250.4 General Requirements for Grounding and Bonding Find more on Grounding and Bonding
You basically perform work on two types of systems: solidly grounded or ungrounded. The general requirements for grounding and bonding these systems can be summarized as follows.
Solidly grounded systems
You ground high-voltage system windings to the earth to help limit high voltage imposed on the system windings from lightning, unintentional contact with higher-voltage lines, or line surges. You must ground metal parts of electrical equipment to the earth by electrically connecting the building or structure disconnecting means [225.31 or 230.70] with a grounding electrode conductor [250.64(A)] to a grounding electrode [250.52, 250.24(A), and 250.32(A)].
Metal parts of the electrical installation are grounded to the earth to reduce voltage imposed on them from lightning so as to prevent fires from a surface arc within the building or structure. Grounding electrical equipment to earth doesn’t serve the purpose of providing a low-impedance fault-current path to clear ground faults. In fact, the Code prohibits the use of the earth as the effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].
Grounding metal parts to earth is often necessary in areas where discharge (arcing) of the voltage buildup (static) could cause dangerous or undesirable conditions. Such an occurrence might be the failure of electronic equipment being assembled on a production line, or a fire and explosion in a hazardous (classified) area. See 500.4 FPN 3. However, grounding metal parts to earth doesn’t protect electrical or electronic equipment from lightning voltage transients (high-frequency voltage impulses) on the circuit conductors. To protect electrical equipment from high-voltage transients, you must install transient voltage surge-protection devices in accordance with Article 280 at service equipment and Article 285 at panelboards and other locations.
To remove dangerous voltage from ground faults, you must bond metal parts of electrical raceways, cables, enclosures, and equipment to an effective ground-fault current path with an equipment grounding (bonding) conductor of a type specified in 250.118. The fault-current path must have sufficiently low impedance to allow the fault current to quickly rise to a level that will open the branch-circuit overcurrent protection device. In addition, you must bond electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, and other metal-piping systems, as well as exposed structural steel members that are likely to become energized, to an effective ground-fault current path. A permanent, low-impedance fault-current path is one that facilitates the operation of the circuit overcurrent device. The earth is not considered an effective ground-fault current path.
Ungrounded systems
You must ground metal parts of electrical equipment to the earth by electrically connecting the building or structure disconnecting means [225.31 or 230.70] with a grounding electrode conductor [250.64(A)] to a grounding electrode [250.52, 250.24(A), and 250.32(A)].
Metal parts of the electrical installation are grounded to the earth to reduce voltage imposed on them from lightning so as to prevent fires from a surface arc within the building or structure. Grounding electrical equipment to earth doesn’t serve the purpose of providing a low-impedance fault-current path to clear ground faults. In fact, the Code prohibits the use of the earth as the effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].
Grounding metal parts to earth doesn’t protect electrical or electronic equipment from lightning voltage transients (high-frequency voltage impulses) on the circuit conductors. To protect electrical equipment from high-voltage transients, you must install transient voltage surge-protection devices in accordance with Article 280 at service equipment and Article 285 at panelboards and other locations.
To remove dangerous voltage from a second ground fault, you must bond metal parts of electrical raceways, cables, enclosures, or equipment together and to the metal enclosure of the system. In addition, you must bond together and to the metal enclosure containing the system to electrically conductive materials that are likely to become energized.
You must install electrical equipment, wiring, and other electrically conductive material likely to become energized in a manner that creates a permanent, low-impedance fault-current path from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second ground fault occur on the wiring system.
A single ground fault can not be cleared on an ungrounded system because there’s no low-impedance fault-current path to the power source. However, in the event of a second ground fault (line-to-line short circuit), the bonding path provides a low-impedance fault-current path so that the circuit-protection device will open to clear the fault.