On Thursday, July 11, 2013 07:31:59 AM Jim Lux wrote:
> Considering that the overvoltage transient/fields propagate at close to
> the speed of light, the difference in time is more like microseconds for
> any reasonable sized house. (Even the Spelling/Ecclestone 50k square
> foot mansion is only a few hundred feet across, and that's <1
> microsecond light time)
True... I should have said micro.
> In the steady state case (60 Hz power line falls on the antenna), the
> conductor has to have "low enough" resistance to keep the potential
> difference low enough to be safe. The code required green wires and
> bonding wires (variously 14, 12, 10, 6, etc. AWG) are more than
> sufficient for this. Note that the code requires different sizes more
> for mechanical reasons than electrical.
>
> This is typically either copper strap or large gauge
I prefer strap for mechanical fastening convenience.
> The size is more determined by mechanical concerns: a ground bonding
> wire outside by itself needs to be bigger than one inside a conduit.
> Larger wires are also less likely to "flashover" to adjacent conductors
> because the radius of curvature is larger: a reason to use round wire,
> rather than flat strip, for lightning conductors, by the way. A 1/4"
> diameter copper wire has a higher breakdown voltage than the edge of a
> 0.020" strip of copper flashing. (5-6 kV vs 1kV)
HV is always more likely to flashover from a sharp edge than a rounded edge.
However in the specific case of a lightning rise, all ground points bonded to
strap would hopefully be at the same potential relative to each other (or at
least a low enough differential to prohibit this from occurring.) In the AC
case, this would indeed be true.
> The bonding wire needs to be big enough to carry the maximum expected
> fault current without melting. The worst case for this is not
> lightning, but a local medium voltage power line shorting to your
> wiring, because it could source several hundred amps for seconds, before
> something trips. Lightning has really high peak currents, but the pulse
> only lasts 50 milliseconds, so there's not much energy dissipated in the
> grounding conductor. (AWG 10 is big enough to handle all but the largest
> lightning strokes without melting)
True. That's why I prefer larger conductors for the 'just in case' scenarios.
Thanks for the corrections and observations Jim.
73 Bill W4WHW
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