[TowerTalk] Fwd: The Need for Grounding

Thu Jan 14 15:24:42 EST 2016

As I see it:

First semester circuits. The first book I opened looked simple until I 
saw the way they solved the problems Instead of the way I had always 
solved those circuits with Ohms Law.  They were replaced with 
summations.  I saw why Calc I and II were prerequisites, not 
co-requisites for the EE program.  I couldn't afford the extra time to 
first take the math courses. I changed to Computer Science where the 
same Calculus was required, but could be taken at the same time but that 
route added great difficulty to the degree. I only had to take two more 
courses for a minor.

  Never saw so much calculus other than in Calc I and II.  That voltage 
loss from tower to house becomes summations of the capacitance to 
ground, inductance in the cables, and finally resistance.
The reverse EMF caused by the fast current rise raises the resistance in 
any conductor, far above the measured DC resistance.  In some instances 
a single ground rod with low, measured resistance might not even see 
most of the strike, because of the reverse voltage created by the high 
current, and fast rise time of a single pulse. It can show why multiple 
ground rods in a network are preferred, or the large concrete UFER can 
work better than a few ground rods.  After 25 years, the calculations 
are beyond me.  True lightning protection is well beyond reaching a 
simple, required DC resistance to ground.  If it were, lightning 
striking a substantial tower wouldn't get of the tower part way down.

That single, 20 foot ground rod with far less than the required DC 
resistance to ground could easily prove to be a poor choice over a 
network of 8' rods that just meets the required resistance.

Unfortunately analyzing a lightning strike is far more than a simple 
Ohms law problem.  That's why we see "Rule of Thumb" used so often.

  73,

Roger (K8RI)

On 1/14/2016 Thursday 12:55 PM, David Robbins wrote:
> 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 at 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 at 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 at 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
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

-- 

73

Roger (K8RI)

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus