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Author Topic: Station grounding.....RF grounding.....???  (Read 10920 times)
WX7G
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« Reply #15 on: July 17, 2012, 11:54:25 AM »

I am not saying a single ground rod is to be used. I am saying that it is best to avoid a lighting path though the house. And to do that one cannot have the coax entering at one end of the structure and the AC power entering at the other end of the structure. Every cable must enter the structure at one point and be bonded together and to ground.

I use SPICE to analyze building/antenna grounding and this can be used to explore different grounding and cable routing configurations. The current through different paths is simulated along with potential differences, effective source impedances, stress on transient suppression devices, and by using cable transfer impedance the differential voltage induced in cables is simulated.
« Last Edit: July 17, 2012, 12:19:34 PM by WX7G » Logged
WX7G
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« Reply #16 on: July 17, 2012, 02:05:19 PM »

On page 13 of this document the preferred single point entrance for all cables is described.

"Ideally, the ground at our electrical service entrance and all other grounds serving telephone, cable service, satellite, antennas and our radio equipment should be tied together. If the tower ground system is not too far away from the equipment, the ground systems should be tied together using wide copper strap and appropriate bonding clamps. Our feedlines and control lines should enter the home at the service entrance and have their respective protectors bonded to this ground as well. This forms the basis of a single point ground system, since the single origin point of the ground system is the service entrance location and ground."


http://ad4c.us/Other%20assorted%20manuals/Installing%2520Lighnting%2520Protection.pdf

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K1CJS
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« Reply #17 on: July 18, 2012, 04:45:15 AM »

I see what you're trying to say, Dave, but that is an 'ideal' situation.  It just doesn't exist.  Most wired utility entries are near each other--NEAR--each other, but they don't enter the building at the same point.  AAMOF, there isn't a house in existence that doesn't have wiring of one form or another on the outside of any of its areas--wiring that feeds off of the electrical panel and is has some sort of box or connection on the outside of the building. 

Before (on another thread) you were advocating running all coax cables to the service ground outside the house, grounding them there, and then running them back along the outside of the house to the shack.  Now you seem to be backpedaling.  Now you're saying they all have to enter the building at one point.  You really should make up your mind! 

Oh, and another thing--a lightning bolt will rip right through a building to find a ground point that it wants.  Do you really think that having a single entry point and one common ground rod is going to prevent that from happening? 

It still seems like you're interpreting the one IEEE paper on grounding in your own way and basing all your recommendations on that.  Ideal situations just don't exist in the real world.  73. 
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AA4PB
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« Reply #18 on: July 18, 2012, 06:01:39 AM »

--a lightning bolt will rip right through a building to find a ground point that it wants.  Do you really think that having a single entry point and one common ground rod is going to prevent that from happening? 

Lighning follows Ohms's law. The current will flow through all parallel paths to ground BUT most of the current will flow in the path that has the least resistance (or impedance). If you can provide a low resistance path outside of the house (lower than any internal path) then most of the current will flow in the outside path. If you can provide a "single point ground" with all equipment grounded to ONLY that point then the voltage on all equipment will rise together and little or no current will flow through the equipment paths. This is a well known technique used by people who regularly design cell sites and other facilities with large antenna towers.

Unfortunately, it is often difficult (expensive) to do in a residence that was never designed with such an installation in mind.
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WX7G
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« Reply #19 on: July 18, 2012, 06:56:03 AM »

K1CJS you don't seem to understand what I've been trying to communicate, perhaps poorly. I suspect that if I were able to post a diagram it would be clear. In a nut shell it is to hang the house AC and antenna cables off a single ground point. There can and should be be other ground rods between the antenna and the AC service ground but from that point into the house no other ground points should be used. The antenna coax is routed to the AC service ground then into the building or along the outside of the building to the shack with no other ground points between the AC service ground and the shack.

This is based on my professional analysis and recommendations as well as industry practice. The IEEE paper I cite I found yesterday and I posted it because it contains diagrams showing this method.  

