This is a late reply but I can fill in a couple details on this subject. In agriculture, low voltage ground differences can be a significant problem. Typically, the cow standing on the wet floor drinks from a water pan fed from plumbing connected to ground hundreds of feet away. Small voltages discourage drinking (I taste even 1.5 volt batteries to see if they are fresh, slightly vinegar tasting). It has been found that animal production can be significantly hurt by cattle and other animals drinking or eating less due to the problem. The National Electrical Code as a result requires equipotential planes (article 547.10) to encompass the entire agricultural building and included metallic equipment.
Regarding grounding of transmission lines, it is an important part of shock, fault and lightning protection, just as within buildings. Systems are typically three phase starting with Y and ending with delta. Delta - delta systems are less desirable due to harmonics, but other than Y-delta systems are often used for other reasons. For transmission, the neutral is only used as a ground point, not a power conductor. It protects against primary to secondary faults (particularly helpful in step-down stages), primary to ground faults and flashovers, and steadies transmission line impedances. The ground is then run above the three phases to intercept lightning and is grounded again at each tower (obviously tower grounding is important). At the customer end the neutral becomes a power conductor for single phase loads and is typically grounded at each customer's service entry. This means many opportunities for parallel ground/neutral connections. Both leakage currents from transmission lines and power currents from neutral connections mean different grounds may differ by several volts. These are the voltages which become a problem in agriculture. Many of us have had to deal with these ground currents in troubleshooting power quality and ground currents in computer rooms. The difference in ground voltages is not usually a problem as long as the local environment can be kept to a common ground voltage, whatever it is. >From a public standpoint, few understand the differences between >electrostatic, electromagnetic and magnetic fields, stray voltages, leakage >currents, overcurrents and the other host of electrical problems. These >mysterious forces are easy to blame for any ills, and once a paranoia develops >are very difficult to overcome. Real physical symptoms can develop based on >fear, independent of the wishes of the victim. After all, Pavlov's dog did not >salivate because he wanted to. He responded unconditionally to the perceived >environment. Similarly, those developing real physical symptoms to electrical >environments usually cannot solve their illness by simple logic. Bob Johnson ITE Safety <http://www.itesafety.com> Richard Nute wrote: Hi Brian: If the neutral is grounded at two points or more, then the earth is in parallel with the neutral and will carry some portion of the neutral current that flows between those two points. This current will cause a potential difference (voltage) between two points of the earth near or on the current pathway. The NEC requires the neutral to be grounded at the service entrance to a building. If the supply to two buildings is from the same source, then some portion of the neutral current will flow through the earth between the two buildings. Of course, a copper neutral wire likely has much lower resistance than the earth. Nevertheless, current will flow through the earth, especially if the earth is highly conductive between the two ground points. Best regards, Rich - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
