53569

神经网络昆斯林的新闻组分类2006

	accidentally buried in insulation.  Ie: if you have insulation blown
	in later.]

	There are now fixtures that contain integral thermal cutouts and
	fairly large cases that can be buried directly in insulation.  They are
	usually limited to 75 watt bulbs, and are unfortunately, somewhat
	more expensive than the older types.  Before you use them, you should
	ensure that they have explicit UL or CSA approval for such uses.
	Follow the installation instructions carefully; the prescribed location
	for the sensor can vary.

	There does not yet appear to be a heat lamp fixture that is approved
	for use in insulation.  The "coffin" appears the only legal approach.

Subject: What does it mean when the lights brighten when a motor starts?

	This usually means that the neutral wire in the panel is
	loose.  Depending on the load balance, one hot wire may end up
	being more than 110V, and the other less than 110V, with
	respect to ground.  This is a very hazardous situation - it can
	destroy your electronic equipment, possibly start fires, and in
	some situations electrocute you (ie: some US jurisdictions
	require the stove frame connected to neutral).

	If this happens, contact your electrical authority immediately
	and have them come and check out the problem.

	Note: a brief (< 1 second) brightening is sometimes normal with
	lighting and motors on the same 220V with neutral circuit.  A
	loose main panel neutral will usually show increased brightness
	far longer than one second.  In case of doubt, get help.

Subject: What is 3 phase power?  Should I use it?  Can I get it in my house?

	Three phase power has three "hot" wires, 120 degrees out of
	phase with each other.  These are usually used for large motors
	because it is more "efficient", provides a bit more starting torque,
	and because the motors are simpler and hence cheaper.

	You're most likely to encounter a 3 phase circuit that shows
	110 volts between any hot and ground, and 208 volts between
	any two hots.  The latter shows the difference between a normal
	220V/110V common neutral circuit, which is 240 volts between the
	two hots.  There are 3 phase circuits with different voltages.

	Bringing in a 3 phase feed to your house is usually
	ridiculously expensive, or impossible.  If the equipment you
	want to run has a standard motor mount, it is *MUCH* cheaper to
	buy a new 110V or 220V motor for it.  In some cases it is
	possible to run 3 phase equipment on ordinary power if you have
	a "capacitor start" unit, or use a larger motor as a
	(auto-)generator.  These are tricky, but are a good solution if
	the motor is non-standard size, or too expensive or too big to
	replace.  The Taunton Press book ``The Small Shop'' has an
	article on how to do this if you must.

	Note that you lose any possible electrical efficiency by using
	such a converter.  The laws of thermodynamics guarantee that.

Subject: Is it better to run motors at 110 or 220?

	Theoretically, it doesn't make any difference.  However, there
	is a difference is the amount of power lost in the supply
	wiring.  All things being equal, a 220V motor will lose 4 times
	less power in the house wiring than a 110V motor.  This also
	means that the startup surge loss will be less, and the motor
	will get to speed quicker.  And in some circumstances, the
	smaller power loss will lead to longer motor life.

	This is usually irrelevant unless the supply wires are more
	than 50 feet long.

Subject: What is this nonsense about 3HP on 110V 15A circuits?

	It is a universal physical law that 1 HP is equal to 746
	watts.  Given heating loss, power factor and other inefficiencies,
	it is usually best to consider 1 HP is going to need 1000-1200
	watts.  A 110V 15A circuit can only deliver 1850 watts to a motor,
	so it cannot possibly be more than approximately 2 HP.  Given rational
	efficiency factors, 1.5HP is more like it.

	Some equipment manufacturers (Sears in particular, most router
	manufacturers in general ;-) advertise a HP rating that is far
	in excess of what is possible.  They are giving you a "stall
	horsepower" or similar.  That means the power is measured when
	the motor is just about to stop turning because of the load.
	What they don't mention is that if you kept it in that
	condition for more than a few seconds hopefully your breaker
	will trip, otherwise the motor will melt -- it's drawing far
	more current than it can continuously.

	When comparing motors, compare the continuous horsepower.  This
	should be on the motor nameplate.  If you can't find that figure,
	check the amperage rating, which is always present.

