Re: About plugging things.

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Suppose an appliance is connected to AC through a
simple transformer located next to it. In that case I can
ground one of the wires "powering the gadget". Once
this is done I can connect the metallic frame of the
appliance to the grounded wire. This would insure
my safety. The third wire would NOT be necessary.
No current would flow through my body touching
the frame, even if my feet were grounded.

The protecting fuse burns at once when the life
wire (whose potential is fluctuating between +
and - Vmax) accidentally touches the frame.

But I would not be protected if the transformer were
grounded in the middle of the secondary winding
and the appliance were connected to the end points of
that winding. In that case the frame would have to be
connected to the ground (at the center of the winding)
with the third wire (to protect me).

The frame must be connected to the grounded point
through a minimal resistance R. When R is not small
then the I*R may be sizable and the frame potential
may fluctuate about zero. Touching the frame, while
my feet are locally grounded, would produce a
sizable current through my body.

The R is small when the frame is grounded locally.
It would not necessarily be small if it were connected
to a far away ground point.
Ludwik Kowalski


Without that wire
small. rather than
far away.

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.