you have to remember that its not just the impedance, its the travel time to
ground compared to the risetime of the lightning stroke.
So if there is a bend that isn't too far from some other structure but it is
still far enough above the ground so the air can break down
before the wave travels to the ground and reflects back up to the height of the
bend you can get side flashovers even if the ground is very good.
This could be a common condition say where the lightning lead from the top of a
2 story house bends at the corner of the roof to
go down the side and there is a tree branch nearby... it would be very likely
you could flashover to the branch before the current gets
to the ground and back up.
that is another part to remember, its always double the travel time... the
voltage at a point above ground won't start to drop until the
current gets to the ground and the reflection travels back up... this is of
course assuming that the impedance of the ground is less
than the characteristic or surge impedance of the structure. lightning is fun
stuff, but only if you can handle the time factor and understand
that a conductor can have very different voltages and currents from point to
Apr 12, 2012 01:06:53 PM, email@example.com wrote:
I agree with your reasoning discounting the magnetic coupling effects of
a bend. I always assumed that your item 2) was the reason for that
recommendation, but intuitively it always seemed to me that if the
impedance between the point of the sharp bend and ground was high enough
to make the increased electrical field at the bend comparatively high
enough for a breakover to occur that it wasn't a very good ground path
to begin with.
On 4/12/2012 6:31 AM, Jim Lux wrote:
> On 4/11/12 10:33 PM, Grant Saviers wrote:
>> might be as simple as:
>> "Don't do anything that increases the inductance of the grounding wire" ?
> the original source might say that, but it's not actually true. Until
> you get a full 360 loop, the inductance of a bend isn't much more than
> the inductance of the length of wire in the bend.
> if you think about it, the increase in inductance in a loop is because
> the magnetic field from one part of the wire interacts with the field
> from another. Two pieces of wire at right angles don't couple very
> well, parallel pieces do. A 360 loop has coupling "across" the loop,
> but with an incomplete loop, much of the loop doesn't couple to anything
> In any case, that's why I'm looking for the original source. I suspect
> that it's either:
> 1) how things were wired back in Ben Franklin's time, and it's carried
> on to this day because it doesn't hurt, even if it doesn't help, either.
> (the corollary, it takes more time to explain why not than to just do it)
> 2) A sharp bend produces a place where flashover/breakdown is more
> likely to occur.
> I discovered a few weeks ago that a bunch of the grounding conductor
> guidelines in power substations (something that amateur radio folks are
> unlikely to have to worry about) were based on the behavior of bolts and
> materials commonly used at the turn of the last century. So that got me
> (that, and Ward's article about 468)
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