lightning may not even follow the path of 'least resistance', at least as you
conceive it. connecting a low resistance wire to a ground rod does not mean
that
the lightning current will follow that wire, it will also divide itself up and
go down the feedlines, through the concrete base, and even jump through the air
to nearby
buildings, trees, etc.
also as noted earlier, lightning currents going into a normal ground rod
DECREASE its effect resistance... the higher the current the lower the
resistance
goes as the soil around the rod starts to ionize and conduct more current away
from the rod. a typical 25 ohm rod may get down to 6 ohms at the peak of
a lightning stroke if most of the current is going there.
no, you can not just consider rods chained together as a series of resistors in
parallel... that may work for DC or power frequency currents, but not for rf or
lightning.
especially with lightning you have to consider the propagation time of the
current through the wire under ground which is much slower than through a wire
above ground. also current is being bled off from the horizontal wires also
reducing what each rod further out sees... so as you go farther away from the
tower
each rod becomes less useful in lightning cases.
Jan 14, 2016 12:24:22 PM, ve4xt@mymts.net wrote:
The important thing to remember is electricity doesn’t follow the shortest path
somewhere, it follows the path of least resistance. And it’s important to point
out that during a lightning event, that path is going to change, as the
lightning charge will easily overload a single rod, driving that rod’s
resistance sky high. Since the event is not yet over, the remaining charge will
follow the next-best path, which could be through your radio. Better it be
through the other rods in your network, no?
Is it useful to envision ground rods as resistors, and a chain of ground rods
as resistors in parallel?
Since we all know parallel resistors divide current among them, and as a group
present a resistance that is some fraction of their individual values, is it a
good meme to remind us of the importance of multiple rods?
73, kelly
ve4xt
> On Jan 14, 2016, at 10:35 AM, Edward McCann via TowerTalk wrote:
>
> On point.
> It is unlikely Ohm's Law will ever be repealed,
> by the current crew in the beltway or that if the pretenders to the throne.
> AG6CX
>
> Sent from my iPhone
>
>> On Jan 14, 2016, at 8:22 AM, Hans Hammarquist via TowerTalk wrote:
>>
>> 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@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@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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