O.k., what
keeps lightning from destroying stuff?
Common Bonding: Rule #1 for successful grounding.
Maintaining bonded paths of extremely low impedance from each station equipment to a single point ground is vital in preventing the exchange of lightning energy between equipment in the station. The station equipment individually bonds to this single point of the station ground. No grounding system offers any protection without 100% equipment bonding. This system, effective only when taken as a whole, is the principle known as COMMON BONDING and the SINGLE POINT GROUND SYSTEM (SPGS).
What this means is that all station equipment is at the same potential and the equipment is all tied together at a single point ground. That common tie-point should be immediately adjacent to the equipment.
After connecting to that first ground rod, it must bond to the home's utility service ground.
The bonding and ground
materials, in addition to being good conductors with very low resistance, must also have a large
surface-area. This is what makes them low impedance! For instance,
3" wide 20ga. solid copper strap (flashing) has similar resistance to a #4 solid copper
wire...but the strap has a lower inductive reactance, and is therefore lower
impedance than the solid wire. 6" strap is even better. This is
important!
Bonding Materials:
5/8"x 8' copper-clad steel ground rods, heavy (#2 or #4) solid copper wire and wide copper strapping are good example of recommended material to tie all equipment grounds, antenna grounds, RF grounds, and the station's main service ground together. Only UL-approved connectors should be used to join ground wires, ground rods, etc. The Cadweld-system ( exo-thermal bonding used outdoors only) can be expensive and is used in underground work . When a ground system is properly connected, there exists almost no electrical difference between earth ground and all radio equipment in the station, the SPGS, all of the station's antennas, and the SPGS of the house utility service-mains. All will rise, (float), and fall together.
The GOAL of all this common bonding is to prevent lightning from sensing a difference in potential between any equipment in the station or it's antennas and maintaining similar low impedance with earth ground.
Q: Is that ever possible (the perfect equi-potential ground)?
A: Only until you get struck! Example: When lightning strikes next to a station, the area immediately around the strike-zone (including the surface of the earth) raises from zero volts to hundreds of thousands of volts for a very brief time. "Ground" as we knew it, ceases to be a safe place. At this same moment in time, the station ground system several feet away feels a huge potential difference and will reference thousands of volts from the earth. Even small values in conductor impedance will allow momentary flow of hundreds of amps of current across the ground system. But the direction it comes from does not matter. If the grounding system is common-bonded it will always work! Current equalization can be violent across anything but a well designed and capable ground system. However, drawing current up from the earth to an un-bonded system has consistently devastating results. The bonding is the critical feature, as it allows all bonded equipment to be in the same "boat". That boat rises equally with the lightning energy, and falls equally with it. When properly bonded, the ground system could care less what actual voltage is present on it.
In grounded stations with un-bonded equipment, there have been flash-overs between separately grounded equipments, including explosive current equalization inside the air spaces of the station!
In a proper system design, during a direct strike the bonded equipment itself might feel thousands or hundreds of thousands of volts, but will carry very little current due to the common-bonding and single point ground. As we discussed above however, lightning will create massive transient voltages on it's own, and current transfer will always happen between parts of the ground system during a strike. Subsequently, the bonded equipment cases and bonding conductors could carry some equalization currents. Sufficiently sized conductors must be able to carry the expected amount of current for their safety purpose. Always err on the side of "large surface areas" there. Nothing you do is more important than this bonding of all equipment to a single point ground! With proper bonding, equipment might survive even with no ground at all. Without proper bonding, no ground system is good enough to keep lightning from causing damage.
Although unfortunate, some damage can result in any high current situation. While unlikely, a direct strike of major magnitude will find the weak points of any system, and could cause damage in spite of a good design. I will add though, this could be compounded by poor housekeeping in the station. Loose conductors scattered behind equipment for instance, could cause high voltage from a strike to flash-over and damage otherwise protected equipment. So remember to keep your station bonding and lightning ground system as uncluttered as possible. Check at least annually for tightness of mechanical joints.
Before we go any further, don't be discouraged and think lightning is unmanageable. It is manageable. Even the worst magnitude of strike possible would do substantially less damage to a protected station.
The best protection from lightning is the Common-Bonded, Single-Point Ground System. There is simply put, no safer way to operate. Even when ground systems are carrying massive amounts of energy, they are still doing their job. Floating that energy across an equipotential system is a happier place to be.