Suppose there's a fairly isolated well grounded antenna mast. Or a power
pylon in the middle of a field. There may be people or animals wandering in
the vicinity. Sooner or later lightning will strike a high mast.


The question arises at what distance from the base of the mast should a
safety fence be erected to protect people and animals from electrocution. Or
to provide some other greater degree of protection.


The situation can be translated into your back yard. There are then
subsidiary questions such as at what distance should the mast-grounding
system be bonded to the domestic house ground? Or to another mast?


It's obvious that risks, probabilities, choice of safety factors, soil
resistivity, strength of lighting strokes are involved. It's entirely up to
YOU what you do about it.


There are ball-parks within ball-parks. Nevertheless, to keep you in the
right set of ball parks a few simple calculations can be made. Without
numbers bafflegab will prevail.


The local ground system at the base of the mast can be represented or
modelled by a hemispherical electrode impressed into the ground surface.
The electrical characteristics of this simple electrode are very easily
calculated from its radius and soil resistivity.


Suppose the antenna mast with its buried legs and set of ground rods extends
out to a radius Rh metres from the centre of the small system. Even if the
mast consists only of a 2" diameter aluminium tube buried to a depth of 2
feet without any rods you will now have a number Rh. For this exceptional
simple case of a single fat rod call Rh = 1/2 depth. It turns out to be
very non-critical anyway.


A hemispherical bowl of low-resistivity concrete is very well represented by
the mathematical model of course. The concrete becomes the ground electrode.
The resistivity of the material forming the ground electrode does not enter
into the argument because the lightning stroke is assumed to be a current
source.


It is not difficult to show that the voltage at the edge of the electrode,
at a distance Rh from the mast itself, relative to the voltage of the mass
of the Earth at a great distance is given by -


V = S * I / 2 / Pi / Rh volts


where S is soil resistivity in ohm/metres,


I = current in lightning stroke in amps,


and Rh is the hemispherical radius.


However, we are not greatly interested in the volts at the electrode. What
matters is the VOLTAGE GRADIENT along the surface of the soil as the current
streams away from the base of the antenna in all radial directions
uniformly.


It should be noted, and is intuitively fairly obvious, that at a short
distance beyond Rh the current flows away from the antenna mast in radial
directions through the soil regardless of the actual shape and construction
of the ground electrode system.


Suppose a cow is facing the antenna and the distance between the animal's
front and rear legs is 1.5 metres. Insofar as the cow is concerned what
really matters is the voltage gradient along the soil surface. With a little
integration it can be shown the voltage difference Vd between two points on
the soil surface at distances R1 and R2 from the mast is given by -


Vd = S * I / 2 / Pi * ( R1 - R2 ) / R1 / R2 volts,

If soil resistivity S = 100 ohm.metres,

( Soil conductivity = 10 mS )


Stroke current = 50,000 amps,


Rear legs distance from mast, R1 = 6 metres,


Front legs distance from mast = 4.5 metres,


Neglecting the resistance of a cow, the potential difference between the
poor animal's front and rear legs would rise in a matter of milli-seconds to
Vd = 44,000 volts without time being allowed even to say "Moo". Current and
voltage then subside more slowly.


Whereas the radio station owner, at the same location, standing on one leg
wearing a rubber boot would very likely survive unscathed.


As a matter of interest the voltage gradient G at a distance of R metres
from the mast is given by -


G = S * I / 2 / Pi / Square( R ) volts-per-metre.


The foregoing calculations are exact when the ground electrode is a true
hemisphere. There are other applications.

----
Reg, G4FGQ


--
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg,
Please don't spread this around! Here in the 'states', the
EPA has advocated diapers for horses and methane collectors for
cattle. Next they, along with the S.P.C.A., will want to put
grounding electrodes and shorting straps on them too. The
straps
may not be too bad, but how would you like the job of placing
the
electrodes? And where?
'Doc

Just teach your cows to stand on one foot. And if they have horns, don't
forget to put those corona-discharge balls on the tips.



--

"'Doc" wrote in message ...


Reg,
Please don't spread this around! Here in the 'states', the
EPA has advocated diapers for horses and methane collectors for
cattle. Next they, along with the S.P.C.A., will want to put
grounding electrodes and shorting straps on them too. The
straps
may not be too bad, but how would you like the job of placing
the
electrodes? And where?
'Doc

On Sun, 1 Feb 2004 13:04:47 +0200, "don bryant"
wrote:

Man, this one goes back a ways.

But back to the main question...How far should a fence be place around a
tower. Most of the answers given referred to the "step voltage" case. There
is another important factor, Side Flashes. A person or animal should not be
to near a tower since the flash heading to ground can take a parallel path

There are just too many things to be taken into consideration.
There are ground currents which produce some pretty impressive voltage
drops per foot as you go away from the tower base.

There are induced currents in metal objects which of course produces
voltages. Sometimes those volt ages can be as much as several
thousand volts per meter in the conductor from a strike a mile away.
Typically, most damage, as I understand, is done by induced current
and voltage from nearby strikes rather than direct hits.

if one is standing close by. I don;t know the distance, but I would stay
10/15 feet away, just in case. It is side flashes that get people standing
under trees. Plus, there is the danger under trees from fragments of the
tree, as well as part of it falling on you. In Johannesburg, we get terrific

The problem with "nearby" is the ground current and its associated
voltages as well as the "side flashes". Most livestock farmers have
seen cows, and horses laying near an electric fence that have been
killed by lightening. Generally they were killed due to the voltage
difference between their feet which sent a current through their
bodies.

Even a ground fault in the electrical system that applies 110 volts
directly to a ground rod at the base of a vertical can produce lethal
voltage drops over a few feet near the base of the vertical. I had
one grab me and refuse to let go until I fell over and my knees broke
contact with the wet ground. (and I was hanging onto a bare copper
wire tied to a ground rod.)

Which brings me to... Some times with a truely impressive strike that
has hundreds of thousands of amps and a very steep rise time, the
current in the tower rises so quickly, it produces one whale of a
magnetic field that quenches the current flow. When that happens the
lightening has to go some where and it gets off the tower. It may
even get off near the top and follow a guy wire, or just jump through
the air.

A fence around a tower, or even around a lot with a tower in the
center can have some very impressive currents and voltages induced
when the tower takes a strike.

Were I going to fence a tower, then I think I'd fence my entire back
yard.

73

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

lighting and all thatch roof houses are required to have a vertical rod
about 1/4 wave on 40 meters).I know all this because my uncle was a musical
genius, a truly fabulous conductor. He was hit by lighting.
Joke...sorry)Don ZS6BTP