[TowerTalk] Earthing a tower

[K8RI]

On 1/2/2013 12:21 PM, Jim Lux wrote:
> On 1/2/13 8:39 AM, K8RI wrote:
>> On 1/2/2013 10:12 AM, Jim Lux wrote:
>>>
>> I haven't found the one on the NWS event yet, but there is a comment
>> about half way down on
>> http://forums.mikeholt.com/showthread.php?t=131113
>> by  dereckbc referring to a telecom site grounding to minimum spec.
>
> Yeah, I saw that one.. it wasn't clear whether the 4AWG copper wire was
> installed correctly
>
>
>>
>>
>> The NWS event I was referring to was part of a Sky Warn training session
>> a couple of years back, but I have little sense of time so all I can say
>> for sure is that it was probably some time in the last ten years.  I've
>> not found the specific photos I was referring to, but all I did find
>> appear to be related to moisture getting into the metal/concrete
>> interface.
>
>
> that seems to be a common thread in failures.
>
> I suppose that if you had a failure like this (spalled concrete),
> compared to, say, a rod that didn't make good contact with the soil,
> you'd never see the "damage" buried with the rod, while the concrete
> damage is obvious.
>
> And, of course, neither says anything about whether the thing that was
> being protected was damaged, or the relative cost to repair.  You could
> be in a situation where the rod grounding electrode survives, but the
> equipment it's "protecting" is destroyed, or conversely, the concrete
> spalls but the equipment survives.

But looking at the odds: I'd place a bet that if the grounding didn't do 
its job, it's highly unlikely that the equipment survived unscathed. 
Care to place a wager?
>
>
>
>>
>> Third row down on (Depending on your screen resolution) shows a pier pin
>> tower base with a chunk blown out http://www.bing.com/images
>> /search?q=concrete+damage+by+lightning&qpvt=concrete+damage+by+lightning&FORM=IGRE
>>
>>
>>
>>
>
> the one at ecm-web.. the guy anchor with the Ibeam sunk into the pier?
> http://ecmweb.com/content/upgrades-tv-station-tower-ensure-247-operation
>
> later on the bing page I found a better article with better pictures
> http://www.copper.org/applications/electrical/pq/casestudy/a6137/a6137.html
>
> ALong with the comment that the problem was that the rebar wasn't
> adequately bonded, but that the quasi Ufer ground was so much better
> that lightning took that path (causing damage) rather than the
> conventional driven rod.

Yup!

Yah.  The resistivity of the concrete may be high, but when you have a 
big block, the overall resistance is pretty low (many thousands of cubic 
centimeters) while the contact area with the soil is huge!




> And I love this quote:
> "Finally, we pulled out and inspected the grounding electrodes at the
> anchors and found that several of the exothermic welds had failed.
> Several conductors were also broken, possibly by earthmoving equipment
> during construction. With all these problems, plus the poor connections
> and high ground resistance, it’s no wonder lightning ignored this part
> of the grounding system!”
>
> Breaking a weld is quite the chore... what did they do, run a bulldozer
> over it.

Or through it!  Typically the weld is the strongest part.

>
> Another interesting quote at the end of the article:
> "Copper is cheap insurance compared with the equipment damage that could
> occur if the system is inadequate."
>
> This is where the difference between a ham installation and a broadcast
> installation is most evident.

The magnitude is different, but a few hundred dollars to a thousand or 
two more is still pretty cheap insurance compared to a good DX or 
contest station. Think of the major DX stations or multi-op contest 
stations.  Just think of the thousands of top end rigs driving top end 
amps.  Those stations aren't cheap.  They may not represent the average 
station, but even then, when a ham has all he/she can afford into a 
modest station, it really needs the best protection they can afford. IOW 
They can't afford to cut corners here. Cutting corners on grounding is 
really false economy!

>  At a multi million dollar installation
> that has a 24/7 requirement, the decision to spend a few thousand
> dollars on big copper and ring grounds etc is fairly easy.  They spent
> more on labor than on the copper most likely.  But that's a different
> cost and risk model than most hams would use.

With two complete stations (xceiver, amp, & tuner) plus relays, remote 
antenna switches and close to several thousand feet of LMR-600 and over 
a 100 coax connectors a $1000 in the ground system is still cheap insurance.

>
>

With over 600 feet of bare #2 and 32 or 33 8' ground rods with their 
associated CadWelds(TM) I have a fair amount invested in the ground 
system even if I did install it before the price of copper went up!<:-))

It boggles my mind that hams will spend many thousands of dollars, or 
have a station that would cost thousands of dollars to replace and then 
scrimp on the grounding system, tower(s), and/or connectors

73,

Roger (K8RI)