"Vern Heiler" wrote in message
...
Gentlemen of wisdom;



I am a licensed home inspector in North Carolina. I have 30+
years experience in electronics repair (Microwave Radio and Large System
Computer), no real experience in electrical wiring but have acquired some
knowledge.

As a licensed home inspector, I am required to report any
sub-panels that do not have neutral and ground isolated from each other in
the sub-panel.

I have questioned many electricians as to the reason behind
isolating the neutral and ground with answers ranging from "well that's
dangerous" to "because its code". The best answer I have found was
posted
on a web site explaining the possibility of a ground fault following a
parallel path (neutral and ground wires) back to the main panel. The
objectionable current on the ground wire would ruin someone's day by
flowing
to all of the outlets along the way.

I understand that current does not follow only the least resistant path,
however the amperage in each path is inversely proportional to the
resistance. With the average 110 volt circuit run in most residences
less
than 200 feet, and #14 gauge wire 2.5 ohms resistance per 1000 ft. that's
maybe ½ ohm plus connection resistances. So if the #14 gauge wire is
over
fused with a 20amp breaker, and a short to the safety ground occurs, 10
amps
run through the ground and 10 amps through the neutral. With 10amps
across
½ ohm the voltage developed is 5 volts rms across the length of the run.
At
this point a person would have to be standing in the tub, with copper
drain
lines (most are plastic now) and licking the cover plate screw on the
outlet
that should be well out of reach, to feel anything until the breaker
trips.
Not likely to ruin many peoples day.

I am aware of the noise problem that can occur due to current on the
safety
ground and concede that ground loops are a problem with computers,
stereos,
etc.

The problem I have with this is what I see as a more hazardous
condition. Mr. Homeowner is shaving with an electric razor and one hand
on
the faucet. (still see copper supply plumbing) There have been
intermittent
electrical problems and the electrician's helper Dilbert has been called
to
fix the problem. Dilbert finds the neutral buss conductor connection is
corroded in the sub-panel and disconnects it to clean the wire. The 5
watt
razor is opposite 100 watt bulbs etc. in the 220 volt sub-panel. The
razor
now sees voltages approaching 220 volts and goes into afterburner before
melting in the hand of Mr. Homeowner. The GFCI does nothing as current is
equal in hot and neutral.

The above scenario is one that happened to a co-worker of mine
a
few years ago, with exception to the razor. He was standing beside
Dilbert
when the neutral wire was disconnected. Moments later his daughter came
down from her bedroom yelling "all the lights are doing strange things and
there is a smell in the house". The electrician's insurance company
replaced one gas range, five VCR's, four televisions, six clock radios,
one
microwave and one DVD player.

Had the neutral and ground been bonded together in the
sub-panel
the un-balance loads would have been carried by the ground wire back to
the
main panel.

The reason I am sending you this is to find what is wrong with
my concern and to feel better about writing up neutral ground isolation
problems in the sub-panels, which I find all the time.



Thanks for any enlightenment you can shed on me.



I've known about the shock hazard, but I wanted to see if there were other
concerns so I went to UL.com for some info. Check out this:
http://www.ul.com/regulators/panelboards.pdf on page 15, paragraph 50. "If
neutrals are bonded at distribution points, the neutral currents take
parallel paths through neutral feeder conductors and metal raceways. If
neutral feed conductors are open, the full neutral current flows over the
grounded raceway or the grounding conductor system. When this happens, the
steel raceway joints and box connections overheat. This is a fire hazard"

So according to Underwriters Laboratories, there is a fire hazard in
addition to a shock hazard with having the neutral bonded to ground in a
subpanel. I think it only takes 25 - 30 milliamperes to stop a human heart.