[TowerTalk] Lightning Advice

[Roger (K8RI)]

>
>>There have been some good suggestions as to grounding. A good Single Point
>>Ground (SPG) is essential.
>
> ------------ REPLY FOLLOWS ------------
>
> I have heard this stated before, but in the case of lightning, I don't
> understand the reason why. I do understand in the case of AC mains
> safety grounding, but not for lightning.
>
> It would seem to me that a strike should be sent to earth as close to
> the point of the strike as possible, rather than routed off many feet
> to an SPG. Could you please explain?

We have some good engineers on here who know much more about this than I, 
but...

I guess I'll start at the end and work backwards.
First; yes, you do need a good ground at the tower where a good ground 
consists of a series of heavy, bare copper cables CadWelded (TM) or some 
other exothermic bond to a number of ground rods spaced roughly twice their 
length apart. Although I disagree with some of the information on the 
Polyphaser site, they have some very good information on grounding.  There 
is some disagreement as to how far out the tower can be before you don't 
need to tie its ground system into the Single Point Ground (SPG). As the 
coax coming in it also a route for the lightning I prefer to run a good 
ground ( complete with ground rods) along with the coax to a grounded 
bulkhead at the house entrance. That bulkhead is my SPG.

Lightning is often erronously though of as DC, but each strike consists of 
multiple strokes with steep rise and fall times. The steeper the rise and 
fall times (which aren't necessiarily the same Dv/Dt or Di/Dt) the higher 
the fundamental frequency with a wave form rich in harmonics. The current 
varies over many orders of magnitude between lightning strikes and this 
along with the various rise and fall times varies the effect on nearby 
wiring. This is just one of the reasons lightning is so unpredictable.

As I mentioned previously a lightning strike a mile away can induce voltages 
as much as 1000 volts (or even more) per meter in wiring.  A lightning 
strike to a  tower and or antenna system 50 or 150 feet from the station 
will result in not only high currents and voltages on the feed and control 
lines, but very high induced voltages in the house and even between pieces 
of disconnected equipment. Again these induced voltages will have relatively 
steep rise and fall times however the inductance and capacitance of that 
wiring will have an effect (modify) the wave form. Tying the ground systems 
togeter *tends* to, or helps to keep the voltage rises from the induced 
voltages in the wiring AND grounds close to the same. Although we think of 
the house wiring as being the one that raises in potential, so too does the 
ground system with a lightning strike. All the voltages in the house are in 
reference to the neutral and ground system.  If all the grounds are tied 
togeter and tend to rise along with the wiring in the house then there is 
*relatively* little potential difference between the hot wires and ground 
and damage is minimized.

One effect the steep rise and fall times have is to create a lag in the 
voltage rise over distance.  The higher the frequency (steeper rise time) 
the more the difference. If the tower is far enough from the house and the 
rise time steep enough the tower can have reached its full potential before 
the voltage begins to rise at the house. In other words the connection from 
the tower to the house may have little if any effect. The tower no longer 
sees the house as part of its ground system even though they are connected. 
BUT with the wide range of rise times and tremendous range of power in 
lighting strikes it's a good idea to tie them together. For any reasonable 
distance it's essential to tie them together.

Most electrical damage is done by energy coming in on the phone lines or AC 
mains. As these should have their own grounds where they enter the house, 
not connecting those grounds into the SPG allows them to rise to a different 
potential. With all the grounds tied together surge protectors have a much 
easier time.

Having said all this, there is little we can do for complete protection and 
in particular the so called super strikes, but a good SPG *WILL* minimize 
the risk and damage.

I hope I phrased this correctly and it's of some help.

73

Roger (K8RI)

>
> Thank you,
>
> Bill W6WRT
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