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Job Site Accident

The Case of the Mysterious Construction Site Electrocution

Apr 20, 2017 | Michael S. Morse
Missing ground and short circuit from an energized phase conductor lead to “hot” metal surfaces and deadly cocktail for one unlucky construction worker.

Every forensic investigation is different — different facts, new victims, etc. But the premise upon which most electrical injuries and deaths occur are typically based on the same principles.

Placing a lunch cooler underneath a construction site trailer proved to be a deadly decision by a construction worker
For electrical contact to occur, there must be two failures that happen concurrently. The first is often invisible — failure of electrical protection. In many cases, that means that the silent and invisible ground protection has ceased to exist. A missing or broken ground wire is often the common culprit, a situation that is easily remedied but only if the failure is known. The second failure often stems from the fact that somehow the electrical source extends to energize ungrounded (and unprotected) exposed conductive surfaces. When a ground is present and effective, any short from source to ground will cause high current flow and a circuit breaker to trip, thus demonstrating effective ground protection.
Once established, these two failures create a lethal cocktail that leaves exposed metal surfaces to sit quietly while invisibly energized to an injurious or lethal voltage level. While there are many open and obvious dangers on construction sites that are dealt with by codes and regulations, there can also be invisible dangers. Such dangers sit in silence (often for long durations of time), until someone is unlucky enough to find himself in the wrong place at the wrong time. That is exactly what happened on a large residential construction site to one unsuspecting laborer. He went to work like any other normal day, not knowing the hidden electrical hazard that awaited him.

The scene

On the date of the accident, the construction site was muddy from unusually heavy rains. The weather on the day of the incident was clear but very hot. Prior to the bad weather, the site had been quite dry.

To keep their lunches out of direct contact with the hot sun and weather, workers were in the practice of placing their coolers underneath the construction site trailer.

The accident

The timeline and all evidence suggested that the victim had taken a break during the late morning to get a snack. When his coworkers noticed he had been gone for a while, one of them went to look for him, ultimately finding him lying on his back unresponsive under the trailer.

When the coworker grabbed hold of the victim with one hand and one of the trailer’s leveling jacks with the other, he immediately received an electric shock. Realizing that the area was unsafe, he attempted to turn off the trailer’s electrical supply but was further shocked by the metal breaker box supplying power to the trailer. He then yanked the cord that provided power to the trailer hard enough to pull the wire free from the connector plugged into the temporary power box. He was then able to pull the man out from under the trailer. Police and emergency medical support were called. Unfortunately, the victim could not be saved and was declared dead on scene.

The investigation

As there was an obvious electrical problem on-site, all temporary power was shut down. The initial investigation determined that the grounding connection on the cable feeding temporary power to the site had never been completed. As a result, the whole site lacked any ground connection. Bonding was intact between all metal surfaces such that all metal surfaces were connected electrically but lacked any form of ground protection. (This was the first ingredient of the lethal cocktail).

Upon establishing a proper ground to the site and attempting to bring temporary power online, it was then determined that a phase-to-ground fault existed somewhere — this fault created a situation where all metal structures on-site would have been energized to the phase voltage.

Both ingredients of the fatal electrical contact were found to exist, but in this case the situation was far worse than normal. It was not just a ground failure to a single device but a ground failure that made every metal surface an electrocution risk.

The police investigated the incident. During their interviews, one of the workers admitted to receiving electric shocks previously from exposed metal. However, he failed to report the shocks to anyone.

Shortly after the accident, I was retained and participated in a site-wide inspection (along with multiple other experts) with the goal of determining what had happened and locating the source of the fault.

By the date of the inspection (one month post incident), power had been connected and proper bonding and grounding connections were in place. The location or cause of the fault still remained unknown. Measurements were taken across the site, and all metal surfaces tested had proper bonds. The on-site electrical system (as observed) appeared to be functioning safely and with proper ground protection.

Much of the original temporary power system had been placed in underground conduits so as not to create overhead hazards for the many tall vehicles moving on-site. We were told that the underground wiring had been buried to a depth that met local code requirements. During the inspection, the underground conduits were excavated.

While most of the length of conduit was found to have been buried to an acceptable depth, a length of conduit running under a heavy traffic area was found to be just a few inches below the surface. When excavated, the conduit was found to have been crushed. Insulation damage was clearly visible on one of the phase conductors, and a clear fault was observed. It was obvious that during construction, there had been a change in the grading, which brought the conduit close to the surface. Furthermore, heavy machinery passing over the shallow conduit had obviously crushed the conduit and caused the fault. (This was the second ingredient in the lethal cocktail.)

Lessons learned

Although no one will know for sure when the fault occurred, the fact that the effect was so pervasive as to energize all of the exposed metal surfaces on-site suggests that the fault could not have lasted long before workers would have received electric shocks. The hard-packed dry soil preceding the rain may have provided somewhat of a barrier to reduce the risk of shock, but that is also unknown. Certainly, the presence of the rain heightened both the risk of being shocked and that shock being lethal.

For me, the case ended after the inspection. Since there was no question of causation or liability, I assume the case settled quickly and for an undisclosed amount with the usual confidentiality agreement.

This case, like so many others, exemplifies some important lessons in electrical safety. First, one should never fail in due diligence when working with electricity. By neglecting to provide a Code-approved ground connection, the company responsible for providing power to the site had placed all workers at risk. This failure ultimately caused the death of an innocent man who was doing nothing more than trying to take a break in the shade on a hot day at work. I often lecture on how important it is to remember that even the simplest of mistakes can have irreparable ramifications and lethal results.

The second issue is that this incident did not occur without warning. Had the first person who received an electric shock reported the incident, the decedent would have likely lived to work another day. Everyone on construction sites has a duty to themselves and to others. Experiencing an electric shock and failing to realize that it might cause injury or death if left unreported is an act that cannot be forgiven. The loss of life in this case was unnecessary – the unfortunate result of individuals failing in their duty to do a job properly and to be responsible to those with whom they worked.

Morse is a professor of electrical engineering at the University of San Diego. He has reviewed hundreds of electrical injury cases. His research has focused on studying the effects of electricity on the human body. More information on electrical injury can be found at http://www.electricalinjury.com.