Neutral Ground Isolation in Sub-panels.


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 thats 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 someones 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. thats 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 electricians 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 electricians insurance company replaced one gas range, five VCRs, 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.
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Vern Heiler wrote:

It doesn't take much current to cause harm. There's a reason why GFCIs trip around 5-6 milliamps.
Above 15.5 milliamps 99% of adult males cannot let go. 18 milliamps can cause suffocation in adult males. Above 60 milliamps can cause ventricular fibrillation in adults. (http://www.bassengineering.com/E_Effect.htm )

Before doing this, the subpanel should have been shut off. It's the same reason that neutrals must be pigtailed in multi-wire circuits.
Chris
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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:

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.
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Thanks to both you and Chris for the correction on the amount of current. It is obvious that Dilbert needed to find a new carrier. However my concern rests in the fact that if you have a mechanical connection you have the opportunity for failure. The other part of the equation is the resistance to ground of the body and other materials, like rubber sneakers, wood floor, etc. that are so prevalent today. We have all worked on 110 circuits live and sometimes without knowing it until a blinding spark tells you. I find neutrals and grounds tied together all the time, usually due to handyman wiring, but never see discolored wire or any sign of problems. Fear not, I will continue to report the condition when I find it. It's just that I feel I see another problem. Maybe not!

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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.
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The hazard with common neutral and ground in sub-panels is the possibility that if the neutral back to the main panel is interrupted the ground conductor will continue to carry the currrent without any obvious signs. Should the ground conductor then develop a high resistance under these conditions, all grounded items will develop a voltage to ground, up to 120 volts. This would be an extremely dangerous situation which is totally avoided by simply isolating the ground and neutral.
Isolation in the main panel would also be safer in this regard, but there is a greater hazard there that the power feed neutral may become disconnected and having the ground available to carry the neutral current can reduce the resulting voltage unbalance.
All code requirements are actually carefully evaluated risks, remedies, and costs, even though it may not be readily apparent. Code violations rarely make an installation safer or in any other way "better".
Don Young
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Thanks to all. It makes me feel better about writing them up having a better knowledge of why.

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Of equal importance to not bonding the neutral or grounded conductors on the load side of the service is the importance of having a main bonding jumper installed at the building service. Without the main bonding jumper a short to ground will not trip the circuit breaker in many cases. I have found main bonding jumpers missing in several inspections. Let me give you a real life example that happened a Chena Hot Springs Resort near Fairbanks, Alaska in about 1990 that almost got me killed. A competent(?) master electrician had wired an 8 plex and I was performing the final inspection. There was no rough in inspection since we don't have State building permits here and I was the State Inspector. The building was supplied from a generator at 240/120 volts located about 600 feet from the building through an underground service lateral. While preparing to write my inspection report I laid my metal tablet on the Kitchen range of one unit. As I picked it up I accidentally placed my other hand on the adjacent metal sink and received a jolt. I said what goes here? I got my meter out of the car and checked and found 120 volts between the range and the metal sink. Holy, mcMoley, I had just inspected the crawl space and literal wrapped myself around metal pipes while crawling around. I went to the service panel and checked there and again there was 120 volts from ground to the neutral. The ranges had been grounded using the neutral or grounded conductor and not the equipment grounding conductor as now required. I had the master electrician come over to the open service panel and asked him for a short piece of wire. I used the wire to temporarily short the neutral bar to ground and whammo, a 20 ampere circuit breaker popped. That 20 ampere circuit breaker fed a small water pump in the crawl space. We went to it and found that the cover had pinched a hot wire to the metal ground placing all the metal water pipes in the building at 120 volts to whatever. We went back to the panel and sure enough the main bonding jumper was missing. It was just a green screw that made the neutral bus bond to the panel metal can and equipment grounding conductor. There was a ground rod and it was bonded to the metal pipes and to the equipment grounding bus in the service panel. Without the main bonding jumper the fault current had to go through earth all the way to the generator building to trip the circuit breaker and the ground resistance was simply too high. The result was there was 120 volts potential between every range and every metal sink in the entire building. The conclusion of this is to never assume anything about the main bonding jumper. It is the first thing I learned to look for when performing an inspection. Luckily, I lived to tell this story.
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I should have added that the service lateral from the generator was three wire with no equipment grounding conductor.
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Gerald Newton wrote:

Many people probably don't know a major function of a ground wire is to cause enough current to flow when hot faults to ground to trip a breaker. This requires the neutral-ground bond at the service to give a low resistance path back to the transformer. If not bonded, the current flows through the earth back to the transformer. The resistance will very likely be too high to trip breakers.
-- bud--
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On Tue, 6 Feb 2007 17:06:56 -0500, "Vern Heiler"

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
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