We will discuss later what value of earth resistance is considered low enough. You’ll see that there is no general rule usable for all cases.
First, however, consider three factors that can change the earth electrode requirements from year to year:
A plant or other electrical facility can expand in size. Also, new plants continue to be built larger and larger. Such changes create different needs in the earth electrode. What was formerly a suitably low earth resistance can become an obsolete “standard.”
As facilities add more modern sensitive computer-controlled equipment, the problems of electrical noise is magnified. Noise that would not effect cruder, older equipment can cause daily problems with new equipment.
As more nonmetallic pipes and conduits are installed underground, such installations become less and less dependable as effective, lowresistance ground connections.
In many locations, the water table is gradually falling. In a year or so, earth electrode systems that formerly were effective may end up in dry earth of high resistance.
These factors emphasize the importance of a continuous, periodic program of earth-resistance testing. It is not enough to check the earth resistance only at the time of installation.