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Single ground
In almost 45 years of ham operation I have never bothered with a "good"
ground connection. I am in what might be my last location and now have a "real" tower (55' crank up). I understand the principles behind a single ground, or at least I think I do. However, ideal requirements (as often quoted in various codes and standards manuals) need to be matched with what one can actually do. I have several ground rods by the tower and one by the entry to my basement shack. The tower-to-entry-point has two #10 wires in the trench (but not not in the conduit) that connect the nearest of the tower ground rods to the entry-point rod. The distance is about 45'. From the entry point area to the rig is about 10' and has a single #10 wire. Also from the rig area to the electrical panel is another #10 wire that is about 40'. Is this a reasonable arrangement? Is it better than nothing? Worse than nothing? I will shortly place two ICE units at the entry point on the two coax lines from the tower. I am still considering what to do with the control lines --- there are 12 for a SteppIR, 6 for a rotator, and 6 for a remote coax switch. Bill W2WO |
"Bill Ogden" wrote I have several ground rods by the tower and one by the entry to my basement shack. The tower-to-entry-point has two #10 wires in the trench (but not not in the conduit) that connect the nearest of the tower ground rods to the entry-point rod. The distance is about 45'. From the entry point area to the rig is about 10' and has a single #10 wire. Also from the rig area to the electrical panel is another #10 wire that is about 40'. Is this a reasonable arrangement? Is it better than nothing? Worse than nothing? Hi Bill, it's reasonable and better than most, providing the following is also true: The coax shields grounded at the tower (min. at the bottom, best top and bottom), and again at the basement entrance single point ground. Shields must be grounded before connection to an arrestor. I will shortly place two ICE units at the entry point on the two coax lines from the tower. I am still considering what to do with the control lines --- there are 12 for a SteppIR, 6 for a rotator, and 6 for a remote coax switch. Bill W2WO ICE also makes protection for control lines. I have no experience with them in particular, but my six ICE coax arrestors have handled a lot of surge from several strikes less than 100' away, two of which were within 50' of antennas. I would also add that the importance of the bonding between shack single point ground and AC service entry point ground is critical to prevent ground potential rise (GPR) from a nearby strike's energy from going through your equipment via your own ground connections. The path in from ground and out through the back of your equipment AC power cords will always exist, but with proper bonding it will not be a destructive connection. Most stations have this station-ground to mains-ground bonding conductor outside the station, but I don't see the harm of having it inside either *if* it was a very short distance [yours is NOT SHORT]. The shorter the route for this bond the better, whatever path it may take. NEC requires it be less than 20'. Fat chance you say. Alternatives are to provide multiple ground rods along a 20' path of this critical bond. That means change your bonding conductor to run outside, from the station single point ground (rod), via a couple additional rods, to the AC mains service entry ground (rod). 73, Jack Painter Virginia Beach VA |
Bill,
I would reroute the wire from your electrical panel directly to the ground rod just outside your entry. Then from there run a single line to your rig. This keeps the rig out of the middle of the ground connections. If possible your AC line for the rig should run from where that ground rod is. AC protectors should be put at that point along with the coax protectors. Now you have a single point ground system and the rig is not in the middle of things. All grounds are at one point including all your protectors. As far as the control wires, get some large MOV's and place one on each line to ground. Again at your single ground point where your ground rod is at the entrance. With several control lines any surge energy that comes down them will be shared by all. You can get by without gas tubes in this situation as all those MOV's are effectively in parallel. If you look inside a rotator protector like Polyphaser makes that is all you will find in them. The ground connection from your protection devices to your ground rod should be as short and as large as you can manage. 3" wide copper strap if the distance is a few feet. If it is a little longer run place two copper straps edge to edge in parallel or a 6" strap. It is important to have a low resistance / low inductance path to your ground system. In addition you may want to add more ground rods / radials to your entrance ground. Even just buried radials run out in different directions away from that single ground rod that you have will help lots. Placing an additional ground rod at the end of each is still better. The big thing is to run the radials in different directions to get some space between your ground rods. Ideally the distance between rods should be the sum of their lengths. The ground saturates when trying to dissipate energy in one place. Spreading it out increases the amount of energy you can dump in a given amount of time. 73 Gary K4FMX On Thu, 18 Nov 2004 22:51:40 GMT, "Bill Ogden" wrote: In almost 45 years of ham operation I have never bothered with a "good" ground connection. I am in what might be my last location and now have a "real" tower (55' crank up). I understand the principles behind a single ground, or at least I think I do. However, ideal requirements (as often quoted in various codes and standards manuals) need to be matched with what one can actually do. I have several ground rods by the tower and one by the entry to my basement shack. The tower-to-entry-point has two #10 wires in the trench (but not not in the conduit) that connect the nearest of the tower ground rods to the entry-point rod. The distance is about 45'. From the entry point area to the rig is about 10' and has a single #10 wire. Also from the rig area to the electrical panel is another #10 wire that is about 40'. Is this a reasonable arrangement? Is it better than nothing? Worse than nothing? I will shortly place two ICE units at the entry point on the two coax lines from the tower. I am still considering what to do with the control lines --- there are 12 for a SteppIR, 6 for a rotator, and 6 for a remote coax switch. Bill W2WO |
"Jack Painter" wrote in message news:zbbnd.1127$wa1.571@lakeread04... snip The coax shields grounded at the tower (min. at the bottom, best top and bottom), and again at the basement entrance single point ground. Shields must be grounded before connection to an arrestor. Jack - regarding your comment on grounding the shields BEFORE connection to an arrestor: My arrestors are mounted on the common ground panel and the coax is grounded via the coax connector to those arrestors. What is the reason for a separate ground prior to that one. Maybe I misunderstood something, but it seems redundant to have a separate ground a few inches from that one. In response to another's comments regarding protection of the SteppIR, rotor, and other control lines: I use MOVs and .01 bypass caps on all those lines in a box at the base of the tower and have another set of the same at the entry panel box. Those components are mounted via European-style screw terminal strips (12 positions per strip) obtained from Jameco via the Web. Much cheaper than the same from Radio Shack or other sources. MOVs came from Mouser. A lightning strike last year entered my shack via relay control lines which were unprotected at that time. Hopefully, the new arrangement will help. 73, Floyd - K8AC |
This was answered off-line but for the group:
"Floyd Sense" wrote "Jack Painter" wrote in message snip The coax shields grounded at the tower (min. at the bottom, best top and bottom), and again at the basement entrance single point ground. Shields must be grounded before connection to an arrestor. Jack - regarding your comment on grounding the shields BEFORE connection to an arrestor: My arrestors are mounted on the common ground panel and the coax is grounded via the coax connector to those arrestors. What is the reason for a separate ground prior to that one. Maybe I misunderstood something, but it seems redundant to have a separate ground a few inches from that one. Coax shield grounding must be accomplished at the tower top, tower base (on ground level, not even 6" above!) and before the arrestor to comply with protector manufacturer requirements and to be in keeping with best engineering practices that are used nationwide in communication tower designs. The grounding just before the arrestor is for two purposes: 1. preventing unnecessary energy (whether capacitively or inductively coupled) from challenging the gas tube, MOV, coil (or all three) mechanisms of a protector, and 2: to help limit the differing potential available to reverse-path voltage from a nearby strike in ground potential rise conditions. A nearby strike can saturate the ground system, and a station ground can reference hundreds of thousands of volts 'up' from the ground, and 'out' via arrestors, phone, power cables to lower potential felt at some distant point. Grounding cable shield at the station single point ground thus helps maintain equipotential for both directions. Even if the station coax arrestors are mounted on the master ground the additional grounding is still helpful for voltage-division during saturated ground conditions. But in all cases that ground bus mount must never be the only place the coax shelding is grounded! In response to another's comments regarding protection of the SteppIR, rotor, and other control lines: I use MOVs and .01 bypass caps on all those lines in a box at the base of the tower and have another set of the same at the entry panel box. Those components are mounted via European-style screw terminal strips (12 positions per strip) obtained from Jameco via the Web. Much cheaper than the same from Radio Shack or other sources. MOVs came from Mouser. A lightning strike last year entered my shack via relay control lines which were unprotected at that time. Hopefully, the new arrangement will help. 73, Floyd - K8AC Several amateurs have reported successful use of private design MOV on control lines. While this could exceed commercially available equipment specs in some cases, for those less familiar with such designs, they are readily available in package-form to protect control lines. Jack |
On Fri, 19 Nov 2004 07:44:50 -0500, "Floyd Sense"
wrote: "Jack Painter" wrote in message news:zbbnd.1127$wa1.571@lakeread04... snip The coax shields grounded at the tower (min. at the bottom, best top and bottom), and again at the basement entrance single point ground. Shields must be grounded before connection to an arrestor. Jack - regarding your comment on grounding the shields BEFORE connection to an arrestor: My arrestors are mounted on the common ground panel and the coax is grounded via the coax connector to those arrestors. What is the reason for a separate ground prior to that one. Maybe I misunderstood something, but it seems redundant to have a separate ground a few inches from that one. In response to another's comments regarding protection of the SteppIR, rotor, and other control lines: I use MOVs and .01 bypass caps on all those lines in a box at the base of the tower and have another set of the same at the entry panel box. Those components are mounted via European-style screw terminal strips (12 positions per strip) obtained from Jameco via the Web. Much cheaper than the same from Radio Shack or other sources. MOVs came from Mouser. A lightning strike last year entered my shack via relay control lines which were unprotected at that time. Hopefully, the new arrangement will help. 73, Floyd - K8AC Floyd, The real reason for grounding the coax shield at the entrance panel in addition to having the protection device grounded is the voltage drop between the connector and the coax line. The cable to connector connection is usually not a good low resistance - high current joint as far as lightning is considered. During a lightning strike you may have considerable voltage drop across that junction. Sometimes connectors are found to have been welded to the cable or their threads welded due to lightning strikes because of the poor connection at the connector. Other times the junction may get burned open. It can also melt the solder quickly in a soldered on connector which would provide for an immediately poor connection. However, lots of people do not do the extra grounding of the cable at that point. Most are lucky if they get some sort of protection device installed and a ground connected. As Jack mentioned grounding the cable "at the bottom of the tower like is used nation wide in tower designs" is ideal. But unfortunately that is not how it usually gets done. Often the lines come off the tower at 6 to 10 feet above the ground to go to the building in a cable tray. But it would indeed be best if they were taken all the way to ground level before exiting the tower. The reason being that during a strike the tower and associated lines on it develop considerable voltage drop due to the high current being conducted. Coming off the tower above ground is like taping a resistor part way up from the ground end. Allowing more voltage to exit on the lines rather than the potential at the base of the tower where it is closer to ground. The tower usually has considerable inductance for voltage to develop across. Ideally lines should be grounded to the tower not only at the top and bottom but at distances along the tower length as well. This is to avoid flashovers that may puncture the line. Lightning protection schemes are all about voltage drop. Most due to inductance of the tower or other conductors carrying the current. All conductors will have inductance and resistance and therefore voltage drop if they are asked to carry lightning current. Keeping things out of the middle of that path is the trick. 73 Gary K4FMX |
Gary Schafer wrote in message . ..
