lightning

I'm curious, the lightning rod system installed on a building
near by has only 3/8" dia cables leading to ground.
However I've read and seen photo's of galvanized fence
posts being melted from lightning strikes. How does the
3/8" cable suffice?

 

The difference is simple- lighting striking things generally does so
because they have poorly conductive surfaces or rounded surfaces that do not
easily bleed away a charge. These means that the whole bulk of the bolt
will try to pass through when it strikes.
But a lightning rod typically has pointed surfaces which easily break
over and bleed away the charge, and even with a direct hit this can stretch
out the discharge time substantially, lowering the peak current. In fact,
most lightning rods are quite effective at grounding out the local space
charge, which means that it often cannot reach breakover near the rod- only
in extreme cases will it do so.

Cheers!

Chip Shults

 

Thanks

 

Low resistance. The lightning is not supposed to strike the lightning
rod, because it is supposed to be a sharp point on the end, which
discharges the current instead.

Remember that lightning has millions of volts at tens of thousands of
amps, and it can do some _very_ serious damage to anything. Nothing was
said that the lightning rod had to survive a direct hit, as long as it
protects the rest of the bldg.

Oh, sh!t. I smell a w_tom coming, better get out of here...

 

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However design of lightning protection schemes depend on the rods and grids
(if used) to be preferred targets for the stroke. The sharp point, at the
gradients involved, will provide little gain over a rounded point as once
local ionization takes place, the effective "point" is more spherical than
pointed. Modern design considers the concept of strike distance as related
to peak stroke current (Rolling ball idea) and statistical probabilities
of failure of protection to keep the probability of a damaging stroke to
property to an acceptable level.
You are absolutely right in saying "low resistance", however, as this
reduces the chance of breakdown from the grounding leads to "protected"
equipment such as dangling bits over the water in a toilet.

 

A sharp point is supposed to form a conductive corona that looks like a
dull sphere and the lightning will probably go somewhere else, but it does
not always work out that way. Lightning will sometimes hit the rod.

Whatever size wire is usually run from lightning rods will normally
survive, and that size is smaller than 3/8 inch. You get tens of
thousands of amps only for a fraction of a millisecond to around a
millisecond.
The main thing is to have the wire make as staright and smooth a run as
possible, since major sharp bends may have enough inductance to have
significant voltage drop across them from current changing by tens of
thousands of amps in a small fraction of a millisecond. Also don't have
anything conductive too close - the inductance of straght wire is
something like a microhenry every couple feet (varying slightly
inversely with the width of the wire).
In extreme cases, the voltage drop across a grounding conductor may be
enough to spark several inches and through a thin wall.

And I would not keep computers or other electronic appliances too close
to the grounding conductor of a lightning rod. I am afraid the electric
field or a sharply increasing pulse of magnetic field could zap things at
close range.

As for the rod itself: If it gets a direct hit, it will probably be not
quite the same. If it had a sharp point, it probably no longer does. It
may also be shorter, but probably by less than an inch.

Ever notice how some houses have multiple lightning rods? The reason is
that there is usually more than one surge of current over a time period of
a couple tenths of a second, and the hot air channel formed by the first
surge of current may be a few feet downwind when the next surge of current
flows.

- Don Klipstein ()

 

---------------
The above is interesting and may be true but that is not the reason.
The reason for multiple rods is that a single rod has a limited "protection
area" A 3 ft rod at one end of a 30 ft roof won't provide protection for
the far end or even the middle of the roof. A crude estimate for a 3ft rod
is a cone of angle 30 to 45 degrees frm the vertical. Modern design is based
on strike distance (rolling ball concept involved) - reduce the probablility
of damaging strokes to an acceptable low level and don't fret about smaller
strokes which may not be caught.