> From: John Woodgate > Sent: Tuesday, December 11, 2007 9:13 AM > > Tarver, Peter writes: > > >This is a part of the ELF/VLF debate, where contention is > made that HV > >power lines routed near schools and housing causes increases in the > >incidence of cancers and other illnesses, especially in children, or > >the power lines are routed near dairy cattle effecting their milk > >production. > > I disagree; these 'stray currents' are real.
I have made no judgmental statements about what's real or imagined. One of the web sites mentioned in the OP was strayvoltage.org. I recognized this web site from several years ago and remember clearly that their concerns centered around the issues I mentioned above and that their concerns were primarily related to high voltage distribution, though might have included medium voltage distribution near schools. Even though a court has sided with dairy farmers, the debate will continue. > The alternating > currents in the overhead conductors have mutual inductance > (not much, but enough) with conducting (resistive) soil, and > therefore voltage gradients are induced in the soil. Cows are > astonishingly sensitive to these voltage gradients, partly > because of their long 'heelbase' (sorry!), but also due to > their physiology. See IEC TS60479-3. As little as 25 V r.m.s. > can kill a cow, and even very low voltages can put cows under > enough stress to affect milk yields and general health. These were among the points made by strayvoltage.org. Again, I made no judgment about the veracity of any claims, one way or the other in the above. > >On-premise protection devices do not come into play and > grounded supply > >conductors do not exist, because primary distribution is > >three-phase-delta. > > I don't know what happens in USA, but in UK all line towers > are connected together by an overhead grounded conductor at > the top of each tower. To what extent this affects ground > currents under normal conditions is questionable, but it does > complete a big loop: tower - conductor - tower - ground ---- > ground - tower. And if there is a loop, some current is bound > to flow at some time. Grounded supply conductor = Neutral. I don't pretend to know every reason for running these groundING wires between towers, but I understood they provide lightning protection by presenting lightning with a preferential path to ground, rather than through the phase conductors. Regards, Peter L. Tarver, PE [email protected] CONFIDENTIALITY This e-mail message and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail message, you are hereby notified that any dissemination, distribution or copying of this e-mail message, and any attachments thereto, is strictly prohibited. If you have received this e-mail message in error, please immediately notify the sender and permanently delete the original and any copies of this email and any prints thereof. ABSENT AN EXPRESS STATEMENT TO THE CONTRARY HEREINABOVE, THIS E-MAIL IS NOT INTENDED AS A SUBSTITUTE FOR A WRITING. Notwithstanding the Uniform Electronic Transactions Act or the applicability of any other law of similar substance and effect, absent an express statement to the contrary hereinabove, this e-mail message its contents, and any attachments hereto are not intended to represent an offer or acceptance to enter into a contract and are not otherwise intended to bind the sender, Sanmina-SCI Corporation (or any of its subsidiaries), or any other person or entity. - 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
