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
that :
1) Current flowing through the wire means the end of the wire attached
to the appliance is not at exactly the same voltage as the end connected
to ground. And since it's quite common to have different circuits
adjacent to each other (eg 220v range next to a 110v refrigerator), the
different voltages can be a problem. You can get a noticeable (although
usually harmless) shock on as little as 6 volts, especially if your skin
is wet.
2) If there were a break in the neutral wire for any reason, all the
appliances that are connected to it would suddenly become 'hot'. BTW,
I've seen several older homes that were wired with a fuse in the
neutral. In my previous house (built in the 20's), there was a light
bulb in the utility room that would burn very bright when the washing
machine blew a fuse, because the fuse was in the neutral, and the bulb
was wired from that neutral to the other leg of the 220v circuit. When
the bulb was bright, the neutral had roughly 110 volts on it.
So, why is the neutral/common grounded ? Two reasons :
1) There are still lots of devices around that are wired to the old
standard. New devices are allowed to be wired this way under some set
of circumstances, most lights for instance use a 2-wire cord. The plug
is polarized (only fits one way) so the part of the light socket that
you are more likely to touch (the base of the bulb) can be connected to
the neutral.
2) It keeps the voltage between ground and all the wiring in your home
as low as possible. If the neutral were not connected to ground, it
would likely float up to some fraction of the primary voltage on the
power line transformer, which is typically 4000 volts. This is enough
to puncture the insulation on many things and is just not a real good
idea.
BTW, I'm talking about USA wiring standards. Other countries may be
(usually are) different.
Hope that clarifies a little
Randall
McVintage@aol.com wrote:
>
> randallyoung@earthlink.net writes:
>
> > The common serves a
> > different purpose than the ground (aka safety ground), and is also tied
> > back to the line transformer output. In a nutshell, the common carries
> > current, while the safety ground should never carry current.
>
> then why are they both connected to the same bar in the breaker box ?
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