The Ohm's Law that we can all quote E=IR is an extremely simplified version
that is only valid for "lumped models". and of course only for resistance. it
must be expanded to include lumped capacitors and inductors which of course
then leads to differential equation forms when AC come into play.
THEN if you are considering anything other than a 'lumped model' you must also
include radiation and other EM field effects. think of 'lumped' things as
components or circuits you can hold in your hand... more technically you have
to compare the highest frequency being considered vs the size, the size to
be considered 'lumped' must be much less than a wavelength at the highest
frequency. thus, a 10' ground wire is NOT a lumped component at least in
the higher HF bands, so to properly model it you need to consider that it will
have different voltages and currents all the way along and include external
fields
and things around it that it might couple with, etc, etc. coaxial cables
connecting things can not even be considered lumped components in most cases.
antennas are of course not lumped components.
Jan 14, 2016 11:36:39 AM, edwmccann@yahoo.com 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It is unlikely Ohm's Law will ever be repealed,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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