On Fri, 19 Nov 2004 07:44:50 -0500, "Floyd Sense" As Jack mentioned grounding the cable "at the bottom of the tower like is used nation wide in tower designs" is ideal. But unfortunately that is not how it usually gets done. Often the lines come off the tower at 6 to 10 feet above the ground to go to the building in a cable tray. But it would indeed be best if they were taken all the way to ground level before exiting the tower. The reason being that during a strike the tower and associated lines on it develop considerable voltage drop due to the high current being conducted. Coming off the tower above ground is like taping a resistor part way up from the ground end. Allowing more voltage to exit on the lines rather than the potential at the base of the tower where it is closer to ground. The tower usually has considerable inductance for voltage to develop across. I'm one of those who pulls the coax off the tower at around eight feet and hangs it on a carrier wire from the tower to the outside wall near the shack. In the past I've had end insulators at both ends of the carrier wire. Your point about grounding the coax at the base of the tower is well taken but is obviously not possible in these situations. It occurs to me that the same effect can be accomplished by connecting a #6 or #8 solid wire between the the coax shields where they bend away from the tower and the base of the tower. Yes? Taking it a bit further it also occurs to me that the carrier wire could be connected to the base of the tower at the point where the tower connects to the ground rods there, then up the tower and connected to both the coax shields at the eight foot level and the tower again. Then horizontally to the house wall with the coax, then down to the ground rods just outside the shack to which the equipment is also grounded. I'd also connect the coax shields to the carrier wire again at the point where they turn away from the wire and go through the wall. One hefty continuous, unbroken length of copper wire. There would still be voltage differentials involved because there is no escape from the inductances BUT . . . is my thinking in the right direction here? 73 Gary K4FMX w3rv |
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Gary Schafer wrote in message . ..
On 20 Nov 2004 17:59:12 -0800, (Brian Kelly) wrote: .. . . I'm one of those who pulls the coax off the tower at around eight feet and hangs it on a carrier wire from the tower to the outside wall near the shack. Theref are many installations like yours in existance. It was the "common way" to do it some years ago. Not the best though. In the past I've had end insulators at both ends of the carrier wire. Your point about grounding the coax at the base of the tower is well taken but is obviously not possible in these situations. It occurs to me that the same effect can be accomplished by connecting a #6 or #8 solid wire between the the coax shields where they bend away from the tower and the base of the tower. Yes? No that won't do much good. If you ground the coax shield to the tower where it bends away from the tower you will have a much better (lower inductance) to ground with the tower than what the wire would provide. The wire would do almost no good at all when compared to the much larger tower in parallel. Got it. Taking it a bit further it also occurs to me that the carrier wire could be connected to the base of the tower at the point where the tower connects to the ground rods there, then up the tower and connected to both the coax shields at the eight foot level and the tower again. Same as above. Grounding the carrier to the tower will do much more than a wire to the ground rods at the tower. The carrier wire should not be insulated from the tower. It and the coax should both be grounded to the tower at the exit point. Otherwise you can have flashover's to the carrier. OK, cancel useless wire from base of tower. Then horizontally to the house wall with the coax, then down to the ground rods just outside the shack to which the equipment is also grounded. I'd also connect the coax shields to the carrier wire again at the point where they turn away from the wire and go through the wall. One hefty continuous, unbroken length of copper wire. There would still be voltage differentials involved because there is no escape from the inductances BUT . . . is my thinking in the right direction here? Connecting the carrier wire to the coax again at the house is a good idea for the same reason you should connect it at the tower. to prevent flashovers to the cables. The same situation exist on the tower itself with lines running down. That is why they should be grounded to the tower at several points. Especially on a tall tower. OK again. The tower has inductance just like any piece of wire has. Although the tower inductance is less than just a length of wire it still has inductance. When lightning strikes the top, the tower and lines all share the current to ground. The farther up from ground you are the higher the voltage will be with respect to ground. I got that from your prior post. It can be significant. Especially on a smaller tower. It took a few seconds to get your point but yes, it's a matter of how far up the tower the coax departs the tower as a percentage of the tower height. Since I'm planning a short (35-40 foot tubular crankup) tower I'll have both a "high inductance tower" and a high pulloff level in terms of percentage. Not good no matter how one looks at it. Leaving the tower only a few feet above ground with your coax line is putting that line at some point above ground that can have high voltage. The best way is to run the lines all the way to the bottom of the tower, ground them there, and then run underground to the house to your ground rods. Don't forget to also run a ground lead from your house ground to your tower ground system too. That's a given. Bury it along with the cables. That will give you more contact with the earth as well as tying the grounds together. The wire will be there but I doubt that I'll be able to bury it. The whole (small) property is part of a forest of huge old hardwoods several of which are crowded close to the house particularly along the wall thru which I need to feed the coax. You'd have to see it to believe it and it's only six miles from City Hall Philadelphia. Digging a trench is not possible thru the tangle of roots on any approach from the tower to the wall. I'm not looking forward to driving ground rods thru this maze of underground lumber but I'll do it even if it takes some serious power drilling to accomplish. What I could do is run all the cables to the bottom of the tower with shield bonds at the top of the moving section, another one at the top of the fixed section, another bond halfway down fixed section and the last one at the bottom of the tower. Which will also be surrounded by trees. There's a hole below the canopy big enough to accomodate a Hexbeam or some similar very compact HF antenna if I spot the tower correctly. Some contractor is going to have a really bad time digging the hole for tower base. From the base of the tower I'll run the cables and the carrier wire horizontally on the surface for a few feet then back up to the eight foot level to a tree trunk. Six feet would also work and the rest of the run would be per previous. The good news is that the soil is eternally damp highly conductive dark loam . . Gary K4FMX Thanks Gary. |
"Brian Kelly" wrote Gary Schafer wrote No that won't do much good. If you ground the coax shield to the tower where it bends away from the tower you will have a much better (lower inductance) to ground with the tower than what the wire would provide. The wire would do almost no good at all when compared to the much larger tower in parallel. This is correct, and why I mentioned even 6" was "too much". It can be significant. Especially on a smaller tower. It took a few seconds to get your point but yes, it's a matter of how far up the tower the coax departs the tower as a percentage of the tower height. The last was not a correct assumption. The distance across a conductor (and in this case it is also the distance to ground) is what allows inductance to create deadly voltage potentials. Any conductor in series with a lightning strike will exhibit the same characteristics. 6" above ground near the base of a tower can translate to as much as 9800v above ground, with just modest assumptions of a very average return stroke current of 25Ka with a rise time of 40Ka/usec. It has no bearing whatsoever how tall or short the tower is. It's not long (or high above ground) before you could see over 100,000v potential develop where coax leaves any tower too soon. Bury it along with the cables. That will give you more contact with the earth as well as tying the grounds together. The wire will be there but I doubt that I'll be able to bury it. Burying a grounding electrode conductor is normally a code requirement. But that is not what you have in connecting the tower ground system to the station ground, AC mains ground, etc. Those are bonding conductors, and they are in many cases required to be insulated. Not in this case, but I want you to understand the difference between grounding, voltage division from many grounds, and a bonding conductor between your station and the tower. The latter is to maintain equipotential, and will not carry more than just equalizing currents. It will be well within the capability of a #6 insulated wire, should you choose to use that. Personally I would go a little larger but #6 is as largest that NEC or NFPA recommend for a bond in *most* cases. So burying the bonding conductor is not a requirement, although to protect it that is exactly what most facilities do. Neither will burying coax feedlines help in lightning protection, unless you are counting on them by design to be grounding electrode conductors! Pretty foolish but heh, if someone tosses feedlines out a window, they may as well short them to a ground rod and "bring it on". In that case any more than about 5,000v will breakdown the dialectric both inside and outside the coax, and anything nearby may be the next target before it ever reaches the ground rod. The good news is that the soil is eternally damp highly conductive dark loam . . Gary K4FMX That is very good news, and it makes your job easier. But good soil or poor soil, understanding what bonding provides is equally if not more important than having a ground rod at all. To rest on the laurels of highly conductive soil and ignore bonding, would be inviting disaster. Yes commercial tower design does require many shield "bonds" along the height of towers, but as I said, I applied a reasonable approach which the average short tower or mast-only owner could and would be likely to comply with - bonding at the top, bottom and station entrance. I suspect few go even that far. You may or may not be interested in all the surge protection diatribe in my website, but it's there because so many unfortunate souls were mislead in this area. I do think you might benefit from it's coverage of what bonding does to protect both you and your station, and it is a lot harder for most to get a hold of then simple mast or tower grounding. It doesn't have to be. http://members.cox.net/pc-usa/station/ground0.htm 73, Jack Painter Virginia Beach, VA |
On Sun, 21 Nov 2004 23:06:45 -0500, "Jack Painter"
