Books have been written on this subject and excellent reference works are available. In this discussion we will address some common errors or misconceptions.

First, many believe "If it is properly grounded " (good earth ground of less than 10 ohms) you won't have transient problems. In an oil production field in New Mexico, the transmitters were "grounded" by sometimes over a mile of earthed pipe. (It would be difficult to imagine a better "ground" than this). The company was losing 100 transmitters per month. The problem was of course related back to our principal of transient overvoltage occurring on the current loop with respect to ground. (Transient ground current flow in this case).

Sometimes an individual will remove the ground in hopes of avoiding the overvoltage problem. In this case the equipment is still subject to line to line transients and the ground threat normally remains. The ground threat remains because an insulation between two conductors (Equipment - Isolator - Earth) forms a capacitor, which at lightning frequencies may conduct energy into the equipment (See Figure 19).