[TowerTalk] Fwd: Grounding of tower

[K8RI]

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 at tm.net>
> To: towertalk <towertalk at 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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