On Sun, 8 Jul 2007 21:25:22 -0400, Tom W8JI wrote:
>It appears there may be some confusion about what a neutral
>and a ground are.
Tom's reading of NEC is the same as mine.
However -- I have heard that there used to be some exception for
certain heavy appliances. If that execption exists, perhaps
someone could cite the specific paragraph.
There is a VERY good reason for the requirement that the equipment
ground not be a current carrying conductor, and that the neutral
be bonded at one, and only one point. The principal reason is
leakage flux and inductance of the return path for load current.
If the neutral carries all of the load current, all of the flux is
confined in a small area between the phase and neutral conductors.
If, however, ground is a return path, the return current divides
between neutral and "the building" -- that is, all of the parallel
ground paths -- according to ohms law. in that condition, most of
the load current will NOT be in the neutral, so most of the flux
will be leakage flux that spreads out over the entire cross
sectional area of the path. This can establish a massive hum field
(depending on the magnitude of the current following that spurious
path), which, by ordinary magnetic induction, will couple into
lots of wires we don't want it to couple into. Like the shields of
interconnect audio wiring, and the returns of RS232 cables, and so
Jim Brown K9YC
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