wrote: "Brian Kelly" wrote Gary Schafer wrote No that won't do much good. If you ground the coax shield to the tower where it bends away from the tower you will have a much better (lower inductance) to ground with the tower than what the wire would provide. The wire would do almost no good at all when compared to the much larger tower in parallel. This is correct, and why I mentioned even 6" was "too much". It can be significant. Especially on a smaller tower. It took a few seconds to get your point but yes, it's a matter of how far up the tower the coax departs the tower as a percentage of the tower height. The last was not a correct assumption. The distance across a conductor (and in this case it is also the distance to ground) is what allows inductance to create deadly voltage potentials. Any conductor in series with a lightning strike will exhibit the same characteristics. 6" above ground near the base of a tower can translate to as much as 9800v above ground, with just modest assumptions of a very average return stroke current of 25Ka with a rise time of 40Ka/usec. It has no bearing whatsoever how tall or short the tower is. It's not long (or high above ground) before you could see over 100,000v potential develop where coax leaves any tower too soon. A little clarification here. When I said "smaller tower" I was not necessarily referring to a shorter tower but one that has less surface area. (smaller face) The main consideration is the distance up from ground that the cables leave the tower. A lightning strike is a constant current source. If it is a 20ka strike the voltage across whatever it hits is going to raise high enough to conduct 20ka. If you have a low impedance conductor (tower) the voltage developed across it will be less than it would be on a high impedance tower (smaller face tower). That is why large communication towers have less problems with lines coming off at higher points on the tower. More of the strike current makes it to ground via the tower with the larger surface it has. Leaving the tower at some height above ground with the cables is still a division of the voltage like a voltage divider. The higher up you are the higher the voltage you will see with respect to ground. But what determines what that actual voltage goes to is the amount of strike current and the amount of inductance between the cable exit point and ground. Of course the cables leaving the tower will also carry part of the current too. Bury it along with the cables. That will give you more contact with the earth as well as tying the grounds together. The wire will be there but I doubt that I'll be able to bury it. Burying a grounding electrode conductor is normally a code requirement. But that is not what you have in connecting the tower ground system to the station ground, AC mains ground, etc. Those are bonding conductors, and they are in many cases required to be insulated. Not in this case, but I want you to understand the difference between grounding, voltage division from many grounds, and a bonding conductor between your station and the tower. The latter is to maintain equipotential, and will not carry more than just equalizing currents. It will be well within the capability of a #6 insulated wire, should you choose to use that. Personally I would go a little larger but #6 is as largest that NEC or NFPA recommend for a bond in *most* cases. So burying the bonding conductor is not a requirement, although to protect it that is exactly what most facilities do. Neither will burying coax feedlines help in lightning protection, unless you are counting on them by design to be grounding electrode conductors! Pretty foolish but heh, if someone tosses feedlines out a window, they may as well short them to a ground rod and "bring it on". In that case any more than about 5,000v will breakdown the dialectric both inside and outside the coax, and anything nearby may be the next target before it ever reaches the ground rod. It doesn't matter what you want to call a ground conductor. The point here is if it can carry any lightning current you are much better off with it buried in the ground. A bare ground conductor making contact with the soil acts like additional ground rods. Why would you not want that? Burying coax feed lines will help with lightning protection. It greatly increases the inductance of the lines to lightning. It also helps to dissipate the energy to ground by the coupling provided. (ie you get less at the other end) You can't help but view them as "grounding electrode conductors" as you may want to call them. After all they are connected to the tower. They are going to carry lightning current if you want them to or not. Might as well let them dissipate part of the energy to earth. A large part of the lightning is RF. You have to treat it as such. A good lightning ground also makes a very good antenna ground system. ( buried radial system) Think in those terms. 73 Gary K4FMX The good news is that the soil is eternally damp highly conductive dark loam . . Gary K4FMX That is very good news, and it makes your job easier. But good soil or poor soil, understanding what bonding provides is equally if not more important than having a ground rod at all. To rest on the laurels of highly conductive soil and ignore bonding, would be inviting disaster. Yes commercial tower design does require many shield "bonds" along the height of towers, but as I said, I applied a reasonable approach which the average short tower or mast-only owner could and would be likely to comply with - bonding at the top, bottom and station entrance. I suspect few go even that far. You may or may not be interested in all the surge protection diatribe in my website, but it's there because so many unfortunate souls were mislead in this area. I do think you might benefit from it's coverage of what bonding does to protect both you and your station, and it is a lot harder for most to get a hold of then simple mast or tower grounding. It doesn't have to be. http://members.cox.net/pc-usa/station/ground0.htm 73, Jack Painter Virginia Beach, VA |
"Gary Schafer" wrote
It doesn't matter what you want to call a ground conductor. The point here is if it can carry any lightning current you are much better off with it buried in the ground. A bare ground conductor making contact with the soil acts like additional ground rods. Why would you not want that? Burying coax feed lines will help with lightning protection. It greatly increases the inductance of the lines to lightning. It also helps to dissipate the energy to ground by the coupling provided. (ie you get less at the other end) You can't help but view them as "grounding electrode conductors" as you may want to call them. After all they are connected to the tower. They are going to carry lightning current if you want them to or not. Might as well let them dissipate part of the energy to earth. A large part of the lightning is RF. You have to treat it as such. A good lightning ground also makes a very good antenna ground system. ( buried radial system) Think in those terms. 73 Gary K4FMX Hi Gary, the coax feedlines are definitely NOT grounding electrode conductors. Not only are they incapable of such by design and accordingly not authorized as grounding conductors, but they could never remain connected to sensitive equipment if it were so. Neither is the shielding on coax sufficient to provide equipotential bonding so they are not allowable bonding conductors either. If anyone wants to sacrifice their coax by not properly shield-grounding and installing the appropriate number of coax lightning arrestors (this means on the tower also) then they will turn them into very ineffective grounding conductors. Burying might help then, but only because you could guarantee a breakdown in the dialectric and where safer to have that happen than underground. I understand many operators allow this and simply toss them out the window, or ground them before a storm, but there is no good reason for it. Proper installation can allow them to remain connected to the equipment without sacrificing the coax or the equipment. Burying coax does not prevent induction by either capacitive or magnetic induction onto the shields of the coax from a nearby strike. If coax were enclosed in metal conduit that was grounded at each end, there would be protection from this. But proper installation of shield grounding and surge suppression at both ends maintains safe levels of energy on the feedline and allows its connection to sensitive equipment.Of course in rare cases there is sufficient energy (such as a 200Ka+ return stroke current) to overcome any level of protection. But protected stations will certainly fare a lot better in those rare events than the unprotected ones. 73, Jack Painter Virginia Beach VA |
On Mon, 22 Nov 2004 19:26:16 -0500, "Jack Painter"
wrote: "Gary Schafer" wrote It doesn't matter what you want to call a ground conductor. The point here is if it can carry any lightning current you are much better off with it buried in the ground. A bare ground conductor making contact with the soil acts like additional ground rods. Why would you not want that? Burying coax feed lines will help with lightning protection. It greatly increases the inductance of the lines to lightning. It also helps to dissipate the energy to ground by the coupling provided. (ie you get less at the other end) You can't help but view them as "grounding electrode conductors" as you may want to call them. After all they are connected to the tower. They are going to carry lightning current if you want them to or not. Might as well let them dissipate part of the energy to earth. A large part of the lightning is RF. You have to treat it as such. A good lightning ground also makes a very good antenna ground system. ( buried radial system) Think in those terms. 73 Gary K4FMX Hi Gary, the coax feedlines are definitely NOT grounding electrode conductors. Not only are they incapable of such by design and accordingly not authorized as grounding conductors, but they could never remain connected to sensitive equipment if it were so. Neither is the shielding on coax sufficient to provide equipotential bonding so they are not allowable bonding conductors either. If anyone wants to sacrifice their coax by not properly shield-grounding and installing the appropriate number of coax lightning arrestors (this means on the tower also) then they will turn them into very ineffective grounding conductors. Burying might help then, but only because you could guarantee a breakdown in the dialectric and where safer to have that happen than underground. I understand many operators allow this and simply toss them out the window, or ground them before a storm, but there is no good reason for it. Proper installation can allow them to remain connected to the equipment without sacrificing the coax or the equipment. Burying coax does not prevent induction by either capacitive or magnetic induction onto the shields of the coax from a nearby strike. If coax were enclosed in metal conduit that was grounded at each end, there would be protection from this. But proper installation of shield grounding and surge suppression at both ends maintains safe levels of energy on the feedline and allows its connection to sensitive equipment.Of course in rare cases there is sufficient energy (such as a 200Ka+ return stroke current) to overcome any level of protection. But protected stations will certainly fare a lot better in those rare events than the unprotected ones. 73, Jack Painter Virginia Beach VA What isolates the shield of the coax from carrying current? As long as it is connected to the tower at one end it is going to have strike current on it whether you want it there or not. Nothing you can do about it. Paralleling other conductors will reduce it's total current but you still have to deal with it on the coax line. If you don't want to call the coax shield a grounding conductor that's ok but that won't stop the current on it. Who told you that you should put lightning protectors at the tower as well as at the building entrance? What good do you think they do at the tower other than cost more money? If you use a radial system for a ground at the tower or several ground rods, the coax run under ground can do the same thing as a radial as far as dissipating part of the energy. Having a buried radial rather than one run in the air lets the ground soak up a lot more energy if it is buried. There will be much less energy at the far end of a buried radial than one run in the air. A radial run in the air will dissipate little energy to the ground. With buried coax the ground acts like a large choke on the cable also. Exactly what you want. The ground increases the cables natural inductance. This is the same reason that long radials are not as effective as more shorter ones in dissipating lightning energy. The inductance of the long wire gets too high and becomes less effective as a conductor. If you don't think that buried cables helps reduce lightning energy at the other end try running a single insulated feed wire for your long wire antenna underground. See how much attenuation it provides to the signals. Burying the coax does the same thing for part of the lightning energy. 73 Gary K4FMX |
