[TowerTalk] Arrestor placement

Sun Dec 28 23:17:42 EST 2014

On 12/28/2014 11:05 PM, Roger (K8RI) on TT wrote:

One further thought:  I would not place an additional arrestor at the 
base of the tower where it would likely see a much higher voltage than 
at the grounding plate (CPG)  at the house entrance.

73

Roger (K8RI)

>
>
> I agree with David, but I've not read all of the thread.
>
>   You want the arrestor as close to the thing it's protecting as 
> practical.  The common point ground at the station entrance is the 
> best compromise.  I'd phrase it a bit differently than David, but the 
> result is the same.    The effects of a  spike coming in  as induced 
> voltage in the house wiring depend on a number of things. An induced 
> spike will likely create a substantial spike on the power, phone, 
> cable, and the antennas. You have the direction to the source, or 
> lightening bolt. The induced voltages will travel a bit slower on the 
> wiring than the EMP through the air. So distance and direction will 
> determine the phase differences between all these conductors connected 
> to the rig.
>
> The pulse "induced" into the antenna(S) from a nearby strike sees the 
> coax and tower as parallel conductors while the center conductor of 
> the coax acts like one plate of a very long capacitor so the voltage 
> across the coax is much less at the base of the tower than at the 
> top.  With the coax braid grounded to the tower at the top and botton, 
> both are "Earthed" at the base of the tower.  With the coax between 
> the tower and shack the coax agan acts like a long capacitor reducing 
> the center conductor to shield even farther.  This in itself reduces 
> the work the arrestor has to do substantially.  (Another reason for 
> mounting the arrestor at the CPG.
>
> Beyond that being the best place to put the arrestor, lightening 
> protection can never guarantee 100% protection.  The induced voltages 
> come at the rig from all directions with their phase differences and 
> amplitudes varying from almost nothing to those we don't want to think 
> about.
>
> Lightening protection is a crap shoot, BUT, a properly placed 
> lightening arrestor along with a well laid out ground system of 
> sufficient size pushes the odds in our favor. It can be substantially 
> in our favor, however you soon reach the point of diminishing returns 
> where where it takes a LOT invested to only give a little increase in 
> protection.
>
> I had no damage from many direct strikes to the tower (17 visually 
> verified strikes and who knows how many were not seen) along with 
> major close strikes that did costly damage to the neighbors.  The one 
> was a multiple strike that hit all around me, but ignored the tower 
> and rigs...BUT...Last summer a nearby strike got into my CAT 5 gigabit 
> network and did major damage to several computers.  I ended up 
> rebuilding two BIG computers, while the router and switch were 
> fried..  One was a complete rebuild from the motherboard up. The 
> other, a smaller computer, but still larger than what most would have 
> was a complete replacement.  The one computer hooks into the station, 
> but I've found no damage to the station.
>
> Unfortunately I use OEM software which meant not only new hardware, 
> but new operating systems as well.  The good news was the station had 
> no problems, or at least none I've found so far.
>
> 73
>
> Roger (K8RI)
>
> On 12/27/2014 1:58 PM, David Robbins wrote:
>> Yes, it can make a big difference. Lightning arresters are NOT just 
>> protecting you from a strike on your antenna.  They also protect you 
>> from a strike on the power line or even just to the ground nearby 
>> that causes the ground system voltage to rise while the voltage on 
>> the center of the coax doesn't... if the arrester is not very close 
>> to the radio it delays the arrester action which can make your radio 
>> the easiest path from ground to the center conductor... in hv 
>> engineering terms its called a backflashover, which is caused when 
>> lightning hits the ground system causing a flashover to a power 
>> conductor.
>>
>> David Robbins K1TTT
>> e-mail: mailto:k1ttt at arrl.net
>> web: http://wiki.k1ttt.net
>> AR-Cluster node: 145.69MHz or telnet://k1ttt.net
>>
>>
>> -----Original Message-----
>> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf 
>> Of David Gallatin via TowerTalk
>> Sent: Saturday, December 27, 2014 18:49
>> To: TowerTalk at contesting.com
>> Subject: [TowerTalk] Arrestor placement
>>
>> Hello all,Somehow I have gotten the idea that one's lightning 
>> arrestors are best placed at the bottom of the tower. This seems 
>> logical to me. Then I came across another piece that said, no, 
>> arrestors are to be placed at the shack entrance panel. Which is it? 
>> Or does it matter?  73, David, AA9G
>>
>> ex W5DCG and KC9EEV
>>
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>
>

-- 

73

Roger (K8RI)

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