As I said, all of this can be modeled using SPICE to obtain numbers to design with. Lighting protection then moves from rules-of-thumb to real engineering. Insulation coordination, ground rod placement, cable transfer impedance, ground strap length and width, transient device sizing, filtering, and so on are then designed to protect the equipment against the specified lighting current.

The method I advocate for a ham to route the coax per the single point ground entry method is to minimize damage to the house (AC wiring vaporizing) and station. This is the ideal method and offers maximum protection. When this cannot be done and the coax and AC enter the building at different points there will be lighting current through the house wiring. Shunt wires between the AC service ground and the coax ground (where it enters the building) will reduce but not eliminate this current.
« Last Edit: July 18, 2012, 07:06:54 AM by WX7G » Logged
WX7G
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« Reply #20 on: July 18, 2012, 08:35:25 AM »

    Let's run a
simplified numerical example. We will ignore the coax and ground wire shield inductance and displacement current.

Senario 1

Grounded tower
Four ground rods at the base
One ground rod where the coax enters the house
AC service has one ground rod
Six ground rods in the system
Lightning current is 100kA

4/6 of the current is conducted to the four ground rods at the tower. 2/6 of the current is conducted to where the coax enters the house (33.3kA). Half of this is conducted to ground at the rod and half goes through the house AC wiring to the AC service ground. There is 16.7 kA through the house AC wiring.


Senario 2

  • Grounded tower
    Four ground rods at the base
    One ground rod where the coax enters the house
    AC service has one ground rod
    A #6 shunt wire is run from the coax entry ground rod to the AC service ground. The #6 wire is about equal to the three #12 wires in the AC circuit powering the station.
    Six ground rods in the system
    Lightning current is 100kA

4/6 of the current is conducted to the four ground rods at the tower. 2/6 of the current is conducted to the where the coax enters the house (33.3kA). Half of this is conducted to ground at the rod (16.7 kA). Half of the remaining 16.7 kA takes a path through the shunt wire and half takes a path though the house AC wiring. This is 8.3 kA.
 

Senario 3

  • Grounded tower
    Four ground rods at the base
    The coax is routed to the AC service ground then to the station
    AC service has one ground rod
    Five ground rods in the system
    Lightning current is 100 kA

    4/5 of the current is conducted to the four ground rods at the tower. 1/5 of the current is conducted to where the coax and AC service ground meet (20kA). All of this is conducted to ground at the AC service. Current conducted through the house AC wiring is 0 A.
[/list]
« Last Edit: July 18, 2012, 08:52:06 AM by WX7G » Logged
WX7G
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« Reply #21 on: July 18, 2012, 01:56:27 PM »

Scenario 3 would seem to say that additional ground rods between the tower and the AC service ground are not needed. For conduction current through the house they are not, but for displacement current they are.

Take scenario 3 and assign 50 ohms to each ground rod. The 100 kA will develop 1 MV between the five ground rods and the surrounding ground. The AC wiring in a 40 x 40 foot house presents roughly 500 pF to ground. The voltage dV/dt of the 100 kA lightning strike is about 1 MV/us into the 10 ohm ground system. The displacement current into the house is:

i = CdV/dt = (500pF)(1MV/1us) = 500 amps.

500 amps is manageable by the AC input of consumer electronic gear (it must meet a 1 kA surge test).

Add more ground rods and this current is reduced. Space the ground rods at least one ground rod length. Add more ground rods and the voltage differential between the house AC wiring and ground is decreased. Place ground rods at the corners of the house with buried heavy copper wire and the voltage differential between the house AC wiring and the ground under the house will be significantly reduced.

1 MV without field enhancement can break down 14" of air. With field enhancement it might break down 24" and make it from the AC wiring, through the floor to ground.

This and the previous emails are enough perform a first-order (simplified) ground system design for AC power protection given the specs of lightning current and house conduction/displacement current. The frequency of various lightning current strikes are available on the web. I would aim for a house current of 1000 amps and in lieu of another number 100 kA for the lightning . The rest falls into place; the number of ground rods and shunt wires if used.

