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Re: About plugging things.



At 09:49 PM 8/20/00 -0400, David Bowman wrote:
Regarding Nexus Replicant's questions:

>Why is there a ground in electric appliances.

To make sure the metallic case of the appliance is always at ground
potential. This would tend to insure one's safety when touching it.

> Why not just having a
>positive and a negative wire. I would also like to know if both the black
>and red wire are above ground or if one or both are below. Thanks in
>advance to anyone who will answer.

Commercial power lines have supplied AC power to the public since
Westinghouse (using Tesla's superior AC system), understandably, beat out
his competitor Edison (who pushed DC power) in the marketplace around a
century ago.

Think about the meaning of AC.

Let me add that if Mr. Replicant has an appliance that is wired with a red
wire and a black wire, it ought to be a 240V appliance -- usually limited
to ovens and other things amateurs really shouldn't be messing with.

But in the spirit of answering the question that _should_ have been asked,
consider the ordinary household outlet. It is typically wired with a black
wire (hot) and a white wire (neutral) and a bare or green wire (safety
ground). The white wire is a lot closer to ground than the black wire is
-- but you don't want to go around assuming it "is" ground. It is tied to
ground at one location in the house, but IR drops ensure that it won't be
at ground at other locations.

We've all seen lots of appliances with two-wire cords. The green wire,
when present, is never part of the normal circuit; it is just there for
safety. It plays two roles when there is a fault:
-- it forms a voltage divider, greatly lowering the voltage that the
fault can apply to the case of the appliance, and
-- it ensures that if the fault is serious enough to apply a significant
voltage to the case, it will draw enough current to trip the circuit breaker.

In house wiring, the red wire is just as hot as the black wire. It is
(typically) 180 degrees out of phase with the black wire. This gives you
black minus white = 120V
red minus white = 120V
black minus red = 240V

For related info see
http://fribble.cie.rpi.edu/~repairfaq/REPAIR/F_appfaq3.html#APPFAQ_005

==========

Don't mess with this stuff unless you know what you are doing;
otherwise you endanger your life and your associates' lives.