On Tue, 10 Mar 1998 21:38:31 -0500 Sean Breheny writes: > Jorge has mentioned that this phenomenon is >observable when dealing with power transmission line. Power >transmission >line is not coaxial, is it? The stuff that is buried underground and used from about 2 KV up to maybe 50 KV (e.g. for distribution to transformers in neighborhoods) is. It has a copper or aluminum conductor at the core, thick polyethylene insulation, then a shield made out of metal braid and conductive plastic, and finally a black plastic outer covering. The shield is likely not intended to normally carry current. It is connected to ground at both ends for safety reasons. Maybe some single-phase circuits use it as the return conductor. In any case, the coaxial construction makes it difficult for high voltage to appear on the outside of the cable if there is a leakage, cut or hole in the insulation. Such a circumstance will cause the cable to arc and the fuse or circuit breaker to open. This line is of course routinely "hipot" tested before installation by connecting a high voltage between the center and the shield and measuring any leakage. A big spool of it charged to many KV would indeed be dangerous. The polyethylene insulation would prevent the charge from leaking off for a long time as well as maybe causing a phantom "recharge" from dielectric absorption. Of course such cable is intended for use at 60 Hz and is not designed for any particular impedance. >I have always been told that it is single >conductor woven cable, The wire strung overhead between poles is laid up of strands of aluminum on the outside, and steel on the inside. The steel is to make it stronger. Even at 60 Hz, there is enough of a "skin effect" to reduce the effectiveness of the conductors in the center, so the aluminum is outside. Except in special cases, it is bare. Please do not climb power poles and touch the wires. You will get hurt. Really high power lines have several cables in parallel. They are held apart in a triangle or square arrangment to make the apparent diameter of the conductor as large as possible. This improves the impedance and current distribution along the line. When dealing with hundreds of km of cable, AC effects such as phase shifts are very real at 60 Hz. sometimes with a hollow core with a coolent run >through it. In power "substations" the conductors are sometimes placed inside much larger outer pipes. The pipes are filled with sulfur hexafluoride gas. SF6 is a much better insulator than air, as well as being certain to be clean and dry, so the conductors can be in closer proximity than if they were strung out in the open. This is rather expensive so it is done only for extremely high voltages or where the real estate to lay out the station with larger seperations would be even more expensive. I don't think the conductors in transmission systems are hollow or actively cooled in any way. Hollow cooled wire is used to wind generators and big motors. Hydrogen (H2) gas was used as the coolant in one example I saw. It's both very "thin" and light (i.e. easy to move at high velocity through small hollow passages) and absorbs heat well. It doesn't seem that H2 gas and big electrical machines should be used near each other. This is what I remember from several years ago; current practices may be different. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]