"Gary Schafer" wrote
What isolates the shield of the coax from carrying current? As long as it is connected to the tower at one end it is going to have strike current on it whether you want it there or not. Nothing you can do about it. Paralleling other conductors will reduce it's total current but you still have to deal with it on the coax line. Incorrect. First, a strike termination device is placed higher than other equipment with its own down conductor. Then a lightning arrestor and shield bonding are specified at the top of the tower, shield bonding along the path (up to three times) and at the bottom, then more shield grounding and another lightning arrestor at the facility entrance. If you don't want to call the coax shield a grounding conductor that's ok but that won't stop the current on it. Current is maintained at a safe level on the coax center conductor and shielding by the above. Who told you that you should put lightning protectors at the tower as well as at the building entrance? What good do you think they do at the tower other than cost more money? National telecommunication companies who specify them in white papers and engineering plans for lightning protection. I have been studying these systems for 18 months now and find this procedure consistently applied. The specific information is proprietary but all I had to do was ask for it. I found the information available via the USAF and other agencies I normally deal with was somewhat old, so I started asking commercial companies what they currently use, and could I have copies of their plans. That's where this information comes from. That and the National Electrical Code and National Fire Protection Association, October 2004 editions. Studying the NEC 250 grounding and bonding and the NFPA-780 offers more information to safely operate communication sequipment, especially during thunderstorms, than all the amatuer radio operators advice put together. Most of the amatuers giving this advice have no personal understanding of why or how this works, they just repeat stories or instructions they heard from someone else. Probably the biggest collection of dangerous information ever shared is what hams offer about lightning protection. Even the ARRL which makes an incredible effort to educate at the issue, has information so old in many cases it has not been used in best available practice for over ten years. With buried coax the ground acts like a large choke on the cable also. Exactly what you want. The ground increases the cables natural inductance. /clipped Your mistaken on this stuff Gary, we either shed or prevent lightning energy from coax by shield grounding, surge protection devices and sometimes encasement in grounded conduit. No plan or specification calls for earth-burying coax to deliver what you promise, and I believe your theory is electrically impossible, unless as I said over and over, the dialectric breakdown occurs, which means the installation was improper in the first place, or overcome by statistically rare events. 73, Jack Painter Virginia Beach VA |
On Mon, 22 Nov 2004 22:42:20 -0500, "Jack Painter"
wrote: "Gary Schafer" wrote What isolates the shield of the coax from carrying current? As long as it is connected to the tower at one end it is going to have strike current on it whether you want it there or not. Nothing you can do about it. Paralleling other conductors will reduce it's total current but you still have to deal with it on the coax line. Incorrect. First, a strike termination device is placed higher than other equipment with its own down conductor. Then a lightning arrestor and shield bonding are specified at the top of the tower, shield bonding along the path (up to three times) and at the bottom, then more shield grounding and another lightning arrestor at the facility entrance. If you don't want to call the coax shield a grounding conductor that's ok but that won't stop the current on it. Current is maintained at a safe level on the coax center conductor and shielding by the above. Who told you that you should put lightning protectors at the tower as well as at the building entrance? What good do you think they do at the tower other than cost more money? National telecommunication companies who specify them in white papers and engineering plans for lightning protection. I have been studying these systems for 18 months now and find this procedure consistently applied. The specific information is proprietary but all I had to do was ask for it. I found the information available via the USAF and other agencies I normally deal with was somewhat old, so I started asking commercial companies what they currently use, and could I have copies of their plans. That's where this information comes from. That and the National Electrical Code and National Fire Protection Association, October 2004 editions. Studying the NEC 250 grounding and bonding and the NFPA-780 offers more information to safely operate communication sequipment, especially during thunderstorms, than all the amatuer radio operators advice put together. Most of the amatuers giving this advice have no personal understanding of why or how this works, they just repeat stories or instructions they heard from someone else. Probably the biggest collection of dangerous information ever shared is what hams offer about lightning protection. Even the ARRL which makes an incredible effort to educate at the issue, has information so old in many cases it has not been used in best available practice for over ten years. With buried coax the ground acts like a large choke on the cable also. Exactly what you want. The ground increases the cables natural inductance. /clipped Your mistaken on this stuff Gary, we either shed or prevent lightning energy from coax by shield grounding, surge protection devices and sometimes encasement in grounded conduit. No plan or specification calls for earth-burying coax to deliver what you promise, and I believe your theory is electrically impossible, unless as I said over and over, the dialectric breakdown occurs, which means the installation was improper in the first place, or overcome by statistically rare events. 73, Jack Painter Virginia Beach VA Jack, I don't think you really understand all you are reading. It sounds like you are digging up stuff designed to sell a lot of protection devices. As far as the NEC requiring grounding and bonding of structures, They want to be sure that there is a continuous bond on things they are concerned with. They don't always consider what they are grounding. Do you really think that placing a down conductor of #6 wire on a tower with a 6 or 8 foot face (big tower) is going to make any difference in the impedance path that the lightning is going to see. Each leg may be 2 or 3" in diameter itself. The impedance of the tower will be so much lower than that wire. The lightning won't know the difference whether that wire is there or not. Even if the added down conductor did carry a large part of the current it would get coupled to the tower anyway before it reached the bottom. That is what happens to the coax lines in reverse. Any energy that the tower is carrying is coupled to the coax lines whether they are grounded to the tower or not. You ground them at multiple points to prevent flashovers between the lines and the tower. You can not keep the lightning energy off the coax lines or any other lines coming down the tower. They are all mutual. When those lines leave the tower at the bottom they are going to have some energy on them unless you have a perfect ground at the bottom of the tower. A grounded antenna will keep voltage levels on the center conductor at a safe level. No need for a protection device at the top. I agree that there is a lot of mis-information on lightning floating around. But don't cut all the hams short either. Some of them have lots of experience in this area. Think about what you read rather than taking in mounds of propaganda and repeating it. Do you think that you can bury the feed wire for your long wire antenna and have it work very well? What do you think will happen to the RF on it? Will it make it all the way back to your receiver the same as it would if it were above ground? 73 Gary K4FMX |
Just a few thoughts on the subject. Lightning conductors are most accurately modelled as DC to HF transmission lines - which indeed is what they are. In addition to resistance they posses inductance, capacitance, a Zo and propagation constants depending on length and diameter. It takes time for a stroke to propagate down and along a set of conductors. It arrives at different times at different places in the system. The generator is a high impedance, pulsed current source of so many thousands of amps. The voltage developed between a conductor, another conductor, and what's in its environment is Zo times the stroke current. Volts can leap across gaps. Once in the ground current travels at a much slower velocity than along a wire. Voltages developed depend on arrival times at different places. A ground rod is a short length of line. Frequencies of 100's of kilohertz are involved. Even reflected volts and currents occur. Ground conductivity can be allowed for. Very crude approximations are involved. Nevertheless, any information about behaviour DURING a strike is better than none. It may be a matter of life and death. It would be interesting to calculate, for a given strike current, the difference in voltage between the front and rear legs of a cow standing near to and facing a grounded antenna mast. Radio hams, presumably endowed with more common sense, can always wear rubber boots while walking around their backyards carrying a field-strength meter in thunder storms. ---- Reg, G4FGQ |
Just another thought -
If a resourceful ham has no rubber boots he can always stand on one leg and hop. --- Reg |
Gary,
