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The Case of the Imperfect Bird on a Wire

The Case of the Imperfect Bird on a Wire

After a general laborer escapes serious injury following contact with electrical current, forensic investigators liken his experience to a bird on a wire scenario

Most everyone in the electrical industry understands how a bird can perch on a wire energized with thousands of volts and still sing a happy song without suffering any ill effects from the deadly current running below its feet. The “bird-on-a-wire” scenario actually proves one of the cornerstone requirements for electrical injury to occur: For current to flow from one point to another, there has to be a voltage difference between the two points. Fortunately for the bird, both of its feet are at the same voltage on one wire; therefore, no current flows through it, leaving the bird free from electrical injury.

In this month's Forensic Casebook, a general laborer found himself in a similar situation during an electrical accident — considered by investigators as lucky as a bird on a wire but under far less perfect circumstances.

The scene

It was a day like any other for an experienced general laborer. When he went to work that morning, little did he know that he would become the victim of a unique electrical accident. Working for a hazardous materials removal company, he was tasked with dismantling and salvaging an out-of-use train substation. Elevated above the ground on a metal structure, the floor of the station was made of cement. On this particular day, it was marked with puddles from the previous night's rain.

Because all of the electrical equipment inside the station had been de-energized, the victim believed he was working in an electrically safe environment. His job was to move a long, heavy pipe (located partially inside and partially outside of the station) through an opening in the wall so that it could be cut up and carted off.

The accident

In order to move the heavy length of pipe, the laborer attached a hand-operated come-along winch between the pipe and rigid metal framework of the building. Upon completing the connection, he began using the come-along to drag the pipe through the opening into the station. Just moments before the incident occurred, he was operating the come-along with one hand and had the other placed on the body of the come-along — the pipe was elevated above his shoulder. The wire from the come-along was stretched tight between the pipe and the connection point to the building's metal framework.

As the pipe inched inward, the center of gravity shifted relative to the pivot point that existed at the wall of the station where the pipe exited to the outside. At some point, the center of gravity moved from outside to inside, and the pipe quickly rotated downward, contacting the victim's shoulder. The outside portion of the pipe rotated upward, until it contacted the energized power lines used to power the trains.

For possibly as long as 30 terrifying seconds, current flowed from the wires, through the pipe, through the come-along, into the framework of the building, and then off to ground. The amount of current flow was so great that it heated up the come-along and caused the winch cable to become red hot and melt in two. When the cable finally snapped, the pipe shifted, and the contact was broken. The current had caused so much resistive heating that the cable ended up welded to the building's frame.

For the duration of the contact, the victim found himself unable to let go of the come-along. After the accident, the victim described the experience in detail: His hands were locked in place, and his calves contracted to the point that they felt like they would explode. When the come-along cable snapped and the circuit was broken, the victim fell free. Although he suffered some minor burns, it could not be determined if the burns were electrical in nature or caused by contact with the heated come-along or the come-along wire when it snapped after melting.

The victim was wearing non-insulating work gloves and steel-toed, rubber-soled work boots, as is standard for this type of work. Although the laborer had a prior history of carpal tunnel syndrome (most likely resultant from a long hard-working career), he was not prepared for the fate that awaited him. After the accident, his wrist problems became so severe that he ultimately had to undergo bilateral wrist fusion.

Investigation and analysis

Although there were others working on the same task in the substation at the time, no one witnessed the accident. Therefore, both the plaintiff (the victim who worked for the hazardous removal company) and defendant (the construction company hired by the city to remove the debris from the site who subcontracted this job out to the victim's hazardous removal company) had to rely on expert witnesses to determine if there was a causative link to the injury. Somewhat after the fact, I was brought into the case as a forensic expert by the plaintiff.

At this point in time, the case had already been substantially litigated, and the issue of liability had been determined. In other words, as a matter of law, it was held that the defendant was liable for damages. (I presumed this was because of the presence of the energized high-voltage wires in such proximity to the work area, which thus created undue risk and led to the incident just described.) Therefore, the only issue to be litigated was the amount of damages.

My task was to reconstruct the accident and explain how the victim could have been in contact with a circuit that was energized to some number of thousands of volts for a period long enough to melt the come-along wire and yet not suffer any significant external injury or burns. Although the voltage of the power lines was never absolutely identified, a search of the literature revealed that the primary voltage to power the train system was typically on the order of thousands of volts.

