[TowerTalk] Dealing with Lightning and MCI

[Tom Rauch]

> Date:          Wed, 13 May 1998 00:28:58 +0000
> From:          David Robbins <k1ttt@berkshire.net>

> Tom Rauch wrote:
>
> > Voltage drop is a function of resistance and current. E=I*R
> 
> don't forget the integral of the electric field along the path.
> this is especially important in this case where the electric
> field can be very strong under a cloud.

That's correct Dave, and at the same time don't forget that field is 
a nearby infinite impedance field easily distorted by even the 
poorest conductors.

Every day we work in and walk through a 300 v / meter field above the 
earth's surface. We not only can't "feel" that field, we can't even 
measure it with an instrument that only requires picoamperes of 
operating current.

> > How much current do you suppose flows into the tower?
> > 
> > Let's assume the tower has a resistance of 1 ohm from end to end, and
> > 1000 ohms to earth. How much current has to flow in the path to hold
> > the tip of the tower at 1000 volts potential more than the earth
> > surrounding the tower?
> 
> in the strong field under the charged cloud the ground potential
> (even without a tower) will rise as the mobile charges in the 
> ground move in response to the charges at the base of the cloud.
> this potential can be quite large over a small area.  

It's true the charge difference can be quite large, but we can't 
easily measure it or observe any ill effects from it.

We also can't stop it from happening, no matter what we do.
 
>if you had
> two ground rods separated by a couple hundred feet and ran insulated
> wire back to a meter you could measure this potential change as
> charged clouds approach.

It is immeasurable because the charge in the wires moves the 
same direction as the charge in the earth. You might, with an 
extremely sensitive meter and no other voltages , be able to 
measure the change, but not with the test setup you suggest. Currents 
induced in the earth from many other sources would mask the 
very gradual change caused by the slow charge migration, and the 
wires themselves would have the same electric field potential as the 
ground and the meter circuit would be like two batteries with positive 
connected to positive.

If a tall tower was well insulated, and you hung a gold foil 
electroscope at the base, you could watch it move. If a well 
insulated wire was mounted horizontally come distance above earth, 
you could watch it's charge change.

> (don't attempt this at home, the voltages
> can get quite large, especially if there is a lightning stroke near
> one of the rods!)

I believe that should read *only* if there is a lightning stroke 
nearby. I have well insulated Beverages that stretch for 500 feet and 
further. I care less about sticking the wires in my teeth and 
stripping them, clouds or no clouds. I won't do that in a 
thunderstorm, however.

The wire would have the same charge distribution as the ground, and 
without appreciable charge movement there would be no potential 
difference between the wire end and earth, if the far end of the wire 
was grounded. If the wire was well insulated, it could rise to a 
potential closer to the potential of the cloud and discharge through 
you to ground, that is why well insulated long wires will arc 
occasionally...especially during wind when friction from air charges 
the wire.

>  because the tower should have a much lower 
> impedance than the ground it will track the voltage at the ground
> connection point, in order to do this current will flow up and down
> the tower as the surrounding field changes.  these currents can
> get large even if the lightning doesn't directly strike the tower
> as the charges in the ground and cloud respond to a nearby stroke.

That's correct, DURING a nearby strike or a direct strike. But 
current is extremely low during the pre-event. Current is so low 
that charges are easily replenished even in something as resistive as 
a tree, or a human.  

> > The tower would require just under one ampere of current
> > CONTINUOUSLY to have a 1000 volt charge!
> 
> not to have 1000 volt CHARGE, but to have a 1000 volt DROP from
> top to bottom.  in a slowly changing field before a lightning
> stroke this wouldn't happen.  the voltage from the bottom to the
> top of the tower would be virtually constant and equal to the
> ground potential.

Correct, I should have said potential difference from top to 
bottom...not charge difference. I was addressing the issue of leg 
joint resistance, and trying to not be confusing.   
 
> this gets to the heart of the problem when you talk about
> lightning and charged clouds and how they affect towers and
> antennas.  when you start talking about events on the scale
> of charged clouds and lightning you have a basic problem.  what
> is 'ground'?  you can measure large voltages between things
> that are supposedly 'ground'.  not only because of the resistance
> of the dirt, but because of the speed of propagation of the
> fields vs the rise times of the waves involved.  over small 
> distances you can limit the differences in the ground potential
> by connecting all your grounds together.

That's right, lightning---contrary to what has been said---behaves 
nothing like dc. It behaves much more like RF than anything else. We 
all know good RF grounds are tough to make, and any lead length can 
be a problem. Woven conductors also are a problem, since they have 
higher RF resistance than smooth round conductors.

> i hope this isn't rambling too much, but this is a complex topic
> and we are just scratching the surface here.

Very good. Thanks for keeping me honest. I hope we agree on the 
measurement with a meter, and my comments were useful.

I'd like to point out that the ONLY time a corona cloud generator 
would work better than a smooth sphere without corona would be if 
the object was totally insulated from ground, and a fair distance 
above ground (to prevent arcs at the base). Even a few tens or 
hundreds of thousands ohms of resistance from the tower to earth 
would replenish charges much faster than corona dissipates them. 
Charge can not be dissipated and removed unless the structure is well 
enough insulated that charges can not be replenished.

It does work VERY well on airplanes, because they have a high 
impedance path to any large charge source.

73, Tom W8JI
w8ji.tom@MCIONE.com

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