As far as the NEC requiring grounding and bonding of structures, They want to be sure that there is a continuous bond on things they are concerned with. They don't always consider what they are grounding. Do you really think that placing a down conductor of #6 wire on a tower with a 6 or 8 foot face (big tower) is going to make any difference in the impedance path that the lightning is going to see. I never suggested such a silly thing! You mistake an earlier reference I made to a bonding conductor. Down conductors are sized according to code standards which provides a minimum for given elevation categories. 3/0 wire which is close to 1/2" in dia is the largest reqirement in NEC sizing table 250.66. In telecommunication applications, down conductors are normally sized to equal or exceed the size of the feed lines, and this means larger sizing yet. It's bonding conductors that burying usually serves no purpose except protection. One bond that is an exception is the station ground to utility entrance ground - that bond will carry ground potential rise current that hopefully bypasses the power connection at the back of station equipment. It must remain low impedance and high current capability, so additional ground rods are required along that bond if farther than 20'. Don't sell the NEC short. NFPA, which oversees the National Electrical Code, doesn't specify maximum protection, it specifies minimum protection standards. Industry does better where it sees cost benefit from doing so. Even if the added down conductor did carry a large part of the current it would get coupled to the tower anyway before it reached the bottom. That is what happens to the coax lines in reverse. Any energy that the tower is carrying is coupled to the coax lines whether they are grounded to the tower or not. You ground them at multiple points to prevent flashovers between the lines and the tower. Actually this is bonding Gary, an important distinction to understand. A device is only grounded at a grounding electrode. All other connections are bonding for equipotential, to minimize voltage differences. The principles of bonding are not taught in the amateur radio or any other communications hobby. Only electrical enginners, electricians, and anyone who studies the electrical code and reference materials on bonding and grounding will understand this. It's hard to even communicate about lightning protection until the basics of protection by equipotential are understood, and this doesn't come from a casual read or anything learned in ham-world postings. At least not from what I've seen, which is about everything a search engine can find. You can not keep the lightning energy off the coax lines or any other lines coming down the tower. They are all mutual. When those lines leave the tower at the bottom they are going to have some energy on them unless you have a perfect ground at the bottom of the tower. A grounded antenna will keep voltage levels on the center conductor at a safe level. No need for a protection device at the top. I didn't write the specifications that major companies are using, and neither did the companies selling lightning protection, although I agree there is influence there. Professional Engineers write these to protect equipment, personnel, and maintain operations, maybe not in that order. If they specify arrestors at tower tops, they must be trying to avoid damage that was not protected without them. It is no guarantee that damage is eliminated by their presence, I'm just relaying their usage - now. I agree that there is a lot of mis-information on lightning floating around. But don't cut all the hams short either. Some of them have lots of experience in this area. I don't cut all hams short. Richard Harrison (resident guru here on RRA) and others are thanked on my website for great information, their experiences, and helpful answers to questions. Many hams provided me with details about damage to equipment, and their humble honesty cannot be thanked enough. I have written many the best of the amateur websites discussing grounding and offered suggestions and references to improve their quality . Here are a few excerpts you or others may have missed from the NEC: Article 810.51 of the current NFPA 70, which states in part... "810.58 Grounding Conductors - Amateur Transmitting and Receiving Stations. Grounding conductors shall comply with 810.58(A) through (C). (A) Other Sections. All grounding conductors for amateur transmitting and receiving stations shall comply with 810.21(A) through (J). (B) Size of Protective Grounding Conductor. The protective grounding conductor for transmitting stations shall be as large as the lead-in but not smaller than 10 AWG copper, bronze, or copper-clad steel. (C) Size of Operating Grounding Conductor. The operating grounding conductor for transmitting stations shall not be less than 14 AWG copper or its equivalent." and Article 810.15, which states "810.15 Grounding. "Masts and metal structures supporting antennas shall be grounded in accordance with 810.21." finally, 810.21, which states "810.21 Grounding Conductors - Receiving Stations. Grounding conductors shall comply with 810.21(A) through (J). (A) Material. The grounding conductor shall be of copper, aluminum, copper-clad steel, bronze, or similar corrosion-resistant material. Aluminum or copper-clad aluminum grounding conductors shall not be used where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum shall not be installed within 450 mm (18 in.) of the earth. (B) Insulation. Insulation on grounding conductors shall not be required. (C) Supports. The grounding conductors shall be securely fastened in place and shall be permitted to be directly attached to the surface wired over without the use of insulating supports. Exception: Where proper support cannot be provided, the size of the grounding conductors shall be increased proportionately. (D) Mechanical Protection. The grounding conductor shall be protected where exposed to physical damage, or the size of the grounding conductors shall be increased proportionately to compensate for the lack of protection. Where the grounding conductor is run in a metal raceway, both ends of the raceway shall be bonded to the grounding conductor or to the same terminal or electrode to which the grounding conductor is connected. (E) Run in Straight Line. The grounding conductor for an antenna mast or antenna discharge unit shall be run in as straight a line as practicable from the mast or discharge unit to the grounding electrode. /clipped 73, Jack Painter Virginia Beach VA |
Here is a quote from your previous post where the discussion was about the tower and lines carrying strike current. Jack Painter First, a strike termination device is placed higher than other equipment with its own down conductor. Your reply when questioned about placing a down conductor in parallel with the tower was: I never suggested such a silly thing! You mistake an earlier reference I made to a bonding conductor. As far as down conductors go you need to read farther to see what they are using them for. We are talking about towers here. No down conductors needed. If you are talking about a building or wooden pole mounted antenna then that's a different story. The applications of grounding and bonding principles are not reserved for an elite society of engineers and electricians as you might like to think. They are free and available to all. There are no secrets involved. Quoting a load of authoritative directives is not a substitute for understanding. It is a play of semantics when you say " an important distinction to understand the difference between bonding and grounding". I would guess that it does not take too much study to understand that real ground can not be half way up a tower. So whether you are "bonding" or "grounding" a cable on a tower, the end result is unmistakably the same. It seems that some of the discontinuity may come from lack of basic understanding of RF principles. Lightning is not just a direct current event that requires only consideration for high currents. As far as buried cables go: I will ask the same question again that you avoided from last time but instead provided mounds of quotes that do not address the point. "Do you think that you can bury the feed wire for your long wire antenna and have it work very well? What do you think will happen to the RF on it? Will it make it all the way back to your receiver the same as it would if it were above ground?" This is a very relevant to "buried coax lines". 73 Gary K4FMX |
On Tue, 23 Nov 2004 11:27:03 +0000 (UTC), "Reg Edwards"
wrote: Just another thought - If a resourceful ham has no rubber boots he can always stand on one leg and hop. --- Reg Hams seem to be resourceful don't they! :) 73 Gary K4FMX |
"Gary Schafer" wrote Here is a quote from your previous post where the discussion was about the tower and lines carrying strike current. Jack Painter First, a strike termination device is placed higher than other equipment with its own down conductor. Your reply when questioned about placing a down conductor in parallel with the tower was: I never suggested such a silly thing! You mistake an earlier reference I made to a bonding conductor. You're displaying dimensia Gary. You accused me of suggesting a #6 wire would be useful as a down conductor and I never said such a ridiculous thing. You misread the posts, or still don't understand most of the terminology that the entire electrical, fire protection, lightning protection and communication industry use to refer to bonding and grounding components. in news:qBdod.15535$D26.3848@lakeread03... I said, and I quote: "Burying a grounding electrode conductor is normally a code requirement. But that is not what you have in connecting the tower ground system to the station ground, AC mains ground, etc. Those are bonding conductors, and they are in many cases required to be insulated. Not in this case, but I want you to understand the difference between grounding, voltage division from many grounds, and a bonding conductor between your station and the tower. The latter is to maintain equipotential, and will not carry more than just equalizing currents. It will be well within the capability of a #6 insulated wire, should you choose to use that." I'm not being condescending, if you're still confused there please say so, and I will try to explain it better. As far as down conductors go you need to read farther to see what they are using them for. We are talking about towers here. No down conductors needed. If you are talking about a building or wooden pole mounted antenna then that's a different story. Its clear to me who needs to do the reading here. Your last shot was also wide of the mark regarding down conductors on towers. They are used on communication towers in exactly the same fashion as they are on any structure, to provide a dedicated path to a grounding electrode for the charge received by a strike termination device. A tower down conductor is bonded to the tower frames in many places, that's the same as a down conductor on any structure is bonded to metal stairways, handrails, roof flashing, etc. on its way to ground. The applications of grounding and bonding principles are not reserved for an elite society of engineers and electricians as you might like to think. They are free and available to all. There are no secrets involved. Please don't take my comments so far out of context. But you're right it is reserved, its reserved for those who pay the fees and time to subscribe to organizations that license the printing of the codes, and constantly discuss and explain changes, applications and plan future requirement for them. Quoting a load of authoritative directives is not a substitute for understanding. It is a play of semantics when you say " an important distinction to understand the