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Delayed Accident Investigations

Posted By David Young, Saturday, November 01, 2003
Updated: Thursday, February 14, 2013

About twenty times each year, I am consulted on accidents involving our electric power delivery system. Most of them are accidents involving property damage. The following are some examples of this type of accident: a house fire that appears to be of electrical origin, a truck is damaged when hitting wires, a truck hits a anchor guy which results in two poles breaking and three spans of wire and equipment falling to the ground, appliances burning up when a high voltage line drops on to a lower voltage line. On the average about six times each year, I am asked to investigate electrical contact accidents involving injury or death. Fortunately, with most accidents, I’m called in immediately. In those cases, except for the victim being removed from the scene by emergency medical personnel, the scene usually hasn’t changed much. The circuit is usually still out (de-energized). The conductor at the point of contact has an obvious pit or metal spatter. The ladder the victim climbed to his death on is still in place. The power fuse that blew when the ladder hit the phase is still hanging in the cutout door. The grass at the foot of the ladder is still smoldering. The neighbor who watched him do it but didn’t say anything is still there. People are still standing around trying to figure out why this guy did what seems to be such a stupid act. In most of these cases, it is easy to determine what happened from the evidence because most of the evidence is still there.

There are also cases where I’m brought months after the accident. In one case, I was asked to investigate an accident that happened three years previously. A man said that he heard a loud bang and received an electrical shock while getting into his car. The car was parked at the curb next to a utility pole. The loud bang came from the failure of one of the high voltage lightning arresters protecting a three-phase transformer bank at the top of the same pole. The victim said that he received the shock from a wire that was attached to the pole and running vertically up the pole. While getting into the car, the victim brushed up against the wire because it stuck out six or eight inches from the pole. Several doctors examined the victim shortly after the accident. The doctors concluded he was not injured.

Three years later, he filed a suit against my company. Except for the victim’s account, the only information we had was the troubled man’s report and his vague recollection. The wire he described as the source of the shock sounded like a grounding conductor. Fault current flowing in the grounding conductor due to the lightning arrester failure could have established a voltage in the grounding conductor from which he was shocked. I checked the weather records and found that the weather at the time of the accident was sunny. However, there had been a violent thunderstorm the night before the accident. Lightning may have partially damaged the arrester and started it into thermal runaway and eventual failure the following day. I visited the scene and found a single grounding conductor running from the ground lead disconnectors of the high voltage lightning arresters and the tanks of the transformers down the pole to a driven ground rod. The bottom eight feet of the grounding conductor was covered with a well-weathered wood molding guard in good condition stapled to the pole with what looked like copper staples.

The nonmetallic guard covering the grounding conductor is required by Rule 93D1 and 4 of the 2002 National Electrical Safety Code (NESC) for grounding conductors of lightning arresters on non-multi-grounded circuits. The guarding partially insulates the grounding conductor to protect the public from voltages caused by fault current flowing in the grounding conductor. To confirm they were copper staples, I first tested them with a magnet. To record the fact as evidence, I then scraped the copper oxide off one of the stables and photographed the staple to show the copper color. This information was important because I remembered that we stopped using copper staples about ten years previously. I later produced evidence showing proof of our switch to galvanized steel staples. If it were our grounding conductor that shocked the victim, our people would not have used an old wood molding and copper staples to cover the grounding conductor after the accident. I suspected there must have been another grounding conductor on the pole at the time of the accident.

I dug around the ground line of the pole and found the top of a second ground rod. About forty inches below the bottom of the transformer tanks, typical communications cable attachment height, I found a ten inch piece of #6 AWG solid copper wire attached at one end to our grounding conductor. Wires like that are often used by communications companies to bond their grounding conductor to our grounding conductor as required by Rule 97G. However, in this application, there should not have been a bonding because the high voltage source was a delta source for which Rule 97A requires separation between the grounding conductors of surge arresters of circuits greater than 750 volts and grounding conductors of secondary circuits of less than 750 volts. Mixed in with all the climber gaff holes on the pole, I also found a series of paired small holes like bird tracks running up the side of the pole. The size and spacing were very similar to the staples used by the communications companies to run bare grounding conductors. The ground rod, the staple holes and the bonding conductor all indicated that there was once a communication cable and associated grounding conductor on this pole. The communications cables on that street were now on separate poles on the other side of the street. I called the communication companies who owned the cables on the other side of the street and asked them if they once had attachments on the pole in question. The cable TV company said they moved their cable to the telephone company-owned poles on the other side of the street about two years ago.

My report was passed on to the plaintiff’s attorney. At my deposition, the plaintiff’s attorney agreed to drop the suit against my company if I would testify on their behalf against the cable TV company. The cable TV company settled out of court

In another case, I was asked to investigate an accident several months after the event. An elderly woman had tripped on a coil of wire while walking on a sidewalk at night. The woman suffered a broken hip as a result of the fall. My company had no record of the accident. A relative of the woman had taken photographs of the scene on the day following the accident. I visited the scene of the accident; the wire was gone and I found nothing that would help me identify the owner of the coil of wire. The photographs however proved to be the only evidence I needed.

I closely examined the photographs under our 7X-450X zoom binocular microscope. Shown in two of the photographs was the coil of wire on the sidewalk, with one end of the wire extending up the pole to the cable TV conductor. Using the micrometer stage of our microscope I was able to accurately measure the diameter of the wire and the adjacent pole images in the photograph. I returned to the scene and measured the diameter of the pole as seen from the camera position. From the actual pole diameter and the ratio of the wire diameter to the pole diameter images from the photograph, I was able to accurately determine the diameter of the coiled wire.

The wire diameter was half the size of any conductor my company uses. Also shown in the photographs was a house with boarded up windows within service distance of the pole. All the other houses on the street had cable TV aerial services except the boarded up house.

My report was passed on to the plaintiff’s attorney. At my deposition, the plaintiff’s attorney agreed to drop the suit against my company if I testified on their behalf against the contractor who was hired to board up the house and disconnect the cable TV and electric service.

National Electrical Safety Code and NESC are registered trademarks of the Institute of Electrical and Electronics Engineers (IEEE).


Read more by David Young

Tags:  November-December 2003  Other Code 

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