Washington Administrative Code › Title 296 - Labor and Industries, Department of › Chapter 296-45 - Electric power generation, transmission, and distribution › Section 296-45-903 - Appendix B-Protection from step and touch potentials-Nonmandatory

Current through Register Vol. 24-14, July 15, 2024
Protection from Hazardous Differences in Electric
				Potential
I.
Introduction
Current passing through an impedance impresses voltage across
				that impedance. Even conductors have some, albeit low, value of impedance.
				Therefore, if a "grounded"1 object, such as a crane
				or deenergized and grounded power line, results in a ground fault on a power
				line, voltage is impressed on that grounded object. The voltage impressed on
				the grounded object depends largely on the voltage on the line, on the
				impedance of the faulted conductor, and on the impedance to "true, " or
				"absolute, " ground represented by the object. If the impedance of the object
				causing the fault is relatively large, the voltage impressed on the object is
				essentially the phase-to-ground system voltage. However, even faults to
				grounded power lines or to well-grounded transmission towers or substation
				structures (which have relatively low values of impedance to ground) can result
				in hazardous voltages.
2 In all cases, the degree of the
				hazard depends on the magnitude of the current through the employee and the
				time of exposure. This document discusses methods of protecting workers against
				the possibility that grounded objects, such as cranes and other mechanical
				equipment, will contact energized power lines and that deenergized and grounded
				power lines will become accidentally energized.
II.
Voltage-gradient
			 distribution
A.
Voltage-gradient distribution curve
.
Absolute, or true, ground serves as a reference and always has
				a voltage of 0 volts above ground potential. Because there is an impedance
				between a grounding electrode and absolute ground, there will be a voltage
				difference between the grounding electrode and absolute ground under
				ground-fault conditions. Voltage dissipates from the grounding electrode (or
				from the grounding point) and creates a ground potential gradient. The voltage
				decreases rapidly with increasing distance from the grounding electrode. A
				voltage drop associated with this dissipation of voltage is a ground potential.
				Figure A is a typical voltage-gradient distribution curve (assuming a uniform
				soil texture).
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				here to view image
Figure A - Typical Voltage-Gradient Distribution
				Curve
B.
Step and touch potentials
. Figure A also shows that workers
			 are at risk from step and touch potentials. Step potential is the voltage
			 between the feet of a person standing near an energized grounded object (the
			 electrode). In Figure A, the step potential is equal to the difference in
			 voltage between two points at different distances from the electrode (where the
			 points represent the location of each foot in relation to the electrode). A
			 person could be at risk of injury during a fault simply by standing near the
			 object.
Touch potential is the voltage between the energized grounded
				object (again, the electrode) and the feet of a person in contact with the
				object. In Figure A, the touch potential is equal to the difference in voltage
				between the electrode (which is at a distance of 0 meters) and a point some
				distance away from the electrode (where the point represents the location of
				the feet of the person in contact with the object). The touch potential could
				be nearly the full voltage across the grounded object if that object is
				grounded at a point remote from the place where the person is in contact with
				it. For example, a crane grounded to the system neutral and that contacts an
				energized line would expose any person in contact with the crane or its
				uninsulated load line to a touch potential nearly equal to the full fault
				voltage. Figure B illustrates step and touch potentials.
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				here to view image
Figure B - Step and Touch Potentials