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Electrical contractors are now often required to be familiar with not only the National Electrical Code (NEC), which applies to service installations, and equipment and appliances in occupancies, but also with the National Electrical Safety Code (NESC) provisions, which apply to electrical supply lines and substations. While the NEC contains rules that are subject to rigid interpretation, the NESC, originated in 1915, governs equipment and installations that are much more frequently influenced by the vagaries of the environment. Therefore, additional safety factors are often judiciously applied to the rules of the NESC, depending upon local operating conditions. This article describes how the NESCis applied in analyzing an accident involving overhead lines.
In 1982, a truck loaded with logs was being driven near Ellsworth, Maine. On a small country road about one mile east of a state road, the logs snagged several telephone conductors. The driver climbed onto the load of logs, which were about 14½ feet above the ground. While he was lifting the telephone conductors, the side of his head contacted a single-phase, bare No. 4 ACSR 7.2 kV primary tap line dating from 1956 or earlier. He suffered burns about his scalp, both hands, and left forearm. The fault current flowed entirely from the primary line through the truck driver’s body, exiting into the telephone lines. No flashovers occurred between the truck’s tire rims and the ground.
The inadequate vertical clearances of the telephone and power lines above the ground, including their excessive sags, were the principal factors leading to the accident. Rule 232 of the 1948 NESC required a minimum vertical clearance of the power line above the ground of 18 feet for a conductor running along a road in a rural district, and 20 feet if the conductor crossed the road. In this case, both poles between which the primary conductor was suspended were installed on the same side of the road. However, because of the curving character of the country road and the use of a 93-inch outrigger or alley arm on one of the poles, which had been installed in order to keep the conductor away from tree crowns on both sides of the road, the span was suspended well past its centerline. Thus, the minimum clearance of 20 feet applied in this case.
The same NESC rule required that a communication conductor be maintained at a minimum clearance of 18 feet. Communication conductors could have minimum clearance requirements as low as 8 feet when the ground under them was accessible only to pedestrians, or 13 feet if they ran beside a public road in a rural district.
Although the contact occurred away from midspan, where the conductors are usually at their lowest, in this case, the lowest point was at the accident site because of the sloping character of the country road.
This road was barred from general public access by a chain strung at its intersection with the state road. Even if the road had been privately owned, the NESC would still have applied to it. Most distribution lines, for which easements have been obtained, are strung over private property.
Per Rule 202 of the 1948 NESC, clearance requirements were considered minimums, which should be exceeded if required by local conditions. In this case, however, even the minimum clearances were not met.
The primary conductor’s clearance at the site of the contact was measured at 17 feet, which is 3 feet below the required minimum. Given the contact of the primary tap line on the side of the truck driver’s skull, an additional clearance of only one-half foot was needed to completely clear the high-voltage conductor. Thus, there would still have been a safety margin of 2½ feet. The communication conductor clearances were measured at 11½ to 13½ feet above the ground, a shortfall of 4½ to 6½ feet, thus ensuring that they would be snagged by a load of logs that were 14½ feet above the ground.
Conductor sag or droop was another factor to be considered in the post-accident analysis. Sag tables are prepared by wire manufacturers, and generally adopted by utilities for determining proper installation. These tables specify the maximum permissible sag for a given type of wire, span length, temperature, and wind and ice loading. In this case, the ACSR conductor’s sag table requirements were not met. The sag was excessive by about 6 feet and the primary conductor would have been 23 feet above the ground at the contact site if the proper sag had been present. Moreover, the heights of the primary conductor at its points of attachment on the two poles could have permitted proper minimum clearance and sags to be obtained.
The situation was worse for the telephone lines, because the heights of attachment above the ground of these lines to the same poles were so low that minimum required vertical clearances could not be obtained, even if the conductors had been perfectly taut.
This article illustrates a typical application of the NESC to overhead line planning, in which isolation of power lines is frequently relied upon rather than solid dielectric insulation. EC
MAZER is a consulting electrical engineer who currently specializes in electrical safety issues. His telephone number is (202) 338-0669, and his e-mail address is [email protected].