The Nature of Surges
Lightning is a high-magnitude, short-duration surge of current between clouds and ground that occurs when an air path is ionized by high electrostatic charges associated with thunderstorms. The common Franklin lightning protection system consists of metal rods called air terminals at the top of the building, connected by wires called down-conductors to a ground system, which usually consists of a ground rod driven into the soil at each down-conductor location. Lightning strokes that would otherwise hit the building are attracted to and intercepted by the air terminals and conducted safely around the building to ground by the down-conductors. An alternative system uses assemblies of small diameter rods called dissipater electrodes at the top of the building. These dissipation arrays are believed to prevent lightning strikes altogether by bleeding off the static charge as it accumulates to prevent buildup of a field under which the air would break down and a stroke occur.Lightning strikes to overhead power lines, or to the ground in the vicinity of underground lines, can introduce surges directly on the power system in the absence of a strike to the building itself. Electrical faults and switching operations on high-voltage transmission systems can also produce surges with respect to ground on the power system. Transient voltage surge suppressor (TVSS) devices are often used throughout a building's electrical system to divert these surges and they also require an effective connection to ground.
To understand the role of grounding in surge protection, let's return briefly to the grounding electrodes we discussed last time. The connection resistance to ground of a single 3/4 in. diameter, 10-ft-long driven ground can be as low as 10 ohms and as high as several thousand ohms, depending on soil conditions. We're doing well to get the overall resistance of the buried ground grids used in electrical substations below 1 ohm. Now consider that the average lightning stroke is approximately 30,000A with the maximum strokes exceeding 200,000A. This magnitude of current, flowing through even the best "substation quality" connection to ground, will produce an extremely high voltage between the grounding conductors and earth that is remote from the ground point.
Bonding As Well As Grounding
Since there is nothing we can do to prevent lightning current from producing high voltage, why aren't the systems in our buildings constantly being destroyed by surges? The answer lies in the principle of the Faraday cage described above. The occupant isn't harmed by the high voltage discharge because the entire surface of the cage is elevated uniformly to that voltage. Although the occupant is in contact with a surface that may be thousands of volts above ground potential, all surfaces they can contact are at the same potential and therefore no current can flow through their body.
We create the building systems equivalent of a Faraday cage by bonding all of the grounding points and all of the metallic systems within a structure together. If we can eliminate voltage differences between systems within the building, the voltage of the building itself can be elevated above ground by a surge current with no ill effects.
