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## Re: [TowerTalk] Ground wire impedance

 To: towertalk@contesting.com Re: [TowerTalk] Ground wire impedance K4SAV Wed, 12 Jul 2006 19:42:21 -0500
 ``` > Strap width Inductance uH > 0.5 in 0.40 > 1 in 0.36 Given that a 0.5 inch wide strap is about the same as a wire.. what about multiple wires in parallel.. If they are spaced apart far enough, the inductances will be parallel. -------------------------- Good thought, but this might be hard to implement at the bonding point if you are thinking of a ground rod/radial system. If you space two #4 wires 6 inches apart you can get 0.72 times the inductance of a single wire. If you space the wires 3.3 ft apart you can get 0.6 times the inductance of a single wire. Interesting link on "quarter shrinking". Jerry, K4SAV -------------------------- Jim Lux wrote: > At 02:40 PM 7/12/2006, K4SAV wrote: > >> All this grounding talk has got me thinking again. >> >> Most of us know that the impedance of a wire is increased by its skin >> resistance, and that a wire with more skin area (such as a strap) will >> provide a lower impedance. But how much lower? I decided to break out my >> spread sheet I made for calculating these things and take a look. The >> data is tabulated below. >> > > > > >> --------------------- >> Calculated data: >> #4 wire, 10 ft length, L = 0.43 uH (straight wire in free space) >> Note: A wire in the ground will appear as a higher inductance than shown >> here, because of the decreased velocity factor of the medium. >> Z(L) represents impedance calculated from inductance only. >> >> Freq Z(L) DC res Skin res >> 330 Hz 8.86e-4 2.49e-3 8.86e-4 >> 920 Hz 2.47e-3 2.49e-3 1.48e-3 >> 1660 Hz 4.46e-3 2.49e-3 1.99e-3 >> 10 kHz 2.69e-2 2.49e-3 4.88e-3 >> 100 kHz 0.269 2.49e-3 1.54e-2 >> 1 MHz 2.69 2.49e-3 4.88e-2 > > > Interestingly, too. compare for a AWG10 wire, which has a DC > resistance of about 10e-3 ohms for 10 ft. I would assume skin > resistance would scale with diameter (at least for higher frequencies) > 6 gauges is half the diameter, so at 1 MHz, call it 0.1 ohm. > Inductance will be almost the same as for AWG4, I think. > > So if the wire is at all very long, the inductance will dominate, even > for a very thin wire. > > I suppose this is important from the "limiting the voltage rise" part > of lightning protection, but still, the wire has to carry the current > without melting, but, there, we can see that an AWG 10 wire can take a > fairly hefty current without fusing, especially for a short pulse. > (The figure you usually see for fusing calculations is the "action" > which is the integrated current squared.. given in A^2*seconds) > > Fusing current for AWG 10 copper for continuous current is about 400 > amps. > But for a short pulse, if you plug 50 microseconds into the Onderdonk > equation, you get a fusing current of 56 kA for a AWG 16 wire. Fusing > current goes as the area in this equation, so an AWG 10, with 4 times > the area, would be 200+ kA. > > Having blown up a fair number of wires ranging from AWG10 to AWG40 > with fast high current pulses, I'd say the real limit, on larger wire > (>AWG16) is going to be mechanical stresses on the wire from the > magnetic field. {google "quarter shrinking" and "exploding wires" for > more info) > > > >> I didn't have a spreadsheet already made up to calculate the skin >> resistance of a strap, but I do have one to calculate its inductance. >> Since the inductance is the predominate parameter, it's probable all you >> will need anyway. The calculations are for a strap thickness of 0.05 >> inches, and a length of 10 ft. Since the thickness doesn't effect the >> inductance very much, it wasn't included as a variable parameter. >> Compare these numbers to a #4 wire, same length, which was 0.43 uH. >> >> Strap width Inductance uH >> 0.5 in 0.40 >> 1 in 0.36 > > > > > Given that a 0.5 inch wide strap is about the same as a wire.. what > about multiple wires in parallel.. If they are spaced apart far > enough, the inductances will be parallel. > > > > > > >> The formula for the wire inductance and strap inductance came from the >> Polyphaser book, Grounds for Lightning & EMP Protection. >> >> One other note of significance: None of these calculations include >> resonant effects. For wires that are long compared to the frequencies >> being considered, resonance effects can increase the impedance by a huge >> amount compared to an impedance value calculated from wire inductance. >> >> Jerry, K4SAV >> _______________________________________________ >> >> >> >> _______________________________________________ >> TowerTalk mailing list >> TowerTalk@contesting.com >> http://lists.contesting.com/mailman/listinfo/towertalk > > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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