Remove voltage on metal parts of an electrical system to protect against lethal shock.
Here's a “true or false” question for you. “Connecting metal parts of a 120V electrical system to a ground rod removes dangerous voltage imposed on the system from a line-to-ground fault by opening the overcurrent protection device (OCPD).” If you answered “true,” you're wrong. To see why, let's review three fundamentals:
Electrons leaving the power supply are attempting to return to the source. They are not trying to go to ground.
The time it takes for an OCPD to open is inversely proportional to the magnitude of the fault current. The greater the ground-fault current, the less time it takes for the device to open.
To protect against lethal shock, you must remove unsafe voltage on metal parts of the electrical system and the building. Do this by opening the circuit's OCPD in less than 1 sec.
To open the device in less than one second, be sure the fault current returning to the source must quickly rise to about 6× (fast-acting fuses) to 10× (other devices) the rating of the circuit OCPD.
Note: A GFCI protection device is not an OCPD. By design, it opens the circuit where the fault is as little as 5mA.
To remove dangerous voltage in less than 1 sec, make sure the fault current path is permanent and electrically continuous, capable of safely carrying the maximum fault likely to be imposed on it, and has a low impedance to allow the operation of overcurrent devices [Secs. 110-10 and 250-2(d)]. You create a low-impedance path when you bond together the metal parts of the electrical system [Sec. 250-90] and bond them to the power supply system grounded (neutral) conductor. Sec. 250-24 requires this operation for service equipment, while Sec. 250-30(b) calls for bonding for separately derived systems.
Naturally, the earth cannot be the intended return path to clear a fault because its high resistance (one billion times that of copper, per IEEE Std. 142 Sec. 2.2.8) will only allow a few (1 to 10) amperes to flow back to the source [Secs. 250-2(d) and 250-54].
You should ground power supply systems to the earth for two reasons. First, it prevents the destruction of electrical components from lightning, and it stabilizes the system voltage during normal operation (Sec. 250-28).