From Glen K9STH, a former engineer for a large utility company in Texas:
I wrote a fairly lengthy dissertation yesterday about dissipation devices that, somehow, was "dissipated" from this thread!
There are those persons who consider dissipation devices "snake oil" and there are those, like me, that have found them to be very effective in reducing the chances of taking a strike. You notice that I say "REDUCING" the chances of taking a strike because there is nothing that can absolutely prevent a lightning strike, at least now known to man!
I spent over 10-years as an "in house" telecommunications consultant to TXU (basically the electric company for almost half the State of Texas) specializing in grounding and lightning protection, and then another, over 10-years, as an independent consultant dealing with the same things.
Lightning starts from the "ground up" and NOT from the sky downward. What are often called "feelers" start from things on the ground and then, when those "feelers" get long enough, those "feelers" are met by the main charge coming from the sky. When the chances of "feelers" being formed are reduced, then the chances of taking a direct lightning strike are reduced.
The purpose of a dissipation device is to "bleed off" the charge from the Earth bound item. With these devices, a corona can be formed which indicates that the charge is being bled off. When there is a corona, there is no feeler! Yes, it is possible for a "feeler" to happen when there are dissipation devices installed. This is because there is a greater charge being developed than the dissipation devices can adequately dissipate.
Dissipation devices are not normally installed on the actual antennas. They are installed on the supporting structure, usually a tower. On a tower, dissipation devices need to be installed on each leg of the tower, just like each leg of a tower needs to be independently grounded at the base. When the height of the tower exceeds around 100-feet, then additional dissipation devices need to be installed below the top of the tower. These additional devices need to be installed every 50-feet to 75-feet along the legs of the tower.
My first "professional" use of dissipation devices was at the Athens, Texas, TXU microwave tower. This tower was infamous for taking lightning strikes with varying levels of damage to equipment. This tower, like all of the TXU owned two-way radio towers and microwave towers had, within a year, or so, had the grounding system upgraded, by an independent firm, to conform with the "latest and greatest" grounding techniques available. However, damage from lightning strikes were still happening. In about a month, there were 3-lightning strikes on the tower causing over $100,000 total damage. Since TXU "self insured" for any single occurence resulting in less than $1,000,000 damage, the cost of replacing the equipment had to come out of the local manager's budget. As a result, the local manager contacted me to see what could be done. I had been trying to get management to consider using dissipation devices after attending a seminar about using the devices. I got the local manager to purchase dissipation devices and they were installed by TXU radio / microwave technicians.
A couple of weeks after the dissipation devices were installed, I got a telephone call from the manager. He told me that they had another lightning strike. When I asked him how much damage, he told me that 2-each telephone "protector blocks", costing about $1.00, had been damaged. Of course, that lightning strike had been on the telephone lines and not to the tower. After the dissipation devices were installed, there were no more direct strikes to the tower.
Because of the experience with the Athens tower, management decided to install dissipation devices on all of the company owned two-way radio towers and microwave towers (well over 150-towers throughout the State of Texas) and on all of the data centers throughout the system.
These towers ranged from less than 100-feet above ground to over 500-feet above ground and a number of those towers, although the grounding system had been upgraded, were taking regular lightning strikes with damage to equipment. Although a good number of these towers were not located near "manned" locations, there were devices, located at each tower, that could detect lightning strikes and then report the strike, over SCATA radio links, to major, company owned, dispatch centers.
A certain telecommunications engineer was assigned to oversee the implementation of the dissipation devices. This particular person was of the "snake oil" group where dissipation devices were concerned. The dissipation devices were first installed on towers that had a history of repeated lightning strikes, sometimes multiple strikes within a single month. As the dissipation devices were installed, the number of lightning strikes on each tower dropped to ZERO! That is, no strike on the tower, nothing reported over the SCATA links, and no damage, whatsoever, to equipment. The engineer then became a believer in dissipation devices.
At the Comanche Peak Nuclear Steam Electric Station, the Nuclear Regulatory Commission REQUIRED that dissipation devices be installed all over the 2-containment buildings. Frankly, the buildings almost look like porcupines because of all the dissipation brushes installed. The NRC also required a dissipation system to be installed on the "met tower". The "met tower" is an over 200-feet above ground tower on which instruments to detect radiation levels are installed as well as all sorts of weather equipment as well. Obviously, the NRC "believes in" dissipation devices!
My tower has been installed for going on 42-years. The top antennas are 67-feet above ground and I live 1/2-block from the highest point in the City of Richardson, Texas. The tower is the highest thing above ground level for over a 1/2-mile radius. The tower has NEVER been hit by lightning. However, my direct across the street neighbor has lost 3-trees and a chimney to lightning. The "pole pig", electric distribution transformer, that is located at the back corner of my lot, which is well under 100-feet from the tower and over 40-feet below the top of the tower, has taken a direct hit. Again, no hits on the tower.
My philosophy for lightning protection is to make sure that the best possible grounding system is in place "just in case" the site takes a lightning strike. Then, install dissipation devices to greatly reduce the possibility of a strike.
Dissipation devices must be installed directly on the tower, mast, etc. That is, "metal to metal" contact between the device and the item to be protected. There should NOT be a resistor, etc., between the device and the tower, mast, etc.
There are companies that guarantee if their dissipation devices are installed that a lightning strike will never occur. Then, if a strike does happen, the company will "claim" that the devices were not installed correctly. Even when the devices are installed correctly, which is going to be virtually every time, there is always a chance of taking a strike. The purpose of dissipation devices is to reduce, as much as possible, the chances of taking a strike and not to absolutely prevent a strike.