Subject: How do I convert two prong receptacles to three prong?

	Older homes frequently have two-prong receptacles instead
	of the more modern three.  These receptacles have no safety
	ground, and the cabling usually has no ground wire.  Neither
	the NEC or CEC permits installing new 2 prong receptacles anymore.

	There are several different approaches to solving this:
	    1) If the wiring is done through conduit or BX, and the
	       conduit is continuous back to the panel, you can connect
	       the third prong of a new receptacle to the receptacle
	       box.  NEC mainly - CEC frowns on this practise.
	    2) If there is a copper cold water pipe going nearby, and
	       it's continuous to the main house ground point, you can
	       run a conductor to it from the third prong.
	    3) Run a ground conductor back to the main panel.
	    4) Easiest: install a GFCI receptacle.  The ground lug
	       should not be connected to anything, but the GFCI
	       protection itself will serve instead.  The GFCI
	       will also protect downstream (possibly also two prong
	       outlets).  If you do this to protect downstream outlets,
	       the grounds must not be connected together.  Since it
	       wouldn't be connected to a real ground, a wiring fault
	       could energize the cases of 3 prong devices connected
	       to other outlets.  Be sure, though, that there aren't
	       indirect ground plug connections, such as via the sheath
	       on BX cable.

	The CEC permits you to replace a two prong receptacle with a three
	prong if you fill the U ground with a non-conducting goop.
	Like caulking compound.  This is not permitted in the NEC.

Subject: Are you sure about GFCIs and ungrounded outlets?
	Should the test button work?

	We're sure about what the NEC and CEC say.  Remember, though,
	that your local codes may vary.  As for the TEST button -- there's
	a resistor connecting the LOAD side of the hot wire to the LINE
	side of the neutral wire when you press the TEST button.  Current
	through this resistor shows up as an imbalance, and trips the GFCI.
	This is a simple, passive, and reliable test, and doesn't require
	a real ground to work.  If your GFCI does not trip when you press
	the TEST button, it is very probably defective or miswired.  Again:
	if the test button doesn't work, something's broken, and potentially
	dangerous.  The problem should be corrected immediately.

	The instructions that come with some GFCIs specify that the ground
	wire must be connected.  We do not know why they say this.  The
	causes may be as mundane as an old instruction sheet, or with the
	formalities of UL or CSA listing -- perhaps the device was never
	tested without the ground wire being connected.  On the other hand,
	UL or CSA approval should only have been granted if the device
	behaves properly in *all* listed applications, including ungrounded
	outlet replacement.  (One of us called Leviton; their GFCIs are
	labeled for installation on grounded circuits only.  The technician
	was surprised to see that; he agreed that the NEC does not require
	it, and promised to investigate.)

Subject: How should I wire my shop?

	As with any other kind of wiring, you need enough power for all
	devices that will be on simultaneously.  The code specifies
	that you should stay under 80% of the nominal capacity of the
	circuit.  For typical home shop use, this means one circuit for
	the major power tools, and possibly one for a dust collector or
	shop vac.  Use at least 12 gauge wire -- many power tools have
	big motors, with a big start-up surge.  If you can, use 20 amp
	breakers (NEC), though CEC requires standard 20A receptacles
	which means you'd have to "replug" all your equipment.  Lights
	should either be on a circuit of their own -- and not shared
	with circuits in the rest of the house -- or be on at least two
	separate circuits.  The idea is that you want to avoid a
	situation where a blade is still spinning at several thousand
	RPM, while you're groping in the dark for the OFF switch.

	Do install lots of outlets.  It's easier to install them in the
	beginning, when you don't have to cut into an existing cable.
	It's useful if at least two circuits are accessible at each
	point, so you can run a shop vac or a compressor at the same
	time as the tool you really want.  But use metal boxes and
	plates, and maybe even metal-sheathed cable; you may have
	objects flying around at high speeds if something goes a bit
	wrong.

	Note that some jurisdictions have a "no horizontal wiring"
	rule in workshops or other unfinished areas that are used
	for working.  What this means is that all wiring must be
	run along structural members.  Ie: stapled to studs.