The next step, and more complicated, is designing in feedline double shielding, air gaps, gas gaps, solid state devices, and bandpass filters to attenuate the lighting induced voltage to a level that will not harm the equipment. And designing the system such that the protective devices do not fail either.


« Last Edit: July 18, 2012, 02:01:29 PM by WX7G » Logged
LA9XSA
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« Reply #22 on: July 19, 2012, 03:26:27 AM »

Ignoring the impedance of the house wiring and coax, and the ground system impedance, seems to me to make the scenarios too simplified. Scenario 1 (no outside bonding of AC and antenna ground) would still be a very bad thing, but not because one 6th of the lightning that strikes the tower will go through the house. It wouldn't, I think. In case of a tower strike, you could indeed get excessive lightning current flowing on your house wiring - or the plasma it leaves behind - but not one sixth of the tower strike.

I would be at least as worried about a transient (such as a neighbor's electrical fault, transformer fault, or lightning strike) coming in on the AC wires, going through the house, and seeking your excellent antenna ground through the house wiring.

Having a single entry point for all cables (scenario 3) is indeed the ideal situation, but if you have sufficiently low impedance perimeter ground cables bonding the coax entry and AC entry grounds together, it would reach the same level of protection. The drawback is that it would need more copper wire than having a single entry for all cables; the advantage is that the extra copper could cost less than it would cost to rebuild the house.
« Last Edit: July 19, 2012, 03:32:16 AM by LA9XSA » Logged
K1CJS
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« Reply #23 on: July 19, 2012, 05:02:06 AM »

WX7G, the one flaw in all of your hypothetical scenarios is this--there are already paths for lightning into a building, paths that you can't do anything about.  ANY wiring--wait, lets expand that--any length of metal, be it piping, electrical wiring, cable, telephone drops, etc. going into--or even just attached to the house may well provide that path.  Also, you can provide what you think is the ultimate low impedance ground point to drain that charge--and the charge may ignore that point entirely and find another. 

Even metallic fixtures such as railings and downspouts or anything else not connected to any sort of wiring can act as a conductor of such a lightning charge, which can then jump to an interior metallic fixture that IS connected to a ground.  You can't ground every pipe, cable or wire that goes into a building, nor can you have an outside ground on every outside metallic fixture whether or not it is connected to power or other wiring in a building.

No matter what you think or how many references you provide, your single point ground is still an ideal situation--one that cannot and does not exist in the real world--and therefore the protection you claim your method provides does not exist either.  Lightning may well follow Ohm's law, but it has been proven time and again that it follows its own path as well--and finds its own ground point no matter how good the ground point you provide may be.

This will be my last post in responce to you.  73.
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WX7G
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« Reply #24 on: July 19, 2012, 05:22:01 AM »

While scenario 1 and 2 are not ideal they can easily be improved. In these scenarios there is a lightning current path through the house AC wiring from the radio room to the AC service ground. The current takes the path along the coax to a radio then along the cables connecting this radio to other equipment in the room and down their respective AC power cords. We can reduce the current through the radios from thousands of amps to zero amps by using a single point ground. Here is how it is configured:

A metal coaxial cable entry panel is provided. On this panel is an AC outlet box that has its ground bonded to the panel. This AC outlet box is plugged into a room AC outlet and all of the radio and computer equipment is plugged into this AC outlet box on the panel. If there is 120 VAC for the radios and 240 VAC for the amp outlet boxes for both are located on the metal pane. The antenna coaxial cables terminate their shields to the bulkheld feedthru connectors on the panel. Wired PC communication cables from outside the room also terminate their shields at the panel. The result is that all RF cables and AC of the radios as well as the PC remain at the same relative potential during a lightning strike. They rise and fall together and there is no lightning current between chassis. An RF switch at the panel to short the lines to the radios may provide enough protection. A protection circuit that allows operating radios to survive even in receive mode can be designed but it would be more than most hams would be willing to do.

