Controlling electric shock hazards on the work site requires more than just installing ground cables. OSHA requires that an equipotential zone be established at the work site while performing transmission line maintenance and construction activities. Linemen accomplish this by establishing a single-point work-site ground and then connecting all conductive objects to it with properly sized personal protective ground cables.

Vehicles involved in the work procedure, such as bucket trucks, cranes and winch trucks, are also connected to the work-site ground. Should the line become energized, the equipotential zone provides protection for the linemen both on the structure or on a vehicle. However, the current flowing into the soil through the work-site ground system produces a ground potential rise at the work site. Workmen on the ground standing near the work-site ground, or touching anything bonded to the work-site ground are exposed to shock hazards known as step voltage, touch voltage and transfer-touch voltage. Over the years, Western Area Power Administration (Western) has conducted numerous field tests documenting the effectiveness of equipotential grounding as well as the shock hazards associated with possible ground potential rise situations.

In the past, field tests have focused on the worst condition: accidental energizing of the line from the power system. This produces the largest fault currents for the ground cables to withstand. The large current also produces the greatest possible voltage differential exposure to workers within the equipotential zone. Ultimately, the tests prove that, as long as the ground cables do not fail, the workers within the equipotential zone are safe. However, the step, touch and transfer-touch voltages produced by this scenario can be as high as 20,000 V.

This type of event is quite unlikely and usually lasts for less than one second. Thus, the prevailing wisdom is that workers on the ground should stay clear of the work-site ground and should minimize any contact with vehicles and other equipment connected to the work-site ground.

Several electric shock events have occurred that prompted Western to take a closer look at work-site electric shock hazards. The events were associated with ground potential rise at the work site and had two significant similarities:

• The de-energized line was exposed to electromagnetic induction from a parallel energized transmission line.

• The ground consisted of a single temporary ground rod.

  The severity of the electric shock hazard produced from ground potential rise depends on two factors: the amount of current flowing to ground at the work site and the resistance of the work-site ground system with respect to remote earth. During the field tests, large step, touch and transfer-touch voltages were produced by injecting large currents into the work-site ground. However, a very small current injected into a high-resistance work-site ground also will produce severe electric shock hazards. While the likelihood of the transmission line becoming fully energized is extremely remote, exposure due to currents produced from electromagnetic induction is continuous. Therefore, establishing a low-resistance work-site ground can significantly reduce the electric shock hazard, caused by induced currents, to linemen on the ground.

Over the last year Western's Electric Power Training Center, in cooperation with their line crews, began a project to document the resistance of work-site grounds, explore the available options of reducing work-site ground resistance and compare the effect of high-resistance and low-resistance work-site grounds on shock hazards produced by induced currents. The specific focus of the project was to examine the use of multiple temporary ground rods as the work-site ground.

When using temporary ground rods as the work-site ground, the actual resistance of the ground rod to remote earth is usually not considered or measured. Generally, the quality of the ground rod is judged on its ability to conduct the fault current away from the work site. A temporary ground rod driven several feet into the soil is considered acceptable as long as there is a perceived solid connection to the soil.

The resistance of a temporary ground rod to remote earth is determined primarily by how deep the ground rod is driven and the resistivity of the soil in which it is placed. Generally, the deeper the ground rod is driven, the lower its resistance will be. Soil resistivity is typically measured in ohm-meters and is a measure of the resistance between opposite faces of a 1-m (3.3-ft) cube of soil. Several factors can affect soil resistivity including temperature, moisture, salt content and compactness. Soil resistivity varies geographically as well as seasonally. Typical resistivity values for various soil types are given in Table 1.

The resistance of a temporary ground rod can be measured using the fall-of-potential method. This process is well documented and accepted by the electric power industry. Several equipment manufacturers provide instruments specifically designed for this purpose. For this project, the Megger Direct Reading Earth Tester (Model 250260) was used.

The work sites examined include those in a wheat field in eastern Colorado, on a mountain ridge in central Wyoming, and along an irrigation canal near the mouth of the Sacramento and San Joaquin rivers in central California. The California location consisted of two work sites located about 200 ft (61 m) apart. Anywhere from two to four temporary ground rods were installed at each site. The ground rods were driven at least 6 ft (1.8 m) into the ground, separated by at least 8 ft (2.4 m) and connected together with a personal protective ground cable. Their resistance measurements are provided in Table 2.

The measurements clearly show that the resistance of the work-site ground is reduced through the use of multiple temporary ground rods. The high ground-rod resistance measured on the mountain ridge in Wyoming was expected, but it was certainly a surprise in the Colorado wheat field. A bigger surprise was the large difference in the ground-rod resistance found between the two California work sites that were no more than 200 ft apart and visually identical.

The significant finding for this part of the project is that the only way to accurately determine the quality of a temporary ground rod is to measure its resistance to remote earth. Figure 1 shows the use of the Megger Direct Reading Earth Tester being demonstrated to the line crews at the Wyoming work site.

The primary electric shock hazard at each of these work sites was step, touch and transfer-touch voltages produced by the electromagnetically induced currents from parallel transmission lines flowing from the de-energized line through the work-site ground. The magnitude of these currents varied throughout the day, because of the changing power flow on the parallel lines, but never exceeded 1.5 A at any of the work sites. With this current flowing through the work-site ground and the associated ground potential rise, the voltage profile of the soil around the work site will not be flat. The work-site voltage profile represents the voltage, referenced to remote earth, of the linemen's feet when walking around the work site. Thus far, we have focused on the resistance of the work-site ground at four work sites. The advantage of establishing a low-resistance work-site ground can be seen by comparing the work-site voltage profiles for both a low-resistance and high-resistance work-site ground at the California Site 1 (Fig. 2).

The arrangement of the four temporary ground rods used at the California Site 1 work site is shown in Fig. 3. The resistance to remote earth of this arrangement is 35 ohms. If a single ground rod were used as the work-site ground, the resistance would be 500 ohms. All ground cable connections were made to this single ground rod. The work-site ground resistance could then be changed from 35 ohms to 500 ohms by disconnecting the farthest right ground rod from the other three. The current flowing through the work-site ground system due to the parallel transmission line was measured to be 0.5 A. The effect of the resistance of the work-site ground was evaluated by measuring the voltage between the bucket truck and the soil (transfer-touch voltage) for both work-site ground configurations. The voltage measurement was made between the front bumper and the soil directly below the bumper. The front bumper represented the part of the vehicle farthest from the work-site ground; therefore, it would have the largest transfer-touch voltage.

The ground potential rise of a work site can be visualized as a topographical map where the contour lines represent locations of constant voltage instead of constant elevation. EPRI's “Transmission Grounding Guide” provides a Web-browser-based tool for computing the ground potential near a ground electrode. The calculated work-site voltage profiles for the single ground-rod work-site gound and the four temporary ground-rod work-site grounds are shown in Figs. 4 and 5, respectively. Examining the voltage profiles of the two test cases on a common voltage scale, as shown in Fig. 2, clearly shows the effect the additional ground rods have on reducing the electric shock hazard at the work site. Figure 6 shows a detail view of the voltage profile for the low-resistance work-site ground.