250.4 General Requirements for Grounding and Bonding.
(A) Solidly Grounded Systems.
(1) Grounding Electrical Systems to the Earth. High-voltage system windings are grounded to the earth to help limit high voltage imposed on the system windings from lightning, unintentional contact with higher-voltage lines, or line surges. Figure 250–16
(2) Grounding Electrical Equipment to the Earth. Metal parts of electrical equipment must be grounded 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)]. Figure 250–17
(3) Bonding Electrical Equipment to an Effective Ground-Fault Current Path. To remove dangerous voltage from ground faults, metal parts of electrical raceways, cables, enclosures, and equipment must be bonded to an effective ground-fault current path with an equipment grounding (bonding) conductor of a type specified in 250.118. Figure 250–19
- Metal parts of the electrical installation are grounded to the earth to reduce voltage on the metal parts 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 the earth is often necessary in areas where the 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.
- Grounding metal parts to the 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, proper transient voltage surge-protection devices must be installed in accordance with Article 280 at service equipment, and Article 285 at panelboards and other locations.
- Grounding metal parts to the earth does not create a zero reference point, nor does it reduce the difference of potential (voltage) between the metal parts and the earth. For example, if the voltage on metal parts from the utility primary neutral is 4.5 (stray voltage), grounding metal parts to the earth will not reduce this value. Figure 250–18
To protect against electric shock from dangerous voltages on metal parts, a ground fault must quickly be removed by opening the circuit’s overcurrent protection device. To quickly remove dangerous touch voltage on metal parts from a ground fault, 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.
The time it takes for an overcurrent protection device to open is inversely proportional to the magnitude of the fault current. This means that the higher the ground-fault current value, the less time it will take for the protection device to open and clear the fault. For example, a 20A circuit with an overload of 40A (two times the rating) would take 25 to 150 seconds to open the protection device. At 100A (five times the rating) the 20A breaker would trip in 5 to 20 seconds. Figure 250–20
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