Now, there will be 16.7 kA or 8.3 kA in scenario 1 and 2 taking the path along the AC power ground. It will also take the path of the LINE and NEUTRAL wires and a preferential path (rather than through the power supplies of the equipment) must be provided. A heavy duty MOV protected outlet box can be attached to the metal panel. The ground on it is bonded to the panel.


While the calculations are simplified they serve to illustrate different grounding schemes. A more complex simulation using SPICE that includes cable inductance and resistance yields similar results. And when a yet more complex SPICE simulation using resistive ground cubes is used voltage gradients along the ground can be explored.

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WX7G
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« Reply #25 on: July 19, 2012, 05:38:15 AM »

K1CJS you make a great point about sneak lightning paths. Your comments are helping us come up with practical solutions for ham grounding. I admit that the single point entry for ALL cables is not practical for many amateur installations.

With a commercial building designed from the start for lightning protection all of this can be taken care of. For a house you are right that it can be difficult or impossible. The mitigation in that case is to attempt to maintain the ground under the building at the same potential. To do that the method of placing a ground rod at each corner of the building connected with buried bare copper wire along the perimeter will help. This can significantly reduce current taking sneak paths.

Amateur stations don't seem to be set up for the single point entry ground. AC power enters through the front of the house and antennas are at the back of the house. I think that this non single point entry ground method may provide enough protection even for a direct strike:

1) Place several ground rods at the tower.
2) Use a grounded entry panel at the station wired as described earlier.
3) Using an RF switch short all equipment inputs when not in use.
4) Don't operate during a storm.
5) Place at least one ground rod at each corner of the building bonded together per the NEC with buried copper wire and connected to the AC service ground.

The only thing different about this method vs. the common entry panel method is that the AC power is routed up to the panel and the equipment AC is powered from this point. This breaks the ground loop that the usual method has and reduces the lightning current through the equipment to zero. I have used this method for sensitive measurements in industrial environments having hundreds of amps of audio return current along ground. The equipment is hung off a single ground point rather than becoming the path for the audio current. 

« Last Edit: July 19, 2012, 05:46:27 AM by WX7G » Logged
LA9XSA
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« Reply #26 on: July 19, 2012, 04:28:56 PM »

You can't ground every pipe, cable or wire that goes into a building, nor can you have an outside ground on every outside metallic fixture whether or not it is connected to power or other wiring in a building.
Well on commercial sites like hospitals or communications sites they do this. Every piece of metal on the roof of the building gets connected to lightning conduction wires that go to the external ground ring, until the whole roof looks like a web of wires. In addition, every piece of metal (grates, fans, etc) in equipment rooms get connected to the interior perimeter ground bus. The ground system is also connected to the structural steel of the building. This guarantees that the lowest impedance path is through the ground system, but damage could still occur if a strike overloads the ground system and branches into undesired paths, or that the strike inducts harmful currents in nearby wires.

Picture: A roof at St. Olavs Hospital in Trondheim, Norway. Each fixture has its own ground wire, and the roof is ringed with wire too.
http://i50.tinypic.com/14loux0.jpg

Not something for the average ham, of course. An important point to take from your post K1CJS is that lightning can arc through air - after all it passed hundreds of meters from the cloud already.

WX7G, insisting on a single entry point for AC and antennas can lead to bad solutions if you don't do it right. For example, if the antenna is situated on the other side of the house to the AC service entry, and you decide to run the coax to the AC service entry by hanging it on your exterior wall halfway around the house, a transient could arc through the wall to the house wiring or induce current in it. Having a separate AC and antenna entry, well bonded together, is preferable to a poor execution of a single entry.

Even if done right, running the extra coax could mean more losses that you don't want.

#6 AWG sounds a bit too small for an external ground ring, for bonding the AC and antenna ground. The Motorola R56 standard document (2005) calls for #2 AWG, or perhaps #1/0 AWG in lightning prone areas, with reference to ANSI T1.313-2003 and ANSI T1.334-2002.