This hypothesis seemed independently confirmed, considering the relatively thick come-along cable had rapidly heated and melted in two. The number of variables, such as unknown resistances at the contact points, made it impossible to reach any more conclusions. The current flow in the pathway, which included the pipe, come-along, and structure of the building, was also unknown. However, the facts suggested that it had to be significant to cause such rapid resistive heating in the come-along cable. No other expert was put forth in the case to oppose these conclusions.

Throughout the investigation process, the most difficult question to answer was why the victim had not suffered more serious external injury. Assuming the pipe and come-along were elevated to the voltage of the power lines, one would expect that a contact of such long duration would have caused serious burns in the majority of cases. Instead, the victim suffered only minor external injuries.

Based on the victim's report that the current exceeded the let-go current (approximately 14mA for an adult male), we knew that the current flow caused tetanic (maximal) muscle contraction. Because burns are a function of time, current, and contact point resistance, the lack of burns for such a long duration contact suggested a current flow that was not that great. (Common wisdom says that it takes roughly 1A to cause instantaneous burns, and a much lower current for a longer duration can also cause burns.) This suggested the current flowing through the victim was far less than 1A.

In my final analysis, I concluded that the victim was incredibly lucky. Multiple circumstances had come together that day, ultimately making him an imperfect bird on a wire. Although not fully insulated from a pathway to ground, circumstances had conspired to substantially insulate him from becoming part of a significant pathway to ground. Perhaps it was the elevated nature of the structure, maybe he was lucky enough to be positioned in the right place on the damp cement floor, or possibly it was the resistance in the soles of his shoes. Although no one will ever know for certain, the resistance between his contact point with the energized pipe and come-along — and the pathway from his feet to ground — was substantially resistive so as to allow only a small amount of current to flow.

Unlike the bird on the wire that sees no current flow, the current in this scenario was certainly enough to prevent the victim from letting go of his points of contact. It was also enough to cause maximal (painful and injurious) muscle contractions. However, it was not enough to cause the types of burns that are typical of contacts with wires energized to thousands of volts. The type of lower current contact predicts what is often called secondary injury, which is not caused from resistive heating but rather from the electrically induced muscle contractions and associated struggle to break free from the contact. Such injuries can include soft tissue injury, soft tissue tears, and even broken bones.

The verdict

At trial, opposition counsel inquired if such a contact could have caused arthritis, the underlying diagnosis that prompted the need for bilateral wrist fusion. I testified that electrical injury does not directly cause arthritis. (However, it is well established that soft tissue injuries can cause an activation or exacerbation of pre-existing dormant conditions.)

The jury returned a verdict of only $20,000 in damages. While the jury found that the shock was injurious, they also believed that the injury did not exacerbate my client's prior medical conditions — and that the wrist fusions would have been necessary even without the electric shock.

On appeal, the verdict was overturned, and my client (the victim) was given a second bite at the same apple. The case was completely re-litigated before a new jury with a different judge. Both sides made the same arguments, and I offered the same testimony. The defense position remained that electric shock could not cause arthritis. Because the fusion was a treatment for an arthritic condition, it maintained that the damages from the shock were limited to only that which was associated with the otherwise minor electrical injury. For a second time, the jury concurred and awarded $25,000 in damages.

In reflection

This was a rare case for several reasons: The overt electrical injury was far less than the contact would have predicted, and the same case was litigated twice. Despite its uniqueness, the case was also somewhat typical. The victim was a working man with a history of the kind of injuries/wear-and-tear to his body that are common among laborers who spend a lifetime tackling this kind of physically challenging work.

Both juries chose not to pay for the injuries that they could not directly link to the electrical contact. The link, which required two experts to make, between a miraculously minor electrical contact and the activation of a pre-existing but silent condition was too tenuous for either jury to accept. In essence, both juries simply could not find that the electrical contact was the proximate cause of the wrist fusion — and that all damages resultant from the activation of the existing arthritic condition were not to be paid by the defendant. Based on my forensic expertise, I respectfully disagree with both juries.

The analysis of what, if anything, could have been done to prevent the incident from occurring yields no satisfactory result. Believing that he would not be exposed to live electrical wires, the victim was doing his job and wearing the proper gear. Despite this reality, both sides were in dispute about whether the risk was foreseeable.

At electrical safety conferences, many experts teach that all electrical injuries are preventable. In my opinion, if they all are preventable, then they must also be foreseeable. Regardless, it seems that elevated diligence must be taught as part of the act of making construction environments properly safe. As in this case, when working with any sort of long conductive object (pipes, metal beams, etc.), it's also wise to survey the area within the reach of the object for all risks.

Dr. Morse is a researcher, consultant, and full professor of electrical engineering at the University of San Diego. He can be reached at [email protected].

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