On Tue, 6 Feb 2007 17:06:56 -0500, "Vern Heiler"
wrote:
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 ˝ 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
˝ 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.
There are no 100% perfect ground systems that address every concern.
It is interesting that there is such a variation in grounding (or
earthing) systems throughout the world. Each system is a matter of
tradeoffs and compromises. Wikipedia has a good article that
describes some of the tradeoffs.
http://en.wikipedia.org/wiki/Grounding_system
Note the US and Canada use the TN-C-S scheme. The UK has both the
TN-C-S and the older TN-S system. France, always different, uses the
TT system that requires a special device called a Residual Current
Detector for safety.
In the US system, the idea is that current should never flow on the
safety ground conductor except under fault conditions. If there is a
fault, the magnitude of the current should be such that it A) it is
sufficient enough to trip out the breaker or fuse or B) trip out the
GFI.
Bonding a subpanel to a neutral means that part of the neutral return
current could flow in the protective safety ground conductor, or a
water or gas pipe, or a heating duct, or the ground itself. This is
never a good idea. Why? Well one reason is that wherever there is
current flow, there can be a potential difference between two points
that are supposed to be at the same (ground) potential. This is
dangerous and presents an electrocution hazard.
The US system is also based on the assumption that the probability of
a neutral wire opening up is relatively low. This would cause a
severe shock hazard for certain 220V appliances such as a range or
dryer wired with the older 3 wire plugs. In such a case, the frame of
the appliance would become energized (with respect to ground) since
most US 220V dryers (for example) have motors that run at 120V and
normally require a neutral for the return current. The new
requirement is a 4 wire plug and a safety ground connected to the
frame of the appliance (as I'm sure you know).
Beachcomber