difference between bonding and grounding". I would guess that it does not take too much study to understand that real ground can not be half way up a tower. So whether you are "bonding" or "grounding" a cable on a tower, the end result is unmistakably the same. Neither should it be as oversimplified as your rebuttal. They are not the same thing and to misunderstand it would be to make deadly mistakes when applying those principles, both outside and inside structures where external equipments connect to people. It seems that some of the discontinuity may come from lack of basic understanding of RF principles. Lightning is not just a direct current event that requires only consideration for high currents. This is a subterfuge to divert the attention from the basics we were discussing, and until that is resolved there is no room for discussion about protection design based on frequencies of structures and wiring in the near field, or the AC components of lightning. As far as buried cables go: I will ask the same question again that you avoided from last time but instead provided mounds of quotes that do not address the point. "Do you think that you can bury the feed wire for your long wire antenna and have it work very well? What do you think will happen to the RF on it? Will it make it all the way back to your receiver the same as it would if it were above ground?" This is a very relevant to "buried coax lines". I ignored that question because I thought it had no relevance to anything here. Maybe you can rephrase what you are asking please? We are talking about shielded coax, and the relationship between lightning and the thin outer covering of coax, the coax shield, the inside dialectric, and the center conductor are quite unique and not convertible to a relationship with bare wire feed of some long wire. My coax feedlines are buried - so I don't hit them with the lawn mower! But it doesn't mean anything to lightning to have your coax buried, unless you hire a trencher to sink them deeper than the ground rods. In a near field strike there will be massive and sufficient energy to make all that shield grounding, bonding and placement of arrestors real important. And it won't matter where the coax is if all of those requirements are not met. if you think your coax is protected under your lawn, lay some turf over your radios and protect them the same way. It's a wives tale Gary, just like so many RF-wives tales, only there are more capable folks here to dispel those. 73, Jack Painter Virginia Beach VA |
On Tue, 23 Nov 2004 17:42:24 -0500, "Jack Painter"
wrote: "Gary Schafer" wrote Here is a quote from your previous post where the discussion was about the tower and lines carrying strike current. Jack Painter First, a strike termination device is placed higher than other equipment with its own down conductor. Your reply when questioned about placing a down conductor in parallel with the tower was: I never suggested such a silly thing! You mistake an earlier reference I made to a bonding conductor. You're displaying dimensia Gary. You accused me of suggesting a #6 wire would be useful as a down conductor and I never said such a ridiculous thing. You misread the posts, or still don't understand most of the terminology that the entire electrical, fire protection, lightning protection and communication industry use to refer to bonding and grounding components. Jack, my apologies. You did not suggest a #6 down conductor on such a tower as I reread your original post. But you did suggest a down conductor on the tower, you just didn't mention the size. in news:qBdod.15535$D26.3848@lakeread03... I said, and I quote: "Burying a grounding electrode conductor is normally a code requirement. But that is not what you have in connecting the tower ground system to the station ground, AC mains ground, etc. Those are bonding conductors, and they are in many cases required to be insulated. Not in this case, but I want you to understand the difference between grounding, voltage division from many grounds, and a bonding conductor between your station and the tower. The latter is to maintain equipotential, and will not carry more than just equalizing currents. It will be well within the capability of a #6 insulated wire, should you choose to use that." I'm not being condescending, if you're still confused there please say so, and I will try to explain it better. As far as down conductors go you need to read farther to see what they are using them for. We are talking about towers here. No down conductors needed. If you are talking about a building or wooden pole mounted antenna then that's a different story. Its clear to me who needs to do the reading here. Your last shot was also wide of the mark regarding down conductors on towers. They are used on communication towers in exactly the same fashion as they are on any structure, to provide a dedicated path to a grounding electrode for the charge received by a strike termination device. A tower down conductor is bonded to the tower frames in many places, that's the same as a down conductor on any structure is bonded to metal stairways, handrails, roof flashing, etc. on its way to ground. DOWN CONDUCTOR ON A BUILDING: A down conductor on a building is a whole different deal than on a tower. On a building the "metal stairways, handrails, roof flashing, etc." are bonded to prevent flashovers as the down conductor runs by. The down conductor is run because there is no better path to connect to. DOWN CONDUCTOR ON A TOWER: My comments about a down conductor on a tower were based around the fact that a down conductor on such a tower is useless. Tower joints should be bonded to negate any resistance in the joints but a down conductor the length of the tower is a waste of wire. I was trying to point out that any down conductor, coax cables or anything else running down the tower, including the tower, would all share the lightning energy. It is impossible to isolate any part. The fact that a typical tower has so much mass, and as a result much lower inductance, in comparison to any down conductor that you could hang on the tower negates it's usefulness. And yes, I know that some people do install down conductors on towers. Some people also hang pointy dissipation arrays on the top of their tower too. Some engineers and "communications managers" specify them also. Hams don't have a lock on ignorance in this department. The applications of grounding and bonding principles are not reserved for an elite society of engineers and electricians as you might like to think. They are free and available to all. There are no secrets involved. Please don't take my comments so far out of context. But you're right it is reserved, its reserved for those who pay the fees and time to subscribe to organizations that license the printing of the codes, and constantly discuss and explain changes, applications and plan future requirement for them. I would guess then that the general library is also "reserved". Quoting a load of authoritative directives is not a substitute for understanding. It is a play of semantics when you say " an important distinction to understand the difference between bonding and grounding". I would guess that it does not take too much study to understand that real ground can not be half way up a tower. So whether you are "bonding" or "grounding" a cable on a tower, the end result is unmistakably the same. Neither should it be as oversimplified as your rebuttal. They are not the same thing and to misunderstand it would be to make deadly mistakes when applying those principles, both outside and inside structures where external equipments connect to people. They are the same thing when we are talking about bonding or grounding on the tower itself, which is where that discussion spawned from. When you start jumping to other subjects then they may or may not be the same thing. Sometimes it is difficult to tell what you are discussing as you want to throw so many things into the fire. It seems that some of the discontinuity may come from lack of basic understanding of RF principles. Lightning is not just a direct current event that requires only consideration for high currents. This is a subterfuge to divert the attention from the basics we were discussing, and until that is resolved there is no room for discussion about protection design based on frequencies of structures and wiring in the near field, or the AC components of lightning. No diversion intended. Only a notice to read the other side of the page. I thought we were discussing lightning strike dissipation and preventing it from reaching the shack. How can you ignore the AC components? They are a major part of it. As far as buried cables go: I will ask the same question again that you avoided from last time but instead provided mounds of quotes that do not address the point. "Do you think that you can bury the feed wire for your long wire antenna and have it work very well? What do you think will happen to the RF on it? Will it make it all the way back to your receiver the same as it would if it were above ground?" This is a very relevant to "buried coax lines". I ignored that question because I thought it had no relevance to anything here. Maybe you can rephrase what you are asking please? We are talking about shielded coax, and the relationship between lightning and the thin outer covering of coax, the coax shield, the inside dialectric, and the center conductor are quite unique and not convertible to a relationship with bare wire feed of some long wire. My coax feedlines are buried - so I don't hit them with the lawn mower! But it doesn't mean anything to lightning to have your coax buried, unless you hire a trencher to sink them deeper than the ground rods. In a near field strike there will be massive and sufficient energy to make all that shield grounding, bonding and placement of arrestors real important. And it won't matter where the coax is if all of those requirements are not met. if you think your coax is protected under your lawn, lay some turf over your radios and protect them the same way. It's a wives tale Gary, just like so many RF-wives tales, only there are more capable folks here to dispel those. BURIED COAX: The relevance here was explained in my first post on the subject. Let's try again. I asked if you thought that if you buried the feed wire from your long wire antenna, it can be insulated if you choose but a single wire, if you would get as much signal from your antenna to your receiver as you would if that same feed wire was not buried but run in the air away from ground. What do you think would happen to the RF on that feed wire? Do you think that it would go unattenuated the same as it would if the wire was in the air? Or would you loose most of the signal if it were buried? This is relevant to burying the coax lines coming off a tower and leading into your shack. The coax shield will be carrying large amounts of lightning energy during a strike that it receives when the top of the tower is struck. Even though the coax is grounded at the base of the tower. since there is no perfect ground system you are not going to be able to dump all the energy to the tower ground. For the moment forget about possible induced currents into the cable itself from nearby strikes etc. 73 Gary K4FMX 73, Jack Painter Virginia Beach VA |
