Occupational Safety and Health Admin., Labor § 1910.269
APPENDIX C TO § 1910.269—PROTECTION energized grounded object) is called the FROM STEP AND TOUCH POTENTIALS ground potential gradient. Voltage drops as- sociated with this dissipation of voltage are I. Introduction called ground potentials. Figure 1 is a typ- ical voltage-gradient distribution curve (as- When a ground fault occurs on a power suming a uniform soil texture). This graph line, voltage is impressed on the ‘‘grounded’’ shows that voltage decreases rapidly with in- object faulting the line. The voltage to creasing distance from the grounding elec- which this object rises depends largely on trode. the voltage on the line, on the impedance of the faulted conductor, and on the impedance B. Step and Touch Potentials to ‘‘true,’’ or ‘‘absolute,’’ ground represented by the object. If the object causing the fault ‘‘Step potential’’ is the voltage between represents a relatively large impedance, the the feet of a person standing near an ener- voltage impressed on it is essentially the gized grounded object. It is equal to the dif- phase-to-ground system voltage. However, ference in voltage, given by the voltage dis- even faults to well grounded transmission tribution curve, between two points at dif- towers or substation structures can result in ferent distances from the ‘‘electrode’’. A per- hazardous voltages.1 The degree of the haz- son could be at risk of injury during a fault ard depends upon the magnitude of the fault simply by standing near the grounding point. current and the time of exposure. ‘‘Touch potential’’ is the voltage between the energized object and the feet of a person II. Voltage-Gradient Distribution in contact with the object. It is equal to the difference in voltage between the object A. Voltage-Gradient Distribution Curve (which is at a distance of 0 feet) and a point The dissipation of voltage from a ground- some distance away. It should be noted that ing electrode (or from the grounded end of an the touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from 1 This appendix provides information pri- the place where the person is in contact with marily with respect to employee protection it. For example, a crane that was grounded from contact between equipment being used to the system neutral and that contacted an and an energized power line. The information energized line would expose any person in presented is also relevant to ground faults to contact with the crane or its uninsulated transmission towers and substation struc- load line to a touch potential nearly equal to tures; however, grounding systems for these the full fault voltage. structures should be designed to minimize Step and touch potentials are illustrated the step and touch potentials involved. in Figure 2.
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100
90
80
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60
50
40
Voltage remote30 from electrode
20
10
0
0 2 4 6 8 10 12 14 16 Distance from rod (ft)
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Figure 1ÐTypical Voltage-Gradient Distribution Curve
C. Protection From the Hazards of Ground- touch voltages will develop. The result of Potential Gradients. this analysis can ascertain the need for pro- tective measures and can guide the selection An engineering analysis of the power sys- of appropriate precautions. tem under fault conditions can be used to de- termine whether or not hazardous step and
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Several methods may be used to protect 2. The use of insulating equipment, such as employees from hazardous ground-potential rubber gloves, can protect employees han- gradients, including equipotential zones, in- dling grounded equipment and conductors sulating equipment, and restricted work from hazardous touch potentials. The insu- areas. lating equipment must be rated for the high- 1. The creation of an equipotential zone est voltage that can be impressed on the will protect a worker standing within it from grounded objects under fault conditions hazardous step and touch potentials. (See (rather than for the full system voltage). Figure 3.) Such a zone can be produced 3. Restricting employees from areas where through the use of a metal mat connected to hazardous step or touch potentials could the grounded object. In some cases, a grounding grid can be used to equalize the arise can protect employees not directly in- voltage within the grid. Equipotential zones volved in the operation being performed. will not, however, protect employees who are Employees on the ground in the vicinity of either wholly or partially outside the pro- transmission structures should be kept at a tected area. Bonding conductive objects in distance where step voltages would be insuf- the immediate work area can also be used to ficient to cause injury. Employees should minimize the potential between the objects not handle grounded conductors or equip- and between each object and ground. (Bond- ment likely to become energized to haz- ing an object outside the work area can in- ardous voltages unless the employees are crease the touch potential to that object in within an equipotential zone or are protected some cases, however.) by insulating equipment.
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803
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