The following is from a web site that describes the experiences of a real world broadcaster who has to deal with lightning, big time. Most of the folks on this site are very practical types and I'm posting this for any comments. The NEC requires a single bonding of the neutral/ground, generally at the service entrance. I think this fellow is talking about multiple point bonding of the neutral/ground versus a single point bond as required. I'm not aware of anything that allows complete isolation of the neutral/ground in most systems.
GROUNDING SYSTEMS THAT WORK IN THIS AREA
This area of Florida receives some of the strongest and most frequent lightning strikes of anywhere in the United States. This system of grounding seems to be the best so far for this area (proven over time). I am not quite sure why or how it works, but on the stations where this type of system has been installed we have cut the downtime and equipment losses almost to zero. We have installed this system on new and existing towers up to 500 ft. in the last ten + years with very good results.
We have had many DISCUSSIONS on this subject with other "engineers", that prefer the "single point" grounding systems without any "ground loops", which I'm sure works well somewhere,(but not here). So we pretty much have done everything as "wrong" as possible and it has been working 100% better than before...
Some of the local electrical codes require the neutral and ground buss to be isolated from each other?? I tie them both to the ground system after inspection. I guess "they" will come and get me someday.....
A few examples.....
WXCV had a single point ground (installed by the tower mfg.)with 6 in copper strap tied to a long ground rod and to the tower. From 1983 until 1994 I have replaced everything from the plate transformer to the top beacon (lost the top half). Since installing this system in 1994 we have not even blown a fuse in any equipment on that site. We have had breakers trip but that is what they're supposed to do.
WGUL FM We constructed this station in the May of 1995 and to date we have not had any lightning damage on that site.
WRGO FM We constructed this station in July of 1996 and to date we have not had any lightning damage on that site.
WXOF FM This site was built in the summer of 1996 by a well known tower mfg. It also uses a single point ground, they almost had a good idea, they ran a #6 copper strand around the inside of the building connected to four ground rods in each corner. The only problem was the rods were drilled into the floor INSIDE of the building!! and only connected to the transmitter ground strap that ran outside to the single point ground. This station has been off the air more than it's been on. The wall where the 6 in copper strap goes through is black and burned from lightning. This site will have our system installed by the middle of September. This will be a very good test for this type of grounding system. Installed ground 02.1998, lightning problem solved to date... 2005.
WYKE DT We constructed this DTV station 03-05. We installed our grounding system during construction as normal. Well... It was hit every time a little storm passed over !!! Needless to say I thought the Devil was out to get me. After a few days of pondering the installation and could not find anything out of order in the plant I decided to look elsewhere. I walked out to where our burried power left the pole and everything looked OK. I then continued down the power line feed and found the transformer pole ground rod sticking out of the ground about 4 feet. I contacted our local power company, and quite a few weeks later they showed up. We checked nine spans and none of the power poles had ground rods. I'm sure all of you guys realize that most of the time they plant a power pole the ground wire that runs from the top of the pole is just wrapped around the base of the pole and stuck in the ground. This is not a good thing in Florida or anywhere else for that matter. We determined that the tower and building had such a good ground system that all the surges from the lightning on the power grid was ending up at our ground grid. After driving a truck load of ground rods on the power poles leading into our site the the Devil has left the building. We also have phone lines into the building, and thats another story.... we have B-1s T-1s and a T-3. The interface boxes were usually found on the floor after a storm along with the Optilator boxes. Since there is no way to properly ground phone lines at the terminals we decided to seperate the telco ground from our plant ground. That has worked very well so far.... So just when you think you know it all.... it can still bite you in the butt.......
Update 11.02.2005 ..... Still Going Strong... The only lightning problems to date is connected to phone lines.. Optilator does the trick most of the time with 4 inches of fiber optic isolation. We are using their new T1 boxes and are working great.
Where the hell did this guy work in Florida? We did a bunch of radio equipment shelters on the interstate and one in every prison. These things got the regular Ufer on the buildings and the tower base, bonded to a ground ring in 1/0 and 40' rods on the corners of the ring and radials going out from there. All of this got tied to a big copper plate in the building that was the ground electrode for everything. Single point ground, and very effective.
InspectorE, Just a little note from an "out of towner". I was under the impression that lightning protection came under the NFPA Codes. Also, there seems to be some confusion amongst even Engineers, as to what is a "Ground", is it the Grounding for the tower itself to allay static charges caused by wind currents and/or lightning, OR is it the RF (Radio Frequency) Ground, that all towers require to radiate a decent signal into the air?. I'm currently getting a tower organised for HF work at my new place, let it be said that you can NEVER have enough Grounding, but, at the same time, these systems need to be kept seperate, the last thing you need is a potential lightning strike back-feeding into your electrical system ground.
I think this is one of the most misunderstood subjects in electrical contracting. Dealing with the BONDING of the neutral and ground, this needs to be only at one location at the service. This is code, also good practice. Note that services with onsite transformers multiple bldg will require their own bonding. Lets look a a typical cell phone site. As far as the grounding system, it may have multiple rods, ufer, tower ground rods, and piping. All grounds need to be bonded together, but not in a loop. Although older systems were looped, such as running a ground loop around the bldg and tying all ground rods to this loop, they now recommend the loop be broken, they found loops can act as a choke and resist large current flow. This grounding system, the larger the better. More ground contact the lower the ground resistance. The lower the resistance the faster the charge is dissipated. All grounds need to be BONDED together. Having a antenna grounding system not bonded to the electrical ground is a bad idea (and not code) If the electrical system has a resistance .5 ohm different than the antenna ground, it will seek the better ground thru your equipment. All equipment, radio, electrical, antenna and such, needs to be at the same potential. If a strike occurs , the voltage will rise equally on all equipment, no current flow. All electrical lines should have MOV or surge protection on them as they will control the rise of voltage on the power conductors. I install grounding systems on commercial radio systems. I see many commercial tower sites , most are very poorly done. None of the sites that I have installed, or the many cell sites I have been to that were well installed and BONDED have suffered from large lightning damage. (And this is in Houston TX, like FL LOTS of lightning) Cell sites are a good examples, as BIG $$ is lost if they go down. Commercial radio stations (at least here) spend little money grounding or maintaining there grounding systems. I service many repeater sites located at commercial sites and few are well maintained. Robert
In this case the "ring" was not a ring shaped ground electrode conductor topology, just the shape of the big electrode. There was a single 1/0 going to the ground plate. All of the equipment had their grounding conductors radiating from the ground plate.