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.
Thanks for any enlightenment you can shed on me.