To better understand the concept of equipment grounding, you should review two NEC definitions in Art. 100:
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Ground. A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.
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Grounding conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
Also review the requirement that states in part:
Grounding of electrical equipment. Conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials [Sec. 250-2(b)].
This concept seems simple: Connect metal parts of an electrical system to the earth to limit the voltage-to-ground on the metal parts. But where did the voltage come from and how are you limiting it?
As the Figure shows, you bond metal parts of an electrical system together and then ground the bonding system to earth to limit the voltage-to-ground — thus preventing destruction of electrical components as well as electric shock that can occur from superimposed voltage from lightning and voltage transients [Sec. 250-2(b)].
However, according to data from the insurance industry, failure to properly ground communications systems [Secs. 800-40(b), 810-12(f), 820-40(b) and 830-40(b)] has led to $500 million of property or equipment damage annually due to lightning or surges. Why? The resistance of the ground determines how effectively your grounding system can dissipate high-voltage surges into the earth. The impedance of the earth ground depends on the resistance of the electrodes, termination resistance, contact resistance of the electrodes to the adjacent earth, and the resistance of the body of earth surrounding the electrodes (soil resistivity).
The NEC does not require you to measure the ground resistance of the grounding electrode unless you use only a single ground rod. When you use two or more ground rods, the measured ground resistance can exceed 25 ohms [Sec. 250-56]. Therefore, to achieve and maintain a low-resistive ground, you must use special grounding configurations, design, and equipment and measuring devices. Failure to properly ground the metal parts of an electrical system to the earth can result in electric shock and fire. Electronic equipment can be destroyed by lightning, line surges, or other high-voltage transients.
Grounding metal parts to the earth does not assist in removing dangerous voltage from line-to-ground faults by opening the circuit overcurrent protection device for the systems that operate at less than 600V! Next month, we'll see why.
