY and hardware stores for less than C$10.

Subject: Inspections how and what?  Why should I get my wiring inspected?

	Most jurisdictions require that you obtain a permit and
	inspections of any wiring that is done.  Amongst other more
	mundane bureaucratic reasons (like insurance companies not
	liking to have to pay claims), a permit and inspections
	provides some assurance that you, your family, your neighbors
	or subsequent owners of your home don't get killed or lose
	their homes one night due to a sloppy wiring job.

	Most jurisdictions have the power to order you to vacate your
	home, or order you to tear out any wiring done without a
	permit.  California, for instance, is particularly nasty about
	this.

	If fire starts in your home, and un-inspected wiring is at
	fault, insurance companies will often refuse to pay the damage
	claims.

	In general, the process goes like this:
		- you apply to your local inspections office or building
		  department for a permit.  You should have a sketch or
		  detailed drawing of what you plan on doing.  This is
		  a good time to ask questions on any things you're not
		  sure of.  If you're doing major work, they may impose
		  special conditions on you, require loading
		  calculations and ask other questions.  At this point
		  they will tell you which inspections you will need.
		- If you're installing a main panel, you will need to
		  have the panel and service connections inspected
		  before your power utility will provide a connection.
		  This is sometimes done by the local power authority
		  rather than the usual inspectors.
		- After installing the boxes and wiring, but before
		  the insulation/walls go up, you will need a
		  "rough-in" inspection.
		- After the walls are up, and the wiring is complete,
		  you will need a "final inspection".

Subject: My house doesn't meet some of these rules and regulations.
	Do I have to upgrade?

	In general, there is no requirement to upgrade older dwellings,
	though there are some exceptions (ie: smoke detectors in some
	cases).  However, any new work must be done according to the
	latest electrical code.  Also, if you do ``major'' work, you
	may be required to upgrade certain existing portions or all
	of your system.  Check with your local electrical inspector.

Subject: A word on voltages: 110/115/117/120/125/220/240

	One thing where things might get a bit confusing is the
	different numbers people bandy about for the voltage of
	a circuit.  One person might talk about 110V, another 117V
	or another 120V.  These are all, in fact, exactly the same
	thing...  In North America the utility companies are required
	to supply a split-phase 240 volt (+-5%) feed to your house.
	This works out as two 120V +- 5% legs.  Additionally, since there
	are resistive voltage drops in the house wiring, it's not
	unreasonable to find 120V has dropped to 110V or 240V has dropped
	to 220V by the time the power reaches a wall outlet.  Especially
	at the end of an extension cord or long circuit run.  For a number
	of reasons, some historical, some simple personal orneryness,
	different people choose call them by slightly different numbers.
	This FAQ has chosen to be consistent with calling them "110V" and
	"220V", except when actually saying what the measured voltage will
	be.  Confusing?  A bit.  Just ignore it.

	One thing that might make this a little more understandable
	is that the nameplates on equipment ofen show the lower (ie: 110V
	instead of 120V) value.  What this implies is that the device
	is designed to operate properly when the voltage drops that
	low.

	208V is *not* the same as 240V.  208V is the voltage between
	phases of a 3-phase "Y" circuit that is 120V from neutral to any
	hot.   480V is the voltage between phases of a 3-phase "Y"
	circuit that's 277V from hot to neutral.

	In keeping with 110V versus 120V strangeness, motors intended
	to run on 480V three phase are often labelled as 440V...

Subject: What does an electrical service look like?

	There are logically four wires involved with supplying the
	main panel with power.  Three of them will come from the utility
	pole, and a fourth (bare) wire comes from elsewhere.

 	The bare wire is connected to one or more long metal bars pounded
 	into the ground, or to a wire buried in the foundation, or sometimes
 	to the water supply pipe (has to be metal, continuous to where
	the main water pipe entering the house.  Watch out for galvanic
	action conductivity "breaks" (often between copper and iron pipe)).
	This is the "grounding conductor".  It is there to make sure that
	the third prong on your outlets is connected to ground.  This wire
	normally carries no current.

	One of the other wires will be white (or black with white or
	yellow stripes, or sometimes simply black).  It is the neutral wire.
	It is connected to the "centre tap" (CEC; "center tap" in the
	NEC) of the distribution transformer supplying the power.  It
	is connected to the grounding conductor in only one place (often
	inside the panel).  The neutral and ground should not be connected
	anywhere else.  Otherwise, weird and/or dangerous things may happen.

	Furthermore, there should only be one grounding system in
	a home.  Some codes require more than one grounding electrode.
	These will be connected together, or connected to the neutral
	at a common point - still one grounding system.  Adding additional
	grounding electrodes connected to other portions of the house
	wiring is unsafe and contrary to code.

	If you add a subpanel, the ground and neutral are usually
	brought as separate conductors from the main panel, and are
	not connected together in the subpanel (ie: still only one
	neutral-ground connection).  However, in some situations 
	(certain categories of separate buildings) you actually do
	have to provide a second grounding electrode - consult your
	inspector.

	The other two wires will usually be black, and are the "hot"
	wires.  They are attached to the distribution transformer as
	well.

	The two black wires are 180 degrees out of phase with each
	other.  This means if you connect something to both hot wires,
	the voltage will be 220 volts.  If you connect something to the
	white and either of the two blacks you will get 110V.

	Some panels seem to only have three wires coming into them.
	This is either because the neutral and ground are connected
	together at a different point (eg: the meter or pole) and one
	wire is doing dual-duty as both neutral and ground, or in some
	rare occasions, the service has only one hot wire (110V only
	service).

Subject: What is a circuit?

	Inside the panel, connections are made to the incoming wires.
	These connections are then used to supply power to selected
	portions of the home.  There are three different combinations:
		1) one hot, one neutral, and ground: 110V circuit.
		2) two hots, no neutral, and ground: 220V circuit.
		3) two hots, neutral, and ground: 220V circuit + neutral,
		   and/or two 110V circuits with a common neutral.

	(1) is used for most circuits supplying receptacles and
	lighting within your house.  (3) is usually used for supplying
	power to major appliances such as stoves, and dryers - they
	often have need for both 220V and 110V, or for bringing several
	circuits from the panel box to a distribution point.  (2) is
	usually for special 220V motor circuits, electric heaters, or
	air conditioners.

	[Note: In the US, the NEC frequently permits a circuit similar
	to (2) be used for stoves and dryers - namely, that there
	are two hot wires, and a wire that does dual duty as neutral
	and ground, and is connected to the frame as well as providing
	the neutral for 110V purposes - three prong plugs instead
	of four (*only* for stoves/dryers connected to the main panel.
	When connected to most sub-panels, 4 prong plugs and receptacles
	are required).  In our not-so-humble opinion this is crazy, but
	the NFPA claims that this practice was re-evaluated for the 1992 NEC,
	and found to be safe.  Check your local codes, or inquire as to
	local practice -- there are restrictions on when this is
	permissible.]

	(1) is usually wired with three conductor wire: black for hot,
	white for neutral, and bare for grounding.

	(2) and (3) have one hot wire coloured red, the other black, a
	bare wire for grounding, and in (3) a white wire for neutral.

	You will sometimes see (2) wired with just a black, white and ground
	wire.  Since the white is "hot" in this case, both the NEC and CEC