"Gary Schafer" wrote Jack, my apologies. You did not suggest a #6 down conductor on such a tower as I reread your original post. But you did suggest a down conductor on the tower, you just didn't mention the size. O.k. Gary. And I didn't mean to suggest a down conductor on a tower either, as its true there is little evidence that it carries more than 25-30% of the total energy with the tower carrying the balance. But it sure is specified in a LOT of places. I follow specifications to the letter unless there is clear evidence that in my particular facility it could be harmful or counterproductive. DOWN CONDUCTOR ON A BUILDING: A down conductor on a building is a whole different deal than on a tower. On a building the "metal stairways, handrails, roof flashing, etc." are bonded to prevent flashovers as the down conductor runs by. The down conductor is run because there is no better path to connect to. DOWN CONDUCTOR ON A TOWER: My comments about a down conductor on a tower were based around the fact that a down conductor on such a tower is useless. Tower joints should be bonded to negate any resistance in the joints but a down conductor the length of the tower is a waste of wire. I was trying to point out that any down conductor, coax cables or anything else running down the tower, including the tower, would all share the lightning energy. It is impossible to isolate any part. The odd system or two specifies isolated tower down conductors. A majority proscribe down conductors and bonding the entire length, and another small percentage use an air terminal but bond it directly to to the tower legs and eliminate the down conductor. Please don't use PolyPhaser's toy lightning machine antics as evidence there. Big industry and government spend millions bringing rocket-triggered lightning down towers all the time, and if down conductors are still being specified they must help. How much, and if it applies to a typical ham tower, let the user decide I guess. The fact that a typical tower has so much mass, and as a result much lower inductance, in comparison to any down conductor that you could hang on the tower negates it's usefulness. And yes, I know that some people do install down conductors on towers. Some people also hang pointy dissipation arrays on the top of their tower too. Some engineers and "communications managers" specify them also. Hams don't have a lock on ignorance in this department. That's true, but maybe 25-30% of the energy directed on a faster path via down conductor means something, I have not seen why. The ESE Dissipaters however - another ball of ceiling wax and snake oil. Throughly disproven yet to this minute it ties up the Lightning Standards Group of the IEEE while they fight over the language to discredit it with. There is also at least one insurance company that discounts for use of the CTS (Charge Transfer System). There is no reliable science behind this theory but it hangs on... Please don't take my comments so far out of context. But you're right it is reserved, its reserved for those who pay the fees and time to subscribe to organizations that license the printing of the codes, and constantly discuss and explain changes, applications and plan future requirement for them. I would guess then that the general library is also "reserved". A good library might have at least the NEC-70 (Electrical code). You can buy the NFPA-780 Standard for the Installation of Lightning Protection Systems for about $40 from NFPA directly. The October 2004 edition is the latest. The full NEC-70 is a lot more. Sometimes it is difficult to tell what you are discussing as you want to throw so many things into the fire. Roger that, sorry. My brain is awash with this stuff but it has been the most productive, interesting and generally most fun project I have taken on in years. I thought we were discussing lightning strike dissipation and preventing it from reaching the shack. How can you ignore the AC components? They are a major part of it. I think we were trying to answer a posters question about grounding the coax shield before it meets an arrestor, yes. BURIED COAX: The relevance here was explained in my first post on the subject. Let's try again. I asked if you thought that if you buried the feed wire from your long wire antenna, it can be insulated if you choose but a single wire, if you would get as much signal from your antenna to your receiver as you would if that same feed wire was not buried but run in the air away from ground. What do you think would happen to the RF on that feed wire? Do you think that it would go unattenuated the same as it would if the wire was in the air? Or would you loose most of the signal if it were buried? This is relevant to burying the coax lines coming off a tower and leading into your shack. The coax shield will be carrying large amounts of lightning energy during a strike that it receives when the top of the tower is struck. Even though the coax is grounded at the base of the tower. since there is no perfect ground system you are not going to be able to dump all the energy to the tower ground. For the moment forget about possible induced currents into the cable itself from nearby strikes etc. Lets come back to that then, because it would require DEEP burying to avoid (It is commonly avoided by shield grounding, arrestors, grounded steel conduit, geometry, and distance). Anyway, yes, there would be some attenuation from a cheap wire insulator that was not designed to prevent RF from travel in either direction. But 95% or greater coax shielding with its dialectric inner liner is designed to keep RF from penetrating. While it allows travel on its inner and outer shield covers during some oddities of RF behavior, it does not allow attenuation of the RF signal by design. Then lets agree it's designed to be 50ohm, or 60ohm, if you want to use the anal British system (which we know is actually correct there Limeys). So if the point you were making was just that some inductance from earth is felt (even on coax) - I can't disagree, but neither would this wildly high impedance to even wet earth impress lightning in the least bit. In dry soil it would be invisible for all practical purposes. After all we stress the need to ground (via 8-10' of copper electrode) the coax shield (and the center conductor via arrestors) so what good is insulated earth? That was my point, and unless you count on unprotected coax breaking-down and arcing into the earth along its length, the burying part is just a no-gain event (just like a tower downcomductor???) Substantially all of the energy from a tower or antenna strike is already on the coax and it's first arrestor, top-shield bond, (middle shield bond if you like) and tower bottom shield ground (and another arrestor if you like) at the same time. Its even on the GROUND at the same time. They all rise in near unison within about 5 microseconds and fall in potential within 20-40 micro seconds. Where damage usually occurs is when unprotected coax has a distant ground only, and the voltage remains high its entire length as a result. This is where I believe you might tempt the voltage out anywhere along a buried path, maybe in thousands of little holes. It could just as easily happen laying on the ground though, so I fail to see what surrounding it in dirt can do. Apparently so do engineers who specify controls and installation procedures. That doesn't mean your argument can't be a good one. I just don't see it as that yet. 73, Jack Painter Virginia Beach VA |
One last note on down conductors. You can figure out what the
inductance of a particular gauge of wire is. You can also figure out what the inductance of a particular size tower is. Just looking at the size difference you can see that the inductance of the small wire in comparison to the tower will be many times greater. That equates the many times greater voltage drop on the small wire. I would say that carrying 25% to 30% of the total current would be way optimistic. It is not just the gauge of the wire verses the gauge of the tower legs but how much inductance each presents. The cross section area of the face of the tower makes for less inductance as well as the size of the legs. RADIALS Buried radials make an excellent lightning ground system. An AM broadcast tower ground system is an example. Not only is it an excellent Radio ground but it is also an excellent lightning ground. Running radials out from the base of a tower that you want to ground is a big plus in addition to ground rods I think you will have to agree. It is suggested that many shorter radials be used rather than a few long ones in a lightning ground system. The reason is that the inductance gets too high to do much good as the length gets long. Also long ground rods suffer the same fate. Too long and the inductance is so high. That is one reason why additional shorter rods are recommended rather than very long rods. COAX LINES Only talking about the energy on the OUTER SHIELD of coax here now. With buried coax lines coming from the tower they serve the same as a radial tied to the tower. They will dissipate some of the energy just the same as a radial will. And they do not have to be bare copper shield lines to do so either. This was where I was going with the buried insulated single wire long wire lead. By the time the energy on the shield of the cable reaches the shack, part of it has dissipated by the coupling to earth. Also the impedance seen at that end is much higher than it is at the tower end of the coax. Just like the radial. The ground is a choke if you will. Add ferite beads on a transmission line and you greatly limit the amount of RF current that gets past on the shield. That is what the earth does to the coax line buried. 73 Gary K4FMX |
RADIALS
It is suggested that many shorter radials be used rather than a few long ones in a lightning ground system. The reason is that the inductance gets too high to do much good as the length gets long. 50' is the number generally used as the longest effective run of an individual radial arm for lightning protection. Also long ground rods suffer the same fate. Too long and the inductance is so high. That is one reason why additional shorter rods are recommended rather than very long rods. Up to 100' lengths of 1" copper clad steel ground rod are often sunk in order to obtain 25 ohm resistance. 