If a construction worker is shocked or electrocuted during a project that requires heavy digging, it's usually the result of hitting a buried line. However, in the case of a contract worker who was shocked while helping to lay a sewer line along a paved road in a suburban area, the accident involved an overhead line. The project required the worker in question and the rest of his crew to use a trenching machine and a front-end loader to transport crushed stone to place under the sewer pipe and refill the trench. Things were going fine until low-hanging telephone and TV cables running parallel to the road became snagged on the side mirror of the front-end loader.
At first, the problem seemed easy to fix. The worker was instructed to raise the telephone and TV cables so that the project could proceed. The plan called for him to temporarily pull down the electric utility neutral conductor and tie it together with the telephone and TV cables using a rope, similar to a previously made tie as shown in the Photo at right. The crew agreed that the quickest way to reach the cables and perform this task was to use the front-end loader as an elevator and lift the worker off the ground and up to the cables and conductor.
However, before the worker completed tying a knot in the rope he received an electric shock from the neutral conductor. The shock first burned him and then threw him to the ground. His injuries included severe burns to his neck, leg, and arm. His hand and lower part of his arm had to be amputated.
The worker filed suit against the electric utility, the cable TV company, and the telephone company, and I was called in to serve as an expert witness on behalf of the injured worker.
Assessing the scene. A review of the scene revealed that a single, bare-stranded 7.96kV primary conductor was located at the top of the utility poles in the span in question. Below this single-phase conductor was a single, bare-wire neutral conductor. The neutral conductor was connected to grounding wires only at the utility poles that were equipped with overhead distribution transformers. The poles with transformers were typically spaced at a three-pole interval. This is a standard design configuration for most electric utility distribution lines. It was also noted that the telephone company owned the utility poles.
A post-accident inspection by OSHA personnel confirmed that the TV and telephone cables didn't meet the vertical clearance requirements stipulated in Table 232-1 of the National Electrical Safety Code (NESC). A mid-span vertical clearance measurement at the site of the accident was found to be approximately 11 feet. The clearance requirement as outlined in Table 232-1 of the NESC is 16 feet.
A review of safety procedures. Although these low-hanging cables created the conditions for the accident, they weren't the cause of the electric shock. It was first thought that the worker in the bucket of the front-end loader reached up and contacted the primary conductor with his hand. The typical OSHA rule on working clearances is that a worker must maintain a 10-foot clearance from energized conductors. Per OSHA 1926.403(j)(3)(iii), unguarded live parts above a working space shall be maintained at elevations not less than specified in Table K-3 (Table).
However, reports from witnesses and the worker himself revealed that the neutral conductor, telephone, and TV cables were level with his chest at the time of the accident. Therefore, I concluded that he received the shock from his contact with the bare neutral conductor.
But how could that have happened? The neutral conductor can become energized under several conditions. For example, if the neutral is broken or disconnected before it reaches the utility substation, the remaining portion of the neutral becomes energized by the connections of the primary windings of the distribution transformers between the primary conductor and the neutral.
In this case, analysis of the grounding system for the 13.8kV single-phase radial power line showed that several hundred volts could exist between the neutral and ground at several places along its length. To my knowledge, the electric utility didn't test the neutral or the grounds at the poles near the accident site.
This case was settled before it went to trial. The injured worker accepted an award of several million dollars in lieu of a lengthy court case. If it had gone to court, all of the defendants would most likely have been found liable for not maintaining their overhead cables at the required heights.
Kusko is vice president of Exponent Failure Analysis Associates in Natick, Mass.