# lightning rod question

Discussion in 'Electronic Basics' started by izzi4, Oct 19, 2004.

1. ### Don KellyGuest

---------
Unlikely- A large conifer forest has thousands of tips each of which is
grounded( poorly but this isn't of imprtance until there is a strike) and
there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. Ever had a transformer fail during a
lightning storm in your urban area? Ever had the lights go out or flicker?

2. ### TimRGuest

Back in engineering school one of the professors was famous for
studying lightning. I recall him saying #10 wire was perfectly
adequate, that we had to remember the wave form. A lightning stoke is
a damped sinusoid, it is not like trying to pass 30,000 Amperes DC
through a wire.

Something he never mentioned, but I have run into since, is Ufer
grounding. It certainly gives low ohmic measurements. Do we know how
well it works in real life, i.e. actual lightning strikes?

3. ### Edward GreenGuest

That comment doesn't make much sense to me. What's waveform got to do
with it? What matters is power dissipation.

Or I guess that's what you are suggesting: the pulse is short enough
that much higher instantaneous powers can be tolerated than at steady
state. Why didn't you just come out and say that!? ;-)

4. ### Steve Harris Guest

COMMENT:

Yes. The waveform is "damped sinusoid." In this case it's the first
descriptive word that's important, not the second <g>.

SBH

5. ### Rich GriseGuest

.
What is "Ufer grounding"?

Thanks,
Rich

6. ### Roger JohanssonGuest

The "Ufer" Ground

"The term "Ufer" grounding is named after a consultant working for the US
Army during World War II. The technique Mr. Ufer came up with was
necessary because the site needing grounding had no underground water
table and little rainfall. The desert site was a series of bomb storage
vaults in the area of Flagstaff, Arizona.

The principle of the Ufer ground is simple, it is very effective and
inexpensive to install during new construction. The Ufer ground takes
moisture quickly and looses moisture very slowly. The mineral properties
of concrete (lime and others) and their inherent pH means concrete has a
supply of ions to conduct current. The soil around concrete becomes
"doped" by the concrete, as a result, the pH of the soil rises and
reduces what would normally be 1000 ohm meter soil conditions (hard to
get a good ground). The moisture present, (concrete gives up moisture
very slowly), in combination with the "doped" soil, make a good conductor
for electrical energy or lightning currents.

Ufer techniques are used in building footers, concrete floors, radio and
television towers, tower guy wire anchors, light poles, etc. Copper wire
does not function well as a "Ufer" ground due to the pH factor of
concrete (+7pH is common). The use of steel reinforcement as a "Ufer"
ground works well and concrete does not chip or flake as has been found
with copper. The use of copper wire tied to the reinforcement rods
outside the concrete shows none of these problems. "

7. ### TimRGuest

My question really has to do with whether the "good ground" is real or
not under high current conditions. Clearly you can get an extremely
low resistance measurement and pass your safety requirements. It
seems likely this ground would be fine for reducing static charge
effects and for electronic equipment grounding. I asked because I
wondered how well a Ufer ground can actually dissipate the current in
a lightning strike. If the soil outside the foundation continues to
be high resistance soil, it would seem the dissipation must occur
within the rebar.

I know I'm not being consistent claiming #10 wire is enough, then
wondering if a rebar array is enough. <grin> But is there any data?
Actual lightning hitting a Ufer grounded structure? It sounds like a
ground plane not a true ground, but I'm not an electrical engineer and
may not understand the difference.

8. ### Roger JohanssonGuest

You can combine this Ufer ground with a system of wires in a radial
arrangement out from the central Ufer ground, to spread out the charge.
Like 10 wires, 300 feet long, buried under ground, like spokes in a
wheel, so the charge is easier lead away from the Ufer ground point.

No grounding point is perfect.

You will still have to assume that the voltage of the grounding point
will be raised momentarily when lightning strikes.
How much, that depends on the strength of the lightning hit and the
quality of the grounding system, how fast it can dissipate the charge.

So we always get back to the protection which holds the voltages in the
protected system together, so the voltage differences do not become too
big.

For that you need spark gaps and similar devices.

