[TowerTalk] Fwd: The Need for Grounding

[Hans Hammarquist]

It's simply Ohm's law that still is valid. Yes, voltage across all the inductance adds in too.


In my case, my tower is grounded with a resistance to ground of about 3 ohms (when it was tested some time ago). If the tower is hit, (I guess) the current is about 3 kA with a resulting 9 kV between the tower and ground. Even if my shack is 300 feet from the tower (which it isn't in my case) you will still have about 9 kV between your grounded shack and all the incoming wires from the tower. (You might have 1 ohm resistance total in your cable, but if there is no significant current the voltage drop is nill.)


If yo shack is grounded with, say, 6 ohms ground resistance you will still have about 6 kV to ground. You will have about 1 kA going through your cables. Now, if you have all your equipment well grounded in the same point as the shack all the equipment will also be on the 6 kV potential, maybe a little off as you might see the voltage drop due to variations in the grounding point. Say that difference is 0.1 ohm. You will the see about 100 V which most equipment will tolerate.


Am I on the wrong thinking path? Correct me if I'm wrong.


Hans - N2JFS






Jan 13, 2016 07:45:50 PM, w3yy at cox.net wrote:

The latest posts about grounding, and finally some free time here, prompt me
to ask the following question.

Given lightning's desire to find the quickest way to ground, why doesn't it
expend itself in a single 8ft ground rod at the base of a tower, rather than
passing through another 250ft of transmission and control lines (also buried
in the ground) leading to the shack? I would think that by then it has had
plenty of opportunity to arc to ground itself.

I am not disagreeing with the experts on this subject, but I just don't
fully understand what is commonly recommended. With only a single 8ft
ground rod at the base of my 100ft and 120ft towers which are about 100ft
and 250ft from my house, I have only suffered two minor damages from a
lightning strike in over 40 years. And, I'm not sure that even had anything
do with the towers, but was just an unrelated power line surge.

73, Bob - W3YY


-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of EZ
Rhino
Sent: Wednesday, January 13, 2016 6:05 PM
To: Towertalk Reflector
Subject: Re: [TowerTalk] Grounds, 'remote' towers, 'house' power system

I'm not in disagreement with you Jim, but then why doesn't NEC specify to do
things for lightning protection such as commonly followed by nearly all
commercial tower installations? Such as multiple ground rods, flat strap,
star grounds, etc? (Think Polyphaser's docs). We know that one ground rod
is woefully inadequate for a direct hit. If NEC is all about lightning, why
doesn't is specify using more than one? It sure seems like NEC is about the
bare minimum for AC protection and when it comes to RF and towers, it
doesn't really give much pertinent info at all.

Chris
KF7P






On Jan 13, 2016, at 15:49 , Jim Brown wrote:

On Wed,1/13/2016 2:35 PM, N3AE wrote:
> The NEC is focused on electrical safety and not necessarily the most
effective system for lightning protection.

This is NOT true. The bonding required between your tower and power system
sub-panel is for LIGHTNING protection.

In general, proper bonding is critical for lightning protection, electrical
safety, fire safety, and to minimize hum, buzz, and RFI. Proper bonding is
described in

http://k9yc.com/GroundingAndAudio.pdf

I'm not going to repeat it here for those too lazy to study it.

BTW -- I TAUGHT courses on Power and Grounding for about ten years.

73, Jim K9YC
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