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@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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73
Roger (K8RI)
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