9. ### ClarenceGuest

Don't look now. But Flagstaff is not in a Desert, it is on the slopes of a
volcano!

This is the system used in Feed Mills.

10. ### JamieGuest

Rebarb wire in concrete or a simple incased steel rods
in a welded square patern..
if memory serve i think it was Mr Ufer in from WARII hence
the name Ufer ground.
but simply a steel cage connection system in cased in the
concrete.

11. ### William J. BeatyGuest

Not obvious, since some books (and teachers) still insist that lightning
rods protect your home by discharging the clouds.

Yes, but we need to remember that the ions produced by things on
the ground would only reduce the net charge in the clouds if they
could *fill* the empty space between the cloud and the ground.

Trees and buildings might be producing charged air, which makes the
air more conductive, but this conductive layer of air would be like
a film of insignificant thickness... because we compare it to the
SEVERAL MILES of air between the cloud and the ground. I'm
imagining that, at most, the charged air might move upwards at a
foot per second. As the charged clouds arrive overhead and the
e-fields become strong, the sharp objects on the ground would only
have time to produce a layer of conductive air a few hundred feet
thick. The layer will also be blowing sideways, so we shouldn't
imagine that one lightning rod would make a cloud above itself.
Rather imagine a smoke stack with a plume travelling downwind.

Lightning rods aren't going to have a large effect on the storm.
A lightning bolt (a plasma streamer) is triggered up in the clouds,
and then grows longer, sometimes growing downwards. Suppose it
becomes several miles long and is approaching the ground. Could
some ionized air hovering over the buildings have any effect? Sure.
The movable charges will act as a resistive "coating" which makes the
lightning think that the Earth lacks buildings. The e-fields which
steer the growing plasma streamer would be altered by the conductive
air, so they would not respond to trees and buildings as much as if
there was no meters-thick layer of ions.

But is this what we want? The layer of charged air would keep the
lightning path random, so the streamer wouldn't be guided to a safe
attachment upon a nice thick copper ground-wire. Maybe it's a good
thing that any wind would blow away the ions, letting the streamer
"see" your lightning rod poking upwards.

Now the region BELOW a lightning rod... that's a different issue. If
an incoming plasma streamer approaches a protected building, then the
e-field around the building grows so intense that the lighting rod or
even the ground wires will launch their own plasma streamers up to
intercept the incoming streamer. A lighting rod acts like a "Scud
Missle Launcher" which shoots down any incoming lightning bolts, forcing
the incoming lightning to follow a trail leading back to the ground
wire. With luck, the *TOP* of the lightning rod will launch the
streamer, which prevents lightning strikes upon anything below the
tip of the rod. The farther away you stand from the vertical ground
wire, the more chance there is that the lightning rod wouldn't
emit a streamer that intercepts any lightning bolts aimed for your

12. ### Don KellyGuest

and

---------------
What you clipped may be of more importance than what you included.
" .....and
there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. "

This contraindicates the "discharge" effect.

Also ,in a previous comment:
"Generally the source of the charge is several miles overhead -the rod
doesn't get seen by it. When a leader gets near a rod then it may be a
preferred target for the next step and if it is, then the main stroke will
be to the rod-if not something else gets hit. Design is based on it being
such a target for higher current strokes (but not necessarily lower current
strokes). This is true for protective systems for transmission lines as
well as structures. Catch the damaging strokes know that some of the little
ones will get by."

In other words, the above is in agreement with what you have said quite
well.
Note that design doesn't count on this streamer and the protection zone is
determined by the downcoming leader's strike distance which is related
(empirically) to the main stroke current . The protected range is given by
the "rolling ball approach. Basically a ball of radius equal to strike
distance and just touching ground (or the local ground plane) and the rod
tip. The shadow of protection is outside this radius for currents equal to
or greater than the design current. Weaker strokes can get by. It's a saw
off- you get what you pay for (and hope that the statistics are reasonably
correct).

13. ### Don KellyGuest

Often all you need is a ground plane.

14. ### ACGuest

The purpose of the ball is to stop the windvane falling down. It serves no
electrical function.