	Other possible shop circuits include heater circuits, 220V
	circuits for some large tools, and air compressor circuits.
	Don't overload circuits, and don't use extension cords if you
	can help it, unless they're rated for high currents.  (A coiled
	extension cord is not as safe as a straight length of wire of
	the same gauge.  Also, the insulation won't withstand as much
	heat, and heat dissipation is the critical issue.)

	If your shop is located at some remove from your main panel,
	you should probably install a subpanel, and derive your shop
	wiring from it.  If you have young children, you may want to
	equip this panel with a cut-off switch, and possibly a lock.
	If you want to install individual switches to ``safe''
	particular circuits, make sure you get ones rated high enough.
	For example, ordinary light switches are not safely able to
	handle the start-up surge generated by a table saw.  Buy
	``horsepower-rated'' switches instead.

	Finally, note that most home shops are in garages or unfinished
	basements; hence the NEC requirements for GFCIs apply.  And
	even if you ``know'' that you'd never use one of your shop
	outlets to run a lawn mower, the next owner of your house might
	have a different idea.

	Note: Fine Woodworking magazine often carries articles on shop
	wiring.  April 1992 is one place to start.

Subject: Underground Wiring

	You will need to prepare a trench to specifications, use
	special wire, protect the wire with conduit or special plastic
	tubing and possibly lumber (don't use creosoted lumber, it rots
	thermoplastic insulation and acts as a catalyst in the corrosion
	of lead).  The transition from in-house to underground wire is
	generally via conduit.  All outdoor boxes must be specifically
	listed for the purpose, and contain the appropriate gaskets,
	fittings, etc.  If the location of the box is subject to immersion
	in water, a more serious style of water-proof box is needed.  And
	of course, don't forget the GFCIs.

	The required depths and other details vary from jurisdiction to
	jurisdiction, so we suggest you consult your inspector about
	your specific situation.

	A hint: buy a roll of bright yellow tape that says "buried power
	line" and bury it a few inches above where the wire has been placed.

Subject: Aluminum wiring

	During the 1970's, aluminum (instead of copper) wiring became
	quite popular and was extensively used.  Since that time,
	aluminum wiring has been implicated in a number of house fires,
	and most jurisdictions no longer permit it in new installations.
	We recommend, even if you're allowed to, that do not use it for new
	wiring.

	But don't panic if your house has aluminum wiring.  Aluminum
	wiring, when properly installed, can be just as safe as copper.
	Aluminum wiring is, however, very unforgiving of improper
	installation.  We will cover a bit of the theory behind potential
	problems, and what you can do to make your wiring safe.

	The main problem with aluminum wiring is a phenomenon known as
	"cold creep".  When aluminum wiring warms up, it expands.  When
	it cools down, it contracts.  Unlike copper, when aluminum goes
	through a number of warm/cool cycles it loses a bit of tightness each
	time.  To make the problem worse, aluminum oxidises, or corrodes
	when in contact with certain types of metal, so the resistance
	of the connection goes up.  Which causes it to heat up and corrode/
	oxidize still more.  Eventually the wire may start getting very hot,
	melt the insulation or fixture it's attached to, and possibly even
	cause a fire.

	Since people usually encounter aluminum wiring when they move
	into a house built during the 70's, we will cover basic points of
	safe aluminum wiring.  We suggest that, if you're considering purchasing
	a home with aluminum wiring, or have discovered it later, that you
	hire a licensed electrician or inspector to check over the wiring
	for the following things:

	    1) Fixtures (eg: outlets and switches) directly attached to
	       aluminum wiring should be rated for it.  The device will
	       be stamped with "Al/Cu" or "CO/ALR".  The latter supersedes
	       the former, but both are safe.   These fixtures are somewhat
	       more expensive than the ordinary ones.

	    2) Wires should be properly connected (at least 3/4 way around
	       the screw in a clockwise direction).  Connections should be
	       tight.  While repeated tightening of the screws can make the
	       problem worse, during the inspection it would pay off to snug
	       up each connection.

	       Note that aluminum wiring is still often used for the
	       main servic