The fundamental nature of lightning is that of an effective DC pulse channeled through a single point of contact. Because lightning is essentially a large DC pulse like a square wave with a trailing edge, low-frequency AC pulses are induced in nearby wires and radiated when lightning strikes. This is what produces the characteristic "slope" on a spectrum analyzer of a lightning strike.

Since lightning is a static charge equalizing through a single point, it is easy to see that equalization over an area is the desired result of the strike. A tree gets blown to smithereens, but the charge dissipates and equalizes over the area around the strike -- both above and below the point of contact. Using good conductors, the magnitude of voltage across a given area is minimized. This lowered difference in potential prevents large voltages from appearing across equipment.

By running ground strap across the inside walls of the shelter around the perimeter at the ceiling, middle of wall, and floor levels, the lightning is given a path around the equipment instead of through the equipment. This prevents damage as high-resistance points in the equipment won't result in arc flashover due to high voltage across those points. This is not a Faraday cage, but works in a similar fashion. By giving lightning an alternative path of low resistance and impedance, the vast majority of the current goes around rather than through.

NEC compliance confuses this point, because traditional RF grounding for lightning and power grounding are somewhat at odds. Military manuals and Motorola's guide to R25 installation help. In some cases, the halo ground system has been implemented with the power ground opposite the tower coax window / ground panel. This allows NEC compliance with respect to the power ground, while putting a ground rod at the tower base and connecting the two together via 0000 to 2 AWG copper conductors.

At this point, we will discuss the merits of a copper ground halo above equipment versus using building steel and steel bolts.

A 4" copper ground strap 0.022" thick has a total cross section of 0.088 square inches, equating to a little larger than the cross-section of 0 AWG copper wire. 0 AWG copper wire has a resistance of 0.1 ohms per 1,000 ft. Thus, 1,000 feet of 4" 0.022" thick copper strap has a resistance of nearly 0.1 ohms.

A carbon steel wire 0.5" in diameter has a resistance of 0.00034 ohms per foot, or roughly .34 ohms per 1,000 ft. Thus, three 1/2" diameter bolts are nearly equal to 4" 0.022" thick copper strap assuming no other losses.

This calculation does not factor for the resistance of zinc at steel-zinc-zinc-steel junctions. However, one can see that steel-on-steel contact under pressure, three or more (typically six) bolts are enough to allow the building steel to act as a halo, thus not requiring a copper halo be installed as long as the building steel is attached to the ground system.

1" diameter carbon steel has a resistance of 9.0 x 10^-5 ohms per foot or .09 ohms per 1kft, so one can see why big towers often are not connected together with ground straps from section to section but smaller towers are.

It would be preferable to have the building steel sections welded together, or jumpers cadwelded around each joint.