First, low induced voltages may create high currents if the Loop impedance is low enough. While the cables are known to have a Low impedance, the ground resistance is an unknown factor here. Further data is required.
Second, if the lines would go arcing all day, under nominal load, the arcing would NOT have gone unnoticed. The arcing problem might only happen during fault situations. In case of transient effects, during short circuits in the load, Line currents could not only be easily a magnitude (or 2) higher, but the frequency content of the current may also be higher (during the transient) possibly inducing voltages of up to 100-500 times higher. This sheds another light on the triangle calculation. (at 500 Hz 5000 A 9 Volts will be induced) During transients, the magnetic field will induce mechanical stress (read Movements) in the line cabling. Vibrations, coinciding with the increased voltage (current), and bad contacts may easily start sparking. Even sub volt circuits may create considerable sparks at high current that may eject heated material over substantial distance from the arc. It all comes down to the question of probability of dry grass, bad contact and current transients, multiplied by the number of poles in the region. Gert Gremmen Van: [email protected] [mailto:[email protected]] Namens John Woodgate Verzonden: zondag 10 februari 2008 23:07 Aan: [email protected] CC: 'Emc-Pstc' Onderwerp: Re: possible power line cause of recent San Diego fires In message <001701c86c28$3ff658e0$c700a8c0@PC323541548743>, dated Sun, 10 Feb 2008, [email protected] writes: > > > > >> >An electrical forensic engineer, Ed Clark, has >> >presented a theory of how power lines could have >> >started one or more of last October's San Diego >> >fires. He blames it on loose connections in >> >the power pole support cables. His evidence is >> >compelling. >> >> I'd like to see some numbers, voltage induced in the guy-ground-guy >> triangle by plausible currents in the 69 kV line, and consequent >> current. > >So would I. > A very rough calculation suggests that 100 A in the line (assumed single-phase) induces only about 20 millivolts in the triangle. If the line is 15 m above ground and carries 100 A, the flux density at ground level is 1.3 microtesla. While the flux density increases with height above ground, the width of the triangle decreases, so roughly compensates. The area of the triangle is about 43 square metres, and the frequency is 60 Hz, so, using V = 2*pi*f*B*A, V = 21.2 mV. YVMD! -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk For very important information, please turn over. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - 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
