..."does anyone run simulations or calculations on lightning strikes. A
direct stike is 100 kA with a risetime of 8 us, correct? If so, using an
inductance of 200 nH/ft, the voltage drop along 1 foot of wire is 2.5
kV. And we can
calculate the low frequency impedance of a ground system. Let's say it is 1
ohm. Now we have 100 kV between the tower base and earth ground. ".....
A common lightning strike may be 100KA but that's not considered
typical. The typical number used is now 20KA (used to be 17 KA).
However I saw an article that said the maximum current ever recorded was
500KA, so that probably means there are a few that are even larger than
that but never recorded. Of course if you are designing for protection,
you probably want to use something much greater than typical, and even
100KA may be a little low, depending on how safe you would like to be.
There are a couple of large hurtles standing in the way of doing good
simulations of the currents and voltages present in a tower system
during a strike. Most everything in a tower system can be modeled fairly
accurately in SPICE with the exception of the earth. The non-linear
conduction that happens during the strike where underground arcing and
current saturation take place is very difficult to model, mainly because
I don't think anyone really understands this event very well. If a model
for this event was available, SPICE could calculate all the currents and
voltages present due to conduction.
The one thing SPICE can't do is calculate the induced voltages. For
this you need a field solver, or something like NEC that can do
calculations for electromagnetic fields. NEC unfortunately can't handle
all of the models and complex structures needed to simulate the whole
system. It seems to me that what is needed is a tool that combines the
capability of SPICE with that of NEC. I am not aware of anything that
does this. Maybe someone else can comment on this subject. A field
solver used in PC board design has some of these capabilities but I
don't know if any of these programs can handle a general environment, or
are relegated to PC board use.
Another special capability that would be needed in this tool would be
the ability to predict arcing. Since your tower design should be such as
to prevent arcing, maybe the tool wouldn't have to actually calculate
values due to arcing, for that you just change the design, but it would
have to predict them.
So you can do some simple calculations either manually or using SPICE,
including some large assumptions, and maybe get a gross estimate of some
of the voltages and currents, but the answers are likely to have large
>does anyone run simulations or calculations on lightning strikes. A common
>direct stike is 100 kA with a risetime of 8 us, correct? If so, using an
>inductance of 200 nH/ft, the voltage drop along 1 foot of wire is 2.5 kV. And
>calculate the low frequency impedance of a ground system. Let's say it is 1
>ohm. Now we have 100 kV between the tower base and earth ground.
>Using some ball park numbers like this we should be able to determine the
>currents and potentials at various points in the system. Then appropriate
>Z and shunt Z can be applied to design for survival of equipment.
>I'm thinking of doing it via design rather than blindly by code. I suppose
>that there is nearly 100 years of experience contained in the codes that will
>lead one to success. But perhaps a good understanding of the nuances can help
>one to gain several dB of margin by some simple additions.
>Can someone steer me to some numbers on lightning? Thanks.
> Dave WX7G
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