shop-talk
[Top] [All Lists]

Re: A simple question about electricity......

To: Randall <randallyoung@earthlink.net>
Subject: Re: A simple question about electricity......
From: Douglas Shook <shook@usc.edu>
Date: Wed, 25 Jul 2001 12:57:17 -0700
Randall wrote:
> 
> Jack :
> 
> I'm not sure I can explain this, so bear with me.  The safety ground is
> grounded, well, for safety.  It provides a 'safe' reference point that
> metal appliances can be tied to, so that there's no chance of your being
> shocked by the leakage currents that are pretty much always present (or
> any fault that occurs).  Once upon a time, the neutral (or common) wire
> was used for this purpose, but there are (at least) two big drawbacks to

Hi all,

What Randall explained is exactly right. I thought that an
example might help to amplify what he explained. 

Forget 220VAC for a moment. If you have a 110VAC circuit, that
110VAC potential is between the hot wire and ground.  Each
circuit needs a hot (black) and neutral (white) to complete the
circuit. It is against code to fuse or switch the neutral side as
the device (let's say a motor) would still have potential to
ground (be hot) if switched off. 

By using ground as the neutral, there is no potential between
someone who is standing on the ground (or touching a copper water
line) and the white wire.  There is potential between the black
wire and ground, though.

For that reason, if you had an electrical motor which had a short
between the hot wire and the metal case of the motor, and the
metal case of the motor did not have the green wire connecting
the case to ground (here we are talking about a two wire
connection with a black and white wire), the motor would continue
to run fine, but the case now also is hot.  If someone were to
touch the case, there would be 110VAC potential between the case
and ground, which could give the person a shock.

If, however, we take the same scenario with a grounded circuit (a
hot black, a neutral white, and a green ground connected to the
case), when the hot wire in the motor shorted to the case of the
motor, it would blow the fuse on the hot side as the current
would be shorted directly to ground. The fuse would blow, the
motor would stop and the case would be safe to touch. 

That third wire, the green ground wire, pretty much guarantees
that electrical motors, washing machines, etc., will not be hot
(the metal surfaces will not be hot) to touch as they are always
grounded (there cannot be any potential between the floor you are
standing on and the case of the device you are touching if it is
grounded).

Ground circuits are not intended normally to carry current.  Only
to carry current during a short, and to maintain ~0 VAC potential
between the metal surfaces of appliances and the ground you are
standing on. Ground can be run down metal conduit on two wire
circuits.

For 220VAC installations, the same scenario holds true, except
220VAC potential is achieved between two hot wires, so there is
no neutral wire.  In this case, the white wire is used for
grounding and to provide for 110VAC leg if needed.

For 220VAC, you would have two blacks (or a black and a red) for
the two hot legs, and a white wire for the ground. In this
example, the white carries no current unless there is a short to
it from one of the hot legs, in which case, the short to ground
should blow the fuse instead of letting the case of the motor be
hot.

regards,

doug shook

///
///  shop-talk@autox.team.net mailing list
///  To unsubscribe send a plain text message to majordomo@autox.team.net
///  with nothing in it but
///
///     unsubscribe shop-talk
///
///


<Prev in Thread] Current Thread [Next in Thread>