Earth has an electrical property, conductivity (or low resistance), that is used in the electrical systems of your facility. Earth’s resistance is the resistance of soil to the passage of electric current, and the Earth is a relatively poor conductor; however, if the area of a path for current is large enough, resistance can be quite low and the Earth can be a good conductor.
If there is an instance of overvoltage or lightning strike, you need to ensure that the current will find a safe path to Earth. If the current does not find this path, or the ground cannot carry the fault current long enough, the result is potential damage to the equipment inside your building.
There are a few factors that can change the Earth electrode requirements from year to year;
- A plant or other electrical facility can expand in size. Such changes create different needs in the Earth electrode, and what was formerly a suitably low Earth resistance can change.
- As facilities add more sensitive computer-controlled equipment, the problems of electrical noise are magnified. Noise that would not affect 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, low resistance ground connections.
- In many locations the water table gradually falls. In a year or so, Earth electrode systems that were effective may end up in dry Earth of high resistance.
These factors emphasize the importance of a continuous routine of earth-resistance testing, per NFPA 70B recommendations. It is not enough to check the Earth resistance only at the time of installation.
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