Re: Masts, Lightning and Volts

Reg Edwards wrote:
> 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.

  One could install grounding mesh throughout one's property...

<snip>

> 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.

  Whoops! You assume uniform soil conductivity. That's 
rarely the case. Consider that most dirt has something 
growing in it. Even ordinary lawns hold moisture near the 
surface which will tend to increase the surface current 
density at the expense of deeper conduction.

> 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.

  Granted, for sufficient values of "short".

> 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.

<snip>

> 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.

  Current advice for those caught in thunderstorms is to 
squat with feet together.

> 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.

  Long weekend, huh?

  Mark L. Fergerson