5 ohm would be better but some soil condtions (such as dry sand) may be 1000 ohms. Bentonite and other similar soil-conditioners cannot be used effectively in sand, that is unless they occupy a deep 3' diameter hole. Here's an interesting anamoly in the NEC, which requires grounding electrode resistance to be 25 ohm at the service entrance ground. If that cannot be accomplished, then a second ground electrode must be used. But there is no requirement to test the results after the second grounding electrode is added! It could easily be 400 ohms after two rods sunk in terrible soil, and yet it meets the code. There are few such glaring examples of short sightedness in the NEC, but engineers and contractors both have to be aware of these problems to design and build safe lightning protection systems. According to both NEC and NFPA, the first grounding electrode coming off a structure may be shorter if conditions require, but then must be followed with a second electrode no shorter than 8' long and sunk to a depth of 10'. This is brand new in the 2004 edition btw. The minimum 8' long rod sunk to depth of 10' remains the code *minimum* in all other standards and applications where grounding electrodes are required. Distance between electrodes should be not less than sum of both their lengths. O.K. there is another little anamoly, since this does not take into account having to reach down 100' to find low resistance ground! Of course UFER grounds are also well accepted and commonly specified. This relates to another FAVORITE wives tale of hams, that is that tower foundations will explode if the grounding system goes into its concrete. There is not one documented case of this happening, yet the fable continues. It's true masonry and lightning do not get along well at all, but bonded conductors inside concrete foundations make it an excellent grounding system. It has been used world wide for decades. COAX LINES Only talking about the energy on the OUTER SHIELD of coax here now. With buried coax lines coming from the tower they serve the same as a radial tied to the tower. They will dissipate some of the energy just the same as a radial will. And they do not have to be bare copper shield lines to do so either. This was where I was going with the buried insulated single wire long wire lead. We do now agree with where you were going. But I hope I can show you it's not a place you want to hold any faith in protecting equipment with. By the time the energy on the shield of the cable reaches the shack, part of it has dissipated by the coupling to earth. Also the impedance seen at that end is much higher than it is at the tower end of the coax. Just like the radial. The ground is a choke if you will. Add ferite beads on a transmission line and you greatly limit the amount of RF current that gets past on the shield. That is what the earth does to the coax line buried. I understand what you are saying, thanks. Lets leave the ferrites alone though, because they can't shunt energy into dirt that doesn't have them, and if this was effective for lightning protection we would all use ferrite-shielded coax, right? Back to the coax shield now - are you aware what is the most voltage that standard coax can carry? It's about 5,500v. At close to 4,500v coax dielectric breaks down and inner and outer conductors become "one". Close to 5,500v, the outer insulator of coax fails (similar to house wiring at 6,000v). Above breakdown voltage there will be flashovers to any object of lower potential, including the air in some cases. At anything less than breakdown voltage, you could hold the coax in a gloved hand and not feel anything. And neither would the earth, if that's where it was laying. Let's review: It does not matter one bit whether you bleed off some RF energy out of a coax underground, because up to 5,500v of DC energy is going to get through with absolutely no affect from being near the dirt. And at over 6,000v nothing is going anywhere through that coax any more, it has reached breakdown and arcs over, melting and expelling the remains of its conductive materials all over the place. I have several such samples in my collection btw. Finally, this will conclude my points about not burying coax for lightning protection: Unless coax feedlines are protected by shield grounding and lightning surge protection devices in the proper manner and locations, there is no hope for its survival in even a nearby strike, let alone a direct attachment. If they are properly protected, it does not improve this protection to bury them. Perhaps it would increase the longevity of some surge protection devices and arrestors by limiting some RF energy they would otherwise be presented with. You would have to ask them that question. I have never seen a manufacturer of these devices require feedlines be buried, maybe they are missing this too. They do however require the devices we have talked about. It's of course fine if you want to hide coax, possibly reducing some external RF affects *and* the manufacturer says it is designed for below frost-line burying. Mine are buried too. 73 AND HAPPY THANKSGIVING Jack Painter Virginia Beach VA |
On Wed, 24 Nov 2004 18:40:41 -0500, "Jack Painter"
wrote: RADIALS It is suggested that many shorter radials be used rather than a few long ones in a lightning ground system. The reason is that the inductance gets too high to do much good as the length gets long. 50' is the number generally used as the longest effective run of an individual radial arm for lightning protection. Also long ground rods suffer the same fate. Too long and the inductance is so high. That is one reason why additional shorter rods are recommended rather than very long rods. Up to 100' lengths of 1" copper clad steel ground rod are often sunk in order to obtain 25 ohm resistance. Those kind of lengths are only "last resort" ground systems if there is no other possible way to get any sort of ground. The inductance will be so high on that length that it will do little for lightning. 5 ohm would be better but some soil condtions (such as dry sand) may be 1000 ohms. Bentonite and other similar soil-conditioners cannot be used effectively in sand, that is unless they occupy a deep 3' diameter hole. Here's an interesting anamoly in the NEC, which requires grounding electrode resistance to be 25 ohm at the service entrance ground. If that cannot be accomplished, then a second ground electrode must be used. But there is no requirement to test the results after the second grounding electrode is added! It could easily be 400 ohms after two rods sunk in terrible soil, and yet it meets the code. There are few such glaring examples of short sightedness in the NEC, but engineers and contractors both have to be aware of these problems to design and build safe lightning protection systems. According to both NEC and NFPA, the first grounding electrode coming off a structure may be shorter if conditions require, but then must be followed with a second electrode no shorter than 8' long and sunk to a depth of 10'. This is brand new in the 2004 edition btw. The minimum 8' long rod sunk to depth of 10' remains the code *minimum* in all other standards and applications where grounding electrodes are required. Distance between electrodes should be not less than sum of both their lengths. O.K. there is another little anamoly, since this does not take into account having to reach down 100' to find low resistance ground! Of course UFER grounds are also well accepted and commonly specified. This relates to another FAVORITE wives tale of hams, that is that tower foundations will explode if the grounding system goes into its concrete. There is not one documented case of this happening, yet the fable continues. It's true masonry and lightning do not get along well at all, but bonded conductors inside concrete foundations make it an excellent grounding system. It has been used world wide for decades. Unfortunately it is not just an old wives tale. Over the years I have seen several tower bases cracked or exploded chunks out of them from lightning strikes. To effectively use the tower concrete as a ground is a good thing to do. However it must never be the only ground. Ground rods also need to be installed and connected to the tower or you will have the effects that I have witnessed. The ones that I saw did not have any auxiliary ground connections. COAX LINES Only talking about the energy on the OUTER SHIELD of coax here now. With buried coax lines coming from the tower they serve the same as a radial tied to the tower. They will dissipate some of the energy just the same as a radial will. And they do not have to be bare copper shield lines to do so either. This was where I was going with the buried insulated single wire long wire lead. We do now agree with where you were going. But I hope I can show you it's not a place you want to hold any faith in protecting equipment with. By the time the energy on the shield of the cable reaches the shack, part of it has dissipated by the coupling to earth. Also the impedance seen at that end is much higher than it is at the tower end of the coax. Just like the radial. The ground is a choke if you will. Add ferite beads on a transmission line and you greatly limit the amount of RF current that gets past on the shield. That is what the earth does to the coax line buried. I understand what you are saying, thanks. Lets leave the ferrites alone though, because they can't shunt energy into dirt that doesn't have them, and if this was effective for lightning protection we would all use ferrite-shielded coax, right? Back to the coax shield now - are you aware what is the most voltage that standard coax can carry? It's about 5,500v. At close to 4,500v coax dielectric breaks down and inner and outer conductors become "one". Close to 5,500v, the outer insulator of coax fails (similar to house wiring at 6,000v). Above breakdown voltage there will be flashovers to any object of lower potential, including the air in some cases. At anything less than breakdown voltage, you could hold the coax in a gloved hand and not feel anything. And neither would the earth, if that's where it was laying. Let's review: It does not matter one bit whether you bleed off some RF energy out of a coax underground, because up to 5,500v of DC energy is going to get through with absolutely no affect from being near the dirt. And at over 6,000v nothing is going anywhere through that coax any more, it has reached breakdown and arcs over, melting and expelling the remains of its conductive materials all over the place. I have several such samples in my collection btw. Finally, this will conclude my points about not burying coax for lightning protection: Unless coax feedlines are protected by shield grounding and lightning surge protection devices in the proper manner and locations, there is no hope for its survival in even a nearby strike, let alone a direct attachment. If they are properly protected, it does not improve this protection to bury them. Perhaps it would increase the longevity of some surge protection devices and arrestors by limiting some RF energy they would otherwise be presented with. You would have to ask them that question. I have never seen a manufacturer of these devices require feedlines be buried, maybe they are missing this too. They do however require the devices we have talked about. Well now, what if the DC component of the strike is only 5495 volts? Still useless to remove most of the RF component of the strike? The RF part does contain significant energy you know. After all isn't the whole idea here to keep as much energy out of the shack as possible? The DC part of the strike is usually easier to suppress with a ground connection. The AC part is dependent on the inductance of the conductor to get it to ground. Why not use all available means at hand. Jack, without trying to sound sarcastic I would recommend that you do some reading about transmission line theory . Also some basics on inductance and capacitance and how energy is transferred in their fields. Outside of what the lightning protection people publish. These may help you in your quest to become a lightning guru. It is fine to read all the specifications of the NEC and other agencies that write specs. But you need to remember that they are using one glove size to try and fit all. If you understand a little more about how things work you can come up with your own fixes to fit the situation much better. You obviously have an appetite for this stuff. Spend a little time on the theory side. You have to be able to adapt to and use the surroundings available to best advantage. The perfect system can not always be built. Well I think I have had my run on this thread. Best Holidays Gary K4FMX It's of course fine if you want to hide coax, possibly reducing some external RF affects *and* the manufacturer says it is designed for below frost-line burying. Mine are buried too. 73 AND HAPPY THANKSGIVING Jack Painter Virginia Beach VA |
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