On Feb 16, 7:04*pm, Brian wrote:
If on a 220 single phase circuit the neutral and ground wires attach to the same buss bar in the
panel, why are they both needed ????

Thanks
Brian

Brian

First things first. If it were in fact a 208,220,230,240 single phase
circuit it would have only three conductors.

In that case each of the two conductors that are neither bare
(uninsulated), green, or white would be the nominal phase voltage
relative to ground. The third conductor is the Equipment Grounding
Conductor (EGC). It might take the form of a bonded metal cable
jacket, cable tray, or a metallic raceway, or a wire much like the
other conductors of the circuit. If it is in the form of a wire it
will be either bare (Uninsulated) or green in color or if it is
American Wire Gage (AWG) size four or larger it would be color coded
green at all accessible points. The reason that the third conductor
can take the form of a metal enclosure around or supporting the
energized conductors is that it only carries current during a fault
condition. Provided that the EGC was properly sized and installed it
will carry the fault current so well that the Over Current Protective
Device (OCPD) that is protecting the faulted current carrying
conductor will open and thus clear the fault.

If the circuit is a dual voltage circuit, such as 120/240 volt North
American residential service range, clothes dryer, or feeder circuit
then it will have both 120 volt and 240 volt loads supplied from the
same circuit. In that case a forth conductor is run with the other
current carrying conductors. That grounded current carrying
conductor; which is commonly called the neutral; will be white in
color or may be coded white at all accessible locations. Under most
conditions of use it will be insulated and will not be supplied
through an OCPD.

Up until quite recently it was permissible to bond the chassis of
certain heavy appliances, such as stoves and clothes dryers, to the
neutral conductor that served the loads in that appliance. Under some
narrower circumstances the neutral conductor could be bare but never
green. Since certain uses, found mostly in residences supplied with
their own electrical service directly from the utility, the neutral
could be bare some confusion has developed as to whether it was an EGC
or a neutral. The code however was always clear that certain
appliance chassis could be bonded to the neutral instead of having a
separate EGC. The thinking was that these exception permitted
circuits were never smaller then number ten AWG and were nearly always
run from compression terminals to compression terminals so there was
little risk of an open or high resistance connection. Likewise the
feeder circuits were also run from and to compression terminals.
Experience has shown that, although they are rare, high resistance and
open conductors do occur in these circuits. Since the consequence of
such circuit failures was the energizing of the chassis of large
appliances or of the entire EGC system of separate buildings or feeder
supplied panels at 120 volts relative to ground this practice has now
been removed from the code and is only permissible in existing
installations that predate the respective code changes.

What was wrong with this technique is that it violates a basic
principle of safety engineering that holds that it should always take
two or more failures in the system, at least one of which is easily
recognizable as a condition that requires correction, to create an
immediate hazard to life or health. With the four wire circuit any
open in the neutral causes noticeable undesired operation such as the
dryer not coming on or the stove's light and controls not working.
Most importantly the undesired operation does not occur simultaneously
with an immediate deadly hazard so that the users are unlikely to put
up with the condition long enough for a complimentary fault to occur
which would turn the situation hazardous.

I hope that is helpful
--
Tom Horne