Earth Grids Are Easy
…Or Are They?
A grounding system can be as simple as a single ground rod, a system of interconnected ground rods, or a ground grid comprising interconnected ground electrodes buried in the ground to establish electrical contact with the earth.
Its purpose is to use the earth as a current-carrying conductor to dissipate noise currents and conduct dangerous lightning and power fault currents. Earth is a very good electrical conductor due to its mass and its availability to everyone at all locations throughout the world. This global access to the earth allows grounding systems installed on opposite sides of the world to connect. In fact, this was the case for a project I consulted on where a submarine cable was installed along the ocean floor between Japan and Hawaii; the two site locations use their grounding systems to connect to earth, which provides a return path for the 1.0 ampere of DC current used to power up optical repeaters in the cable.
Earth is also used as a return path for phase-to-ground fault currents, carrying these currents back to their electrical source (typically a utility transformer or generating source). Similarly, utilities use earth and the system neutral as parallel, redundant paths to return neutral currents back to their substation transformers.
Another, perhaps less understood benefit of the ground system is that it can provide a common (near zero) voltage reference to all structures, electronics, and other equipment connected (bonded) to it. Because the grounding system can conduct current at times, its near-zero voltage status will change, often increasing to several thousand volts, when current is passing through it on its way to earth. This ground potential voltage is a function (product) of the current flowing through the grounding system and the impedance/resistance of the grounding system’s contact with earth.
Testing a grounding system to measure impedance and calculate resistance is important since the engineer responsible for designing the grounding system must ensure that the elevated voltage in the grounding system doesn’t create a safety risk to people or equipment coming in contact with (touch potential) or in the vicinity of (step potential) the grounding system.
There are various methods for measuring grounding system impedance, including Fall of Potential (FOP) and various computer-based test methods.