"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.


Several OSHA safety rules for working on electrical circuits have been
violated in your descriptions..
First disconnect the power and check the circuit with a meter to insure is
not energized before beginning work.
Second, if you can't disconnect the circuit, insulate any one and yourself
from coming in contact with circuit components and if possible turn off and
lock out all equipment that may become energized. If equipment can become
energized install barriers to keep persons out of danger.
Reasons for keeping the neutral (or grounded conductor) and the equipment
grounding conductor separated on the load side of the service are explained
in many books. One reason is that the equipment grounding conductor is
designed to carry enough current for a short time (like less than 1/10
second) in order to trip the circuit breaker or open a fuse. The equipment
grounding conductors are not designed to carry current for long periods of
time under normal operating conditions.
Also, you state:
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.

The resistance of No. 14 is about 3 ohms per 1000 ft. For 200 ft. the
resistance is 3 x( 200/1000) = 0.6 ohms
The ground fault current is 110 volts / .6 = 183 amperes
This high amperage is sufficient to trip a 15 ampere circuit breaker or open
a 15 ampere fuse in time. Generally we look for five times the rating of
the circuit breaker or 75 amperes in this case.
You assumed 10 amperes fault current and this is simply incorrect.