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Re: [TowerTalk] Fwd: Grounding of tower

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Grounding of tower
From: K8RI <K8RI-on-TowerTalk@tm.net>
Date: Mon, 06 May 2013 20:27:37 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 5/6/2013 6:48 PM, Hans Hammarquist wrote:
Maybe you can count on a "nice" weld if/when lightning strikes. It, sure, will 
jump that little distance.

Yup, sure will and in the process apply many thousands of volts to equipment tied to that ground. Poor grounds are the cause for many equipment losses. It doesn't have to be a gap, just a few ohms. Just think if a few ohmd resistance in the ground connection can cause a great deal of damage, how about a small gap?

For the moment, let's look at that gap. Say it only has 300 volts breakdown and lets ignore even that. The lightning flashes across the gap creating a momentary plasma of just 1 ohm. The lightning strike is not a strong one, but develops 5000 amps for only a millisecond. During that one thousandths of a second the wiring in the house is E=IR or 1 X 5000 = 5000 volts above ground

Most of us would not consider that to be an acceptable situation. However most home electrical grounds will only protect you from the induced voltage from a nearby strike and only mitigate the damage from a direct hit. The purpose of that ground is to prevent burning the house down, not to protect your equipment.

My antenna system consists of two towers. At present a 100' ROHN 45G and a 40' 25G soon to be a 72' LM470 crank-up. The ground system consists of over 600 feet of bare #2 CadWelded to 32 or 33 8' ground rods. The house electrical system is also tied into this system.

That 45G took 17 visually verified direct strikes the first 6 years it was up. IE Some one reported seeing the strike. The only damage was the removal of all the waterproofing/tape/putty and plating from the connectors "up there. I began to have second thoughts about this location, but there haven't even been any close strikes over the last 4 years. Who knows what this summer will bring.

As the cables came up behind a desk the storm was usually over before you could get to them and there was no way I'd tackle disconnecting then during a storm. So basically the entire station was connected to http://www.rogerhalstead.com/ham_files/Tower29.htm these plus 75, 40, and 160 slopers, 2 large UHF TV antennas and a small "Dish Network" satellite antenna. There is also a 5 computer network tied into the station which also ties into the cable system

There were no equipment failures with antennas covering 160 through the 440 band even though they remained connected at all times!

There is a reason the NEC calls for "bonded" or "CadWelded" connections.

73

Roger (K8RI)




Hans



-----Original Message-----
From: K8RI <K8RI-on-TowerTalk@tm.net>
To: towertalk <towertalk@contesting.com>
Sent: Mon, May 6, 2013 5:40 pm
Subject: Re: [TowerTalk] Grounding of tower


On 5/6/2013 5:13 PM, Hans Hammarquist wrote:
Gentlemen,


It's hard to believe that a screw connection can be approved for burial. Even
if it is allowed, and why wouldn't it, I still would (very strongly)  prefer to
be able to inspect them now-and-then.



I have never had a buried screw connection maintain integrity.  When we
put in an underground feed the electrician used the standard ground rod
clamp.  A year later I removed the dirt and could slide the clamp up and
down the ground rod using only 2 fingers

73

Roger (K8RI)

I used short pieces of 4" schedule 40 PVC pipes around my ground connections.
Lowes has (I am sure) caps that fit over the 4" pipes so grass and other stuff
can cover them. I also like to be able to run the lawn mower over it without
problems.


Hans - N2JFS
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