new wrote:

Great answers guys - you are right ground does not equal neutral - I even
remember being taught that!

Hi,
Ground = safety, last defense line.
Ground loop causes undesirable ground current in many instances.
Tony


Tony
Are you lost, just quarrelsome or both. Have you ever actually measured
a ground loop current in access of one amp. Natural ground gradients
never produce currents of 5.5 amperes. Ground loops occur from voltage
gradients that are measured in millivolts. The only hazard a naturally
occurring ground loop causes is to electronic signal quality between
digital equipment connected to grounds with differing potentials.

250.4 General Requirements for Grounding and Bonding.

The following general requirements identify what grounding and bonding
of electrical systems are required to accomplish. The prescriptive
methods contained in Article 250 shall be followed to comply with the
performance requirements of this section.

(A) Grounded Systems.

(1) Electrical System Grounding. Electrical systems that are grounded
shall be connected to earth in a manner that will limit the voltage
imposed by lightning, line surges, or unintentional contact with
higher-voltage lines and that will stabilize the voltage to earth during
normal operation.


The actual grounding of electric systems is not done for electrical
safety but rather for limiting damage to the system from "lightning,
line surges, or unintentional contact with higher-voltage lines."

The conductors that actually provide a safe level of touch potential on
the exposed non current carrying metallic parts of the wiring system are
called Equipment Grounding Conductors because they connect the exposed
metallic parts to the Grounding Electrode Conductor. The connection
that actually clears any fault is the main bonding jumper. That conducts
the stray current back to the neutral of the service and to the Xo of
the transformer. If the fault that is allowing the current to get into
the exposed metallic parts in the first place is of a low enough
impedance sufficient current flows to trip the Over Current Protective
Device. If the fault impedance is too high for this then the low
impedance pathway provided by the EGCs keeps the touch potential below
dangerous levels.
--
Tom