> 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.
>> <snip>
>
>
>
>
>> ---------------------
>> 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
>> _______________________________________________
>>
>>
>>
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>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
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