I'm not an electrical or radio engineer - my degree is software oriented - but I've read enough about lightning protection to realize how tricky it is to run calculations on it. The idea that a strike will just equally divide itself among a number of ground rods without consideration to their physical distance or the impedance (resistance and reactance) of the connectors between them, or the ground resistance, still strikes me as overly simplistic.
« Last Edit: July 19, 2012, 04:35:30 PM by LA9XSA » Logged
WX7G
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« Reply #27 on: July 19, 2012, 05:14:20 PM »

LA9XSA you make some good points. I had considered the potential difference between the coax shield and the inside of the house. The potential difference is kept relatively low by the shield being tied to the house AC service ground and the slow di/dt of the lighting. Figuring a 15 meter length of coax (12 uH shield L) and a di/dt of 10 kA/us the potential difference between the ends of the coax is 120 kV, or enough to arc 2" though air. Concerning induced current in the house wiring remember that without the single point entry configuration the house wiring lightning current is 16 kA or 8 kA depending on if a shunt wire is used. The induced current due to the coax running along the house will be less than this. I can calculate this if you want. And when a lightning shunt wire is used it too will induce a current into the house wiring. I'll think about all of this some more.

I was analyzing the pseudo single point entry where the RF cables are brought in at one end of the house and the AC is brought in at the other. The analysis is to see what is the advantage of mounting the power outlets on the RF panel vs. not doing so.

With the AC outlets on the panel the lightning current along the coax (the current that bypasses the ground rod(s) at the entry point) takes the only other path, which is down a ground wire to the room AC outlet ground and though the house wiring to the AC service ground. No lightning current takes the path along the coaxial cables or the power cables to the station equipment. The coaxial cables have their shields bonded to the panel and the inner conductors bonded via RF shorting switches. No voltage is induced in the coaxial cables and the radio input will receive 0 volts. .

Now take the case where the the station equipment is connected to the room AC outlet(s) and the coaxial cables are connected to the panel. The lightning current path is along the coaxial cables to the station equipment, along the AC power cables and through the house wiring to the AC service ground. As shown in a previous post this might be 8 kA. Let's say the coax connecting the panel to the radio is a single RG-213 three meters long. The TI (transfer impedance) of RG-213 at lightning frequencies is about 0.004 ohms per meter. 8 kA X 0.004 ohms/m X 3 meters = 1280 volts. The radio will receive a 128 volt pulse having a waveform matching the lightning current.




  

« Last Edit: July 19, 2012, 05:19:09 PM by WX7G » Logged
WX7G
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« Reply #28 on: July 20, 2012, 05:53:27 AM »

LA9XSA, note that the coax that is routed along the building - from the AC service ground to the station - carries no lighting current and will not induce current into the house wiring. It is however at a potential relative to the ground under it and that difference can be calculated for purposes of insulation coordination.

-------------------------------------------------
So we have two distinct scenarios: Route the coax to the station and deal with the lightning current through the house AC wiring or route the coax to the AC service ground then to the station, have no lightning current through the house, and deal with the potential vs. ground on the coax.  

In the first scenario the lightning current through the house is the ratio of the house AC ground impedance divided by the antenna ground impedance X the lightning current. If each ground rod presents the same impedance the formula is the number AC ground rods divided by the number antenna ground rods X lighting current. Antenna ground rods include the rods outside the station coax entrance. To reduce house lightning current we are led to more antenna rods and fewer AC ground rods.

In the second scenario there is no lighting current through the house except for capacitive charging current. The lightning potential on the coax shield can be reduced by using more antenna ground rods. To reduce the coax shield potential during a strike we need to reduce the shield current. That leads us to more antenna rods and fewer AC ground rods.

Either scenario can be designed such that when lightning hits the antenna the house and station survive. Both scenarios point to more antenna ground rods and fewer AC ground rods.




« Last Edit: July 20, 2012, 05:57:05 AM by WX7G » Logged
WX7G
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« Reply #29 on: July 20, 2012, 04:07:19 PM »

The Lightning Rod Man by Herman Melville

http://www.melville.org/lrman.htm
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