Re: Potentially stupid question about AC

[Rubywand <rubywand@swbell.net>]

Supertimer writes ...
> 
 .... 
> DC makes sense.
>
     Sort of. We have a lot of terms for electronic stuff (e.g. "electrons")
and rules that work most of the time (like current = voltage/resistance).

 
> 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).

     The neat attribute of AC is that you can transport power (Voltage x
Current) long distances at a high voltage on relatively small lines. When it
gets where you want it-- like a line going past a row of homes-- you can send
it through a simple step-down transformer at each home to get the lower
household voltage and a high current capability.



> 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?
 ....

     Pretty much.

     The more equipment you have using AC motors, transformers, and circuits
which actually run a capacitor from AC to some chassis, the greater the
potential for 'leakage'-- i.e. for having some capacitively coupled voltage
that 'wants' to go to ground.   


     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.)

     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.

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.)

     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.



Rubywand