Re: Potentially stupid question about AC

[dalloff@gcfn.org (Dave Althoff)]

Gee, I go away for a couple of months, and look what happens...!

I'm all confused about the attribution at this point...but you can figger
it out.  You guys ought to stick with Apple ][ stuff that you really know
about!

: >> So how does AC work?  I know that the current
: >> alternates (back and forth) so negative and positive
: >> reverses sides (either 60 or 50 times a second
: >> depending on the country).

: >> I also know that if
: >> electricity is leaked, in a grounded device the
: >> electricity goes into a wire that leads to a post
: >> that's driven deep into the ground (hence ground
: >> wire).
: >> 
: >> So why is one lead considered "live" and the other
: >> "neutral" if they seem for all intents and purposes
: >> to be the same.  You can plug something in
: >> backward and it'll still work.  Also, why does the
: >> electricity go into the ground if it leaks from a
: >> grounded device?  Does the earth and the power
: >> station form a circuit or something?

What you have is an oscillating potential which is referenced *to ground*.
 In a single-phase 115v circuit, you have a single 'hot' wire and a
'neutral' wire.  The neutral is bonded to ground in the breaker panel...if
you open your breaker panel you will find that the white wires and the
green wires are all tied to the same bus-bar.  

The current comes into the house on the two hot wires...the black and the
red if you like.  Those lines carry opposite potentials which, when
compared with ground, vary continuously between -115v and +115v...so if
you tap both of the lines coming in, you get 230v, varying in polarity
with time.  Each line will rise to +115v, then fall to 0v, then drop to
-115v then rise to 0v relative to ground (I'm talking DC voltage here) in
1/60 second (America and Japan; 1/50 in the rest of the world); the two
lines...coming from the same source which is probably 3-phase which I
really don't properly understand (though I should)...so the two lines are
exactly 180-degrees out of phase with one another.  Okay?


: >     From what I recall about this stuff, when the AC comes off a transformer
: >to your home you usually get 220V with a center tap:
: >
: >
: >
: >S______(hot)_______________ typically Black (220V from hot to hot)*
: >S
: >S                110V
: >S____(neutral)__________    typically White*
: >S
: >S                110V
: >S______(hot)_______________ typically Red*
: >
: >
: >(110V and 220V are the traditional values. The actual voltages are going to be
: >closer to 115V and 230V.)

Exactly.

: >     Except for 220V wiring used by some major appliances, most of your wiring
: >is going to be 110V and come from a "hot" line plus "neutral". The usual color
: >coding for 110V wiring is 
: >
: >Black-- hot*
: >White-- neutral*
: >Green-- an earth ground (via a grounding rod)*
: >
: >*WARNINGS: By the way, it is best not to place a bunch of faith in color
: >coding. Not everyone follows the "usual" coding. Some wiring may reverse black
: >and white.
: >
: >For sure, you do not want to place any faith in any 'big hole', 'little hole'
: >conventions for wall sockets. Just assume they're both hot.

Again...exactly.

: Ok, this is what I was originally confused about.  What is the
: difference between hot and neutral if appliances run if you
: reverse the leads?

The difference is that the current is on the 'hot' line while the other
line is connected to ground, completing the circuit.  The reason the lamp
or appliance works either way is that you still have a complete
circuit...current flows both ways through the circuit, so the direction of
flow isn't all that important.  Motors do not run backwards when plugged
in backwards because their coils are wound in a non-polarized fashion.  In
fact, some motors can turn either direction *with the same wiring*.

: And why is it dangerous to defeat a polarized plug?

Frequently, one side of the appliance will be tied to the neutral line,
which should have 0v potential relative to ground (since it *is* the
ground).  If you plug the appliance in backwards, suddenly the chassis is
connected to the hot side of the power line, so if you are grounded and
you touch the chassis, you get nailed.  This is also a good reason for the
safety ground...most products do not have one side of the plug wired to
the chassis, but the chassis should be bonded to the safety ground.  That
way, if the incoming power cord is damaged, for instance, and the hot wire
comes in contact with the chassis, the current is guaranteed a path to
ground that doesn't involve going through the end user.

: >Avoid counting a lot on voltage measurements to earth ground as a guarantee of
: >no shock hazard. Many meters load down the measured circuit and do not
: indicate
: >true voltage at small currents. Even at low currents, a decent voltage can
: make
: >you jump and break something.
: >
: >Water pipes should _not_ be counted on for use as an earth ground.
: >
: >     I don't know if white (neutral) is commonly tied to a grounding rod at
: the
: >utility pole (where the transformer is), at your home, or at either place.
: Even
: >if it is, you would be likely to measure some voltage from neutral at a wall
: >socket to earth ground because lights, etc. on the same circuit are pulling
: >current through the neutral line and your household wires are not perfect
: >conductors. (That is, the neutral line on a particular socket could behave
: >something like a tap on a resistor connected across a voltage.)

Very good.  Yes, the neutral is typically connected to the ground rod via
the grounding bus bar in the distribution box.  The only exception I can
think of off-hand is seperately derived systems...ie generators...where
there is to be a ground rod or two at the generator, and 5 wires carried
through the distribution panels (I'm thinking US 3-phase here.  Note also
that European 3-phase wiring does not use a neutral wire.)

: >     So, you can often get at least a small shock from touching a white
: >"neutral" wire at a 110V socket if you are at all gounded. Touching the black
: >"hot" wire can deliver a much more serious shock if you are at all grounded.
: >Touching the black and white wires at the same time will definitely deliver a
: >serious shock.

: So both hot and neutral wires carry current, correct?  The
: difference is that the neutral wire is also grounded (at the
: unity pole where the transformer is)?  So how does the
: polarized plugs work (is the something in the appliance
: connected preferentially to one lead rather than the other)?

Yup.  Has to do with the locations of the lines, mostly.  For instance,
the contact on the bottom of a standard incandescent light bulb is
typically connected to the hot wire so that if you touch the exposed screw
while removing the lamp for replacement, you do not come directly into
contact with the hot line on the exposed screw base.

: I'm asking because I used to be called to install household
: fixtures for relatives and I used to always connect the wires
: any which way.  I mean, polarity did not seem to matter
: since the appliances always worked either way.  Now it
: turns out there is a safety concern.  Why?

You're one of the reasons we can't always be sure the small blade is the
hot wire, aren't you?!  8-)

--Dave Althoff, ][.
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