"Danny D'Amico" wrote in message
...
On Thu, 14 Nov 2013 19:39:38 -0600, dpb wrote:

the third wire is the ground but NEC until
relatively recently allowed the ground to also be the neutral

You seem to understand this better than I do.

The way *I* understand a "ground" is that a ground wire carries no current
(unless there is a problem). The "neutral" wire, on the other hand,
*always*
carries current.

That's a pretty big difference (as I understand it anyway).

More specifically, the way I understand a ground wire, is that it goes
from the receptacle in the wall to the main breaker panel, where it
literally is driven directly into the ground (usually by some kind of
bar).

In contrast, the neutral wire, as I understand it, goes to the same
breaker panel, but then it goes from there to the power pole, and then
from that pole it may travel hundreds of feet to a few more power poles,
but eventually, it too is driven straight into the ground.

The difference, as I understand it, is that the ground never carries
current (unless there is a fault), while the neutral is always carrying
current (and therefore it might have a potential on it).

Given that they're not at all the same thing, I then have trouble
understanding the statement that the ground is "also" a neutral.

Again, you seem to understand better than I do, but, the way I
described it above, a ground and a neutral are totally different
things.

So, I don't understand how a ground can 'also' be a neutral.

The power comes into your house by 3 wires. At the pole is a transformer
that has a wire on each end of a coil and one from the center. If you only
used 240 volt devices there would be no need for the neutral wire. If the
power usage on each side of the 120 volt lines were exectally the same there
would not be any need for the neutral going to the pole. As the sides are
not exectally ballanced and often not ballanced very well at all, the
neutral carries the unballanced current. In the house the 120 volt circuits
use one of the hot wires and the neutral for the return.

At your breaker box the neutral is bolted to the frame of the box and it
also goes to the recepticls and other parts of the house. The ground wire
goes to a rod outside the house that is driven into the ground. It is
bolted to the frame of the breaker box. It then also goes whever the power
wires goes to such as the recepticals. In effect the neutral and ground
wire are the same wire, but perform differant functions. Often the neutral
or ground wire may be a differant size than the two hot 240 volt wires. If
the neutral and ground wires are the same size, there is no electrical
reason that you could not use either of them for the ground or neutral. YOu
do not want to do this as it can create great confusion to the people doing
the wiring. That is one reason the neutral is insulated and white and the
ground is either bare or green.

The two hot wires may be red and black or maybe just two black wires as it
does not usually mater which wire is hooked to which side of the 240 volt
device.

YOu are correct, there should not be any current on the ground wire unless
there is a problem. If one of the hot wires shorts to the frame of the
dryer and there is no ground or neutral (on a 3 wire plug) connected to the
frame, it becomes 120 volts to ground and if you or anything conductive gets
between the frame and the real ground or another device that has its frame
grounded, there will be current flowing and could shock or kill.


If in the 3 wire circuit the neutral becomes disconnected down line of the
dryer, the frame of the dryer will become hot with 120 volts minus a small
ammount. That small ammount could be the timmer and light bulb in the dryer
that is still connected to one side of the 240 volt line.
That is the reason for using the 4th wire that is only connected to the
frame of the dryer in later years.