On Tue, 26 Sep 2006, Bruce Rice wrote:
In most areas of the U.S.A. we have 120/240vac. The National
Electrical Code requires the Grounded Conductor (the ground
wire - usually green or bare copper) to be connected to a
buried ground rod and to metallic piping of the water line
(usually at the hot water heater) and to the service entrance
of the natural gas line (where the line enters the house).
Don't even ask about all the variables that can be found,
there are zillions.
The Grounding Conductor (Neutral-usually white insulation) is
connected to the Grounded Conductor in the distribution
panel.
The last paragraph says it all. :) This is THE most important
concept in electric wiring service in regard to the ground
wire. The following web site reveals why this is so.
--------------------------------
<http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html#c3>
[snip]
In the event of an electrical fault which brings dangerous high
voltage to the case of an appliance, you want the circuit
breaker to trip immediately to remove the hazard. If the case
is grounded, a high current should flow in the appliance ground
wire and trip the breaker. That's not quite as simple as it
sounds - tying the ground wire to a ground electrode driven
into the earth is not generally sufficient to trip the breaker,
which was surprising to me. The U.S. National Electric Code
Article 250 requires that the ground wires be tied back to the
electrical neutral at the service panel. So in a line-to-case
fault, the fault current flows through the appliance ground
wire to the service panel where it joins the neutral path,
flowing through the main neutral back to the center-tap of the
service transformer. It then becomes part of the overall flow,
driven by the service transformer as the electrical "pump",
which will produce a high enough fault current to trip the
breaker.
[snip]
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