Not all lightning is created equal. About 30% of all lightning strikes have a peak current of over 10 kA, while about 10% of all lightning strikes have a destructive current of over 50 kA. A percent or two of strikes get over 100 kA, and strikes have been recorded with current peaks well over 200 kA! Even the DC component of lightning creates immensely large voltage differences. Its AC components develop astronomical voltages that saturate any earth ground.
A typical ground rod (8' X 5/8") has an impedance of about 5 ohms in good soil . Dusting off ohm's law, we see that 5 ohms times 20 kA is 100,000 volts! Or is it? Well, we did neglect the inductive reactance of the ground wire at the 100Khz to 100 Mhz associated with lightning. Take notice that the power developed from just average lightning is sufficient to overload most protection systems unless voltage-division (bonding and ground systems) are present.
The fact is that lightning will find a network of paths to ground by arcing over to whatever is at a lower potential. Installed at the utility service entrance, either MOV or Silicon surge-suppression can handle most differential voltage that becomes superimposed over the normal power line voltage. Modern development in interior surge protection involves ignoring common mode which is unable to enter U.S. buildings via power lines, in favor of normal mode protection. Normal mode protection involves the hot to neutral lines only. Normal mode suppression never assumes more than 6,000v because that is the level where an electric meter and all house wiring would simply arc-over, and no more voltage than that can be carried by interior wiring. In June 0f 2004 lightning struck a home 1/2 mile from me, blowing the electric meter off the house! An ineffective service-entry ground rod (in sandy soil) is almost surely the cause of this damage.
Understanding the difference between normal-mode and common mode transients is key to analyzing susceptibility to lightning damage. It is common in commercial and industrial construction to find powerful MOV protectors guarding the utility line entrance to the structure. These often protect against all modes (differential between each of the wires and each wire to ground potential). On large, heavy duty HV entrance lines that's appropriate, but those high current carrying lines do not come into a home. All common-mode (if any) will always stop at the service panel, where neutral and ground are bonded anyway. Thereafter, voltages imposed will be "normal-mode".
As the previous paragraph explains, newer protectors ignore common mode error in favor of normal mode protection. Another way to distinguish differential and common mode transients, is to think of differential mode as a transient injected with a parallel connection and common mode as a series connection. Lightning itself (the actual strike) is common mode.
The local ground connection will not stay at ground potential. If we could freeze time at the instant a lightning strike connects, we would see that the local ground has an immense voltage at the center of the strike due to the limits of the earth's impedance. As we move away from the strike center there is a voltage gradient (step voltage).
Imagine further, concentric circles with the lightning stroke's connection with ground at the center. With your feet about shoulder width apart, do you want to be standing parallel or perpendicular the voltage gradient? The unfortunate sole who makes the wrong choice could be killed by the strike with his two feet at vastly different potentials, while someone next to him standing with both feet on the same voltage gradient line can walk away unscathed.
While holding this picture, we can see that for any direction one faces it is possible for lightning to strike at a point that exposes our feet to ground connections of different potentials. Just as we would be safe standing on one foot, our station can survive with a "single point ground" as well.
Finally, bringing the story back to equipment damage prevention, we need to assume that any two ground sources entering a building can have a large voltage difference when lightning strikes nearby. The common mode transient, as defined here, is a transient voltage difference between two local ground sources. Damage happens when the ground rises to 100's of thousands of volts, and uses our grounded equipment as a path to lower potential somewhere else!
Common mode transients can be of great concern when connecting antennas and their ground radials to the Station ground. If the electric utility ground connection was the only ground entering your building, there would be little problem because everything would go up and down in voltage together, and there could be no electric discharge. But when you bring common mode transients into a different side of the home (the radio station!), you have a major electrical potential on your hands. Bonding direct from station ground to the utility service ground with heavy, low impedance cable or copper strap to the service mains is a must. This bond is critical, as the AC ground wiring in the radio room is of hopelessly high impedance to be a lightning conductor. Lightning will force that path if you don't provide it!
Warning: If you do not bond from station to service entry point, a GPR event (Ground Potential Rise) can use your AC ground wires (assisted by those dangerous ground-referenced "surge protectors") to destroy your equipment by drawing ground current up through the station ground and out through the equipment in reverse along your AC wiring. The same is true of coax arrestors, phone line protectors, etc. They all work horribly in reverse, destroying the very equipment they were designed to protect. Bonding is the only defense against this damage, unless you can afford high voltage isolation transformers or fiber-optic isolation from ground entirely.
Finally, ensuring that all ground conductors are sufficiently sized to carry high current for a short time and common bonding all equipment, will allow a ground system to prevent what would otherwise be "violent current equalizations" during a strike. If the system can't handle the current, destructive flashovers will occur.
Other ground sources are also troublesome. Plumbing is a very low impedance ground source and often enters on a different side of the building than the electric utility ground. If this is the case, care must be taken to ensure that no utilities are connected to this separate ground source. It is however advisable to include (bond) the cold water entrance into the overall station and building ground system. Just don't bond a lightning down conductor from the roof to anything until it reaches it's own ground rod first. Once it does, it is both safe and code requirement to do so.
Note: I discovered that my cable provider failed to
connect their cable shield ground to my service mains ground rod. The installer
initially failed to connect any ground at all, and when I
pointed that out, he connected "his" ground to a water pipe under the
kitchen. I was not aware at the time how a lightning strike
could cause a huge voltage potential between these unbonded ground points, and
destroy not only the cable modem but the computer and everything it was
connected to. That's because it was bonded to nothing, and had
a separate ground from the rest of the house!
*** Check your utilities! *** Bond everything ***