By: Rob Beckers
A good ground is very important, maybe the most important thing, in mitigating lightning damage; the more energy you can divert to ground before it gets into the house or equipment room, the less you have to deal with through lightning and surge arrestors. More importantly, more energy diverted to ground means less can go through your electronics!
A lightning strike moves a massive amount of electrical charge around, and a single grounding point is not going to be able to cope with that flow. Even a few closely spaced ground rods are not really going to cut it; all those electrons need some space to fan out and disperse to ground. Here is what the pros do: They run a star-shaped network of flat copper strap fanning out from the tower, with ground rods spaced along (and tied into the ribbon) at twice the length of the rod. So, for 10 feet ground rods they are spaced 20 feet apart. The reason for using a copper strap is that even though lightning is in essence a direct-current event (most of the time the cloud is negative, the ground positive), there is considerable high-frequency energy involved as well and it tends to stay at the surface of a conductor (“skin effect”). Because of that, a flat copper strap of at least 1 1/2″ diameter and at least #26 AWG (0.0179″) thickness has less impedance (the resistance for high-frequency energy) than a #4/0 AWG (0.46″) round copper conductor.
The copper radials should be buried at least 8″ and preferably 18″ or more underground. A radial in wet soil will work better than the same radial in dry soil. There is also a maximum length where the radial’s impedance will prevent any additional electrical charge from traveling further down the line. Short radials do not work very well either, because of the same charge saturation problem mentioned before. A good radial is at least 50 feet, and no more than 75 feet long. If a radial comes within 4 feet of a metal object, that object should be electrically connected to the radial. More radials is certainly better, but after 4 radials one enters the domain of diminishing returns. Keep in mind that radials do not need to go in a straight line. Gradual bends around obstacles or to follow the terrain are fine.
Sharp bends add inductance, and will cause lightning to look for an easier way to get to ground. Any conductor that is going to carry lightning current should therefore have a bending radius of no less than 8″. This is not the place for neat, 90-degree angle installations. Gentle curves is what works best.
Ground rods in a well-conducting soil have a cylindrical region of influence around them with a diameter that extends roughly twice the length of the ground rod. This is the region in which the ground rod disperses lightning strike energy. Placing round rods closer together than twice their length means their regions of influence overlap. That does not make it a worse grounding system, but there is no gain either and it is a waste of money to place them that close together. So, for 8 foot ground rods they should be 16 feet apart, for 10 foot rods it becomes 20 feet from one rod to the next. The ground rods themselves should be copper clad. There are several reasons for this: The copper cladding makes them last longer underground than bare steel or galvanized rods (stainless rods will last longer still, but they are also much more expensive). What is more, the ground strap or wire is going to be made from copper, and connecting that with a bare steel or galvanized ground rod creates a battery that will quickly consume the steel or galvanized rod through galvanic action. A copper clad ground rod will prevent this from happening, and again, make it last longer. The saying in the business is that the best resistance-to-ground you are going to see from a ground rod is right after installation, it is downhill from there.
Just in case that was not obvious; Ground rods and their connecting ground wires are normally kept fully underground. So, dig a hole before pounding in the ground rod, and drive it below ground level. Grounding wires should be underground in trenches (and backfilled). In fact, for optimal effect the wires of your ground radials should be 18″ underground. Not only will this look better, and keep your lawn tractor from mauling its blade, it will also be electrically more effective.
If possible, the ground rods should use exothermic bonds to connect to the grounding strap or wire (“Cadweld” and “thermOweld” are often used brand-names for exothermic welds). Those last far longer with more predictable and lower connection resistance than the next-best alternative, high-pressure clamps and joint compound (anti-corrosion compound). According to the pros you should inspect high-pressure clamps annually, if you use them. Not easy to do when they are underground. Brazing is an allowed substitute for exothermic welds, though this is a skill that not many can do well.
If you are lucky enough to have a well close to your tower, be sure to tie it into your grounding system. A steel well casing makes an excellent ground! This only works for metal well casings. The newer plastic casings are of no use for grounding.
A grounding system of multiple radials does not have to be ‘in the horizontal’. There are special 50 feet long ground rods (or regular rods with couplers) that are used to make a vertical grounding system. When done properly this works just as well, though installation requires professional equipment. The above link also shows the effects of both the lack of a proper grounding system, and the difference this can make.
All the above assumes you are able to dig into the ground, and install ground radials plus ground rods. If you have rock to deal with, or very dry (non-conductive) soil you should talk to a professional and find the best option for your particular case. Ufer-grounds can be a solution.