[BC] Lightning and grounded masts

[Clive Warner]

> Actually, Grounded towers don't do much better. It seems that when the
charge
> travels down the tower to ground, it induces voltage in the outrigger
wires.
>
> -D

Well, let's see if I can put some figures to that assertion.
The magnetic flux density B, from a strike, is mu*H
or 4*PI*(10 to the -7) * I/2*PI*R where R is the distance from the current
path in metres.

The shunt-fed is of course connected at the top (= 0 metres) and at the
base, probably about 2 metres from the mast. So let's (conveniently) call
the average distance 1 metre.

A strike has current characteristics varying from 10 KA to over 200 KA.
Here I will adopt a fairly high value of 100 KA.

The worst case (lightning strikes contain a full spectrum) is at DC - 17KHz,
so I will choose the worst case.
At 3m from the stroke path, the nearest my tables will go, I see a flux
density of 36 dBGauss.
Graphing with Excel and using regression gives me an estimated value of 45
dBG.
6dBG = 2 Gauss.    45dBG = approx. 160 Gauss.
(See "EMI Control Methodology and Procedures", Don White Assoc., for refs.
above)

Assuming a wavelength of 600 KHz then lamda = 500 metres and one-tenth
lambda = 50 metres.
Therefore we can say the situation is an induced field of 160 Gauss across a
'cable' of 50 metres.
The mast end is earthed, we can calculate the induced voltage using
Faraday's Law.

This states that E = dW/dT where dW is the change in magnetic flux, in
Webers/sq.m, of the event, and dT is the time over which the change takes
place, in seconds.
Unfortunately our field units are in Gauss not Webers so first I must change
that to 0.016 Webers/sq.m
(How I HATE American obsolete units. My heavens I was in 6th grade when we
changed from that crap (cgs) to proper MKS units!)
Now I need to know how many sq.m cutting field lines that I have on the
adjacent conductor. Let's say the downlines are 2m across at the base, 50m
high, and taper to a point at the top. Using the formula for the area of a
triangle gives us half base x height, = 1x50 or 50 sq. metres.
I looked up the approximate pulse duration for lightning and discovered a
big variation in pulse length, but for a high current strike we can work
with 20 microseconds.
Substituting, this gives V = 0.016/0.000002 = 800 volts.
======================================
This looks like a lot less than the equivalent lightning strike on a
base-insulated mast. DWC give 300 MV as the voltage of the same 'sample'
strike. Obviously the LPD discharge horns will limit this, but I suggest
that the breakdown voltage of the discharge horns is likely to be many times
the 800V I calculated above.

Anyone care to challenge my figures?

Empirically, I have worked on many shunt-fed, and never ever saw any damage
caused by lightning.

Best wishes to all
Clive Warner (staggering back upstairs with arms full of giant reference
tomes)
www.citiria.com