Assume for the moment that Building 1 has an earth electrode subsystem consisting of ground rods
interconnected with the cold water system with a net resistance to earth of 3 ohms. With a lightning discharge
of 20 kA, the voltage of the complex will rise to 60 kV with respect to Building 2 and that portion of the earth
not in the immediate vicinity of Building 1. At Building 1, the cable shield voltage will rise along with that of
the building. This voltage pulse will travel down the cable, successively raising the shield potential to as much
as 60 kV with respect to the surrounding earth. Such high voltages cause insulation breakdown in the form of
tiny pinholes where the lightning energy punches through.
As the lightning pulse travels down the cable, its amplitude diminishes due to cable resistance and dielectric
losses. However, the amplitude of the pulse can still be sufficient to damage circuit components in terminating
equipment in Building 2. To minimize this damage, surge arresters compatible with the terminating components
and hardware should always be provided on such cables. Further information on the use of surge arresters is
presented in Volume II, Section 18.104.22.168.
In the event of a lightning stroke, there is a definite personnel hazard posed by the voltage gradient in the soil
in the vicinity of the point where the lightning discharge enters the earth. In homogeneous soil, the current
rapidly leaves the electrode. The current density is highest near the electrode and rapidly decreases with
distance from the electrode. In soil of uniform resistivity, a significant voltage gradient will exist between two
points that are differing distances from the electrode. Figure 3-13 illustrates the nature of this voltage
variation and shows the hazard encountered by personnel walking (or standing) in the area. The voltage
difference across the span of a step can be sufficient to be lethal. As shown earlier, the degree of the hazard is
determined by the magnitude of the stroke current, the grounding resistance of the earth electrode, and the
distance away from the electrode. No control can be exercised over the current; the threat, however, can be
lessened by achieving a low common ground resistance and by minimizing the step potential as discussed in
3.7 BASIC PROTECTION REQUIREMENTS.
To effectively protect a structure such as a building, mast, tower, or similar self-supporting object from
lightning damage, the following requirements must be met:
An air terminal of adequate height, mechanical strength and electrical conductivity to withstand the
stroke impingement must be provided to intercept the discharge to keep it from penetrating any nonconductive
outer coverings of the structure or to prevent it from terminating on antennas, lighting fixtures, transformers,
or other devices likely to be damaged or destroyed.
A low impedance path from the air terminal to earth must be provided.
The resistance of the connection between the discharge path and the earth must be low.
These requirements are met by either (1) an integral system of air terminals, roof conductors, and down
conductors, securely interconnected to provide the shortest practicable path to earth, or (2) a separately
mounted shielding system such as a metal mast which acts as an air terminal, and a down conductor or an
overhead ground wire terminated at the ends (and at intermediate locations, if needed) with down leads
connected to earth ground electrodes. Specific design practices are contained in Volume II.