250.4 General Requirements for Grounding and Bonding Continued
To understand how a ground rod is useless in reducing touch voltage to a safe level, let's answer the following questions:
What is touch voltage?
- At what level is touch voltage hazardous?
- How do earth surface voltage gradients operate?
Touch/Step Voltage: The IEEE definition of touch/step voltage is a€śthe potential (voltage) difference between a bonded metallic structure and a point on the earth 3 ft from the structure.a€ť
Hazardous Level: NFPA 70E, Standard for Electrical Safety in the Workplace, cautions that death and/or severe electric shock can occur whenever touch/step voltage exceeds 30V.
Surface Voltage Gradients: According to ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1], the resistance of the soil outward from a ground rod is equal to the sum of the series resistances of the earth shells. The shell nearest the rod has the highest resistance and each successive shell has progressively larger areas and progressively lower resistances.
Don't worry if you don't understand the above statement; just review the table below with Figure 250.25.
Distance from Rod_______Resistance_____Touch Voltage
1 Ft (Shell 1).....................................68%.........................82V
3 Ft (Shells 1 and 2).......................75%.........................90V
5 Ft (Shells 1, 2, and 3)..................86%......................103V
Many think a ground rod can reduce touch voltage to a safe value. However, as the above table shows, the voltage gradient of the earth drops off so rapidly that a person in contact with an energized object can receive a lethal electric shock one foot away from an energized object that is grounded to the earth.
The generally accepted grounding practice for street lighting and traffic signaling for many parts of the United States is to ground all metal parts to a ground rod as the only fault-current return path. Studies by some electric utilities indicate that about one-half of one percent of all their metal poles had dangerous touch voltage.
Figure 250.26 The common practice of installing a ground rod at a metal pole supporting a luminaire serves no useful purpose.(B) Ungrounded Systems.
Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book), if a ground fault is intermittent, or allowed to continue on an ungrounded system, the system wiring could be subjected to severe system overvoltage, which can be as high as six or eight times the phase voltage. This excessive system voltage can puncture conductor insulation and result in additional ground faults. System overvoltage can be caused by repetitive charging of the system capacitance or by resonance between the system capacitance and the inductances of equipment in the system [7.2.5].
According to IEEE 242, (1) 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(D), and 250.32(A)].
In addition, ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) states, a€śOne of the dangers of an ungrounded system is that system overvoltages can occur during arcing, resonant or near-resonant ground faults [1.4.2].a€ť And, a€śField experience and theoretical studies have shown that arcing, restriking, or vibrating ground faults on ungrounded systems can, under certain conditions, produce surge voltages as high as six times normal.
The conditions necessary for producing overvoltage require that the dielectric strength of the arc path build up at a higher rate after each extinction of the arc than it did after the preceding extinction. This phenomenon is unlikely to take place in open air between stationary contacts because such an arc path is not likely to develop sufficient dielectric recovery strength. It may occur in confined areas where the pressure may increase after each conduction period.
Neutral grounding is effective in reducing transient voltage buildup from such intermittent ground faults by reducing neutral displacement from ground potential and reducing destructive effectiveness of any high-frequency voltage oscillations following each arc initiation or restrike [1.2.14].a€ť Figure 250.27
(2) Bonding Wiring Methods to the Metal Enclosure of the System. To remove dangerous voltage from a second ground fault, metal parts of electrical raceways, cables, enclosures, or equipment must be bonded together and to the metal enclosure of the system.
a€˘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 surface arcs within the building or structure. Grounding equipment to the earth doesn't provide a low-impedance fault-current path to the source 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)].
a€˘Grounding metal parts to the earth doesn't protect electrical or electronic equipment from lightning voltage transients 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 in accordance with Article 285 at panelboards and other locations.
(3) Bonding Conductive Materials to the Metal Enclosure of the System. Electrically conductive materials that are likely to become energized must be bonded together and to the metal enclosure containing the system.
(4) Fault-Current Path. Electrical equipment, wiring, and other electrically conductive material likely to become energized must be installed 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 cannot 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.
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