This section contains grounding requirements for systems, circuits, and equipment. Grounding electrical circuits and electrical equipment is required to protect employees against electrical shock, safeguard against fire, and protect against damage to electrical equipment. There are two kinds of grounding: (1) electrical circuit or system grounding, and (2) electrical equipment grounding. Electrical system grounding is accomplished when one conductor of the circuit is intentionally connected to earth. This is done to protect the circuit should lightning strike or other high voltage contact occur. Grounding a system also stabilizes the voltage in the system so expected voltage levels are not exceeded under normal conditions. The second kind of ground is equipment grounding. This is accomplished when all metal frames of equipment and enclosures containing electrical equipment or conductors are grounded by means of a permanent and continuous connection or bond. The equipment grounding conductor provides a path for dangerous fault current to return to the system ground at the supply source of the circuit should an insulation failure take place. If installed properly, the equipment grounding conductor is the current path that enables protective devices, such as circuit breakers and fuses, to operate when a fault occurs. The figure below illustrates both types of grounding.
The path to ground from circuits, equipment, and enclosures shall be permanent and continuous.
This requirement was extracted from NEC 250-51, Effective Grounding Path, which is more complete and fundamental to the understanding of electrical safety. It states that the path to ground:
It is important to remember the following regarding safe grounding paths:
GROUNDING OF EQUIPMENT CONNECTED BY CORD AND PLUG
Under any of the conditions described below, exposed non-current-carrying metal parts of cord- and plug-connected equipment which may become energized shall be grounded.
Under the conditions described above, exposed non-current-carrying metal parts of cord- and plug-connected equipment must be grounded. Grounding metal parts is not required where the equipment is supplied through an isolating transformer with an ungrounded secondary of not over 50 volts or if portable tools are protected by an approved system of double insulation. To ground cord- and plug-connected equipment, a third wire is commonly provided in the cord set and a third prong in the plug. The third wire serves as an equipment grounding conductor which is connected to the metal housing of a portable tool and a metal grounding bus inside the service entrance equipment. The service entrance equipment is located at the entrance point of the electric supply for a building or plant and contains, or serves other panelboards which contain, branch circuit protective devices such as fuses and circuit breakers. The third wire provides a path for fault current should an insulation failure occur. In this manner, dangerous fault current will be directed back to the source, the service entrance, and will enable circuit breakers or fuses to operate, thus opening the circuit and stopping the current flow.
The figure below illustrates the potential shock hazard that exists when no third wire, grounding conductor, is used. If a fault occurs, most of the current will follow the path of least resistance. If the worker provides a path to ground as shown, some portion of the current will flow away from the grounded white conductor (neutral) and return to ground through the worker. The severity of the shock received will depend on the amount of current that flows through the worker.
The figure below illustrates the advantage of a properly connected grounded conductor. It should be noted that properly bonded conduit and associated metal enclosures can also serve as a grounding conductor.