It seemed like any other day on a large commercial hog farm. Workers went about their daily chores one summer day, tending to the hogs as usual and leaving the farm around 5:30 p.m. At this time, everything appeared to be in order with the 1,800 hogs housed in the finishing building — where hogs are kept and fed prior to delivery to the slaughter house. Bright and early the next morning, however, the same workers arrived as usual — only to find a gruesome discovery. All of the hogs in the finishing building had died overnight as a result of asphyxiation due to carbon dioxide poisoning.
What had led to the untimely death of these poor little pigs? It soon became obvious that the problem stemmed from a failed ventilation system. Because the hogs were kept in pens — unable to move about freely — mechanical ventilation was required to prevent a buildup of carbon dioxide. Overnight, an electrical malfunction had caused loss of power to the finishing building, which led to the failure of the ventilation system and subsequent buildup of carbon dioxide in the space — a lethal combination that ultimately led to the swine's demise. In the investigation that followed, experts were charged with determining what was behind this sudden power loss, which prompted the accident.
The system. To answer this question, a comprehensive analysis of the electrical service was necessary. Electric service to the premises was 120/240V, single-phase, 3-wire from a pad-mounted transformer, owned by the electric utility company. The primary feeder to this transformer was underground, as was the 120/240V secondary feeder to the service entrance equipment of the farm. The secondary service feed from the transformer entered a current transformer cabinet and was then distributed to two fusible disconnect switches that served as the main disconnect devices for the entire farm. Two 3/0 copper cables with PVC insulation in the housing of a safety switch were connected using compression connectors.
The lawsuit. The farm's insurance company subrogated against the electrical contractor that installed the fused disconnect switch, asking for reimbursement of the cost of the lost hogs. The insurance company alleged that the electrical contractor, having installed the fused disconnect that ultimately failed, was responsible for the installation and suitability of the equipment; therefore, it was liable for the accident.
The farm's insurance company hired a forensic engineer (plaintiff's expert) to investigate the cause of the loss of power to the finishing building. I was retained by the defense counsel (representing the electrical contractor) to determine if the plaintiff's findings were correct.
The investigation. Electrical repairs had been completed prior to the arrival of the plaintiff's investigator; therefore, the electrical system was operating normally. The plaintiff's expert found that the left splice on the load side of the wiring leading out of the fused disconnect switch (and providing power to the finishing building) had faulted to ground, prompting loss of 240V to the finishing building and causing the ventilation fans, which operated on 240V, to stop working. The location of the fault is shown in the Figure (sketch made by plaintiff's expert).
The fusible disconnect switch that served the finishing building is seen in Photo 1 on page 20 after the repairs were made. Note the arc mark on the left side of the cover. Photo 2 shows a section of copper cable that was a part of the splice in the left side of the disconnect switch. The corrosion appears white in some places and green in others. Without having a sample analyzed, possible corrosion products included copper chloride (white) and copper chlorate (green). The evidence gathered by the plaintiff's expert indicated some corrosion had occurred before failure of the splice.
Based on my examination of the plaintiff's photos, I found that the right splice (Photo 3 on page 22) exhibited evidence that the connection (splice) was overheating. Photo 4 on page 23 shows the corrosion found on the splice on the right. Photo 5 on page 23 illustrates a close-up of the copper cable that was clamped in the right splice. The wire insulation was examined and observed to be PVC.
The splices and their attached pieces of wire were sent to a materials laboratory for examination. A series of tests were run on the corrosion stains found on the uninsulated portion of the copper electrical cable. The results of these tests indicated that the corrosive agent was most probably chlorine that outgassed from the PVC insulation and formed hydrochloric acid in the presence of humid air. Hydrochloric acid rapidly corrodes copper.
In order to evaluate the corrosion that was observed in the plaintiff's photographs, I needed to examine the physical evidence. As a result, the defendant's attorney demanded that the plaintiff produce the cable section shown in Photo 2. Neither the plaintiff's expert nor the plaintiff's attorney could produce such evidence.
Although the missing link was not available, certain facts were still clear to me. As a result of my work, I determined that chlorine gas appeared to attach to and corrode the copper wires, resulting in the deterioration of the splices. One of the plaintiff's photos indicated that the cause of the failure was the corrosion of the copper wiring by the chlorine gas. I demanded to examine the piece of evidence that the plaintiff's expert had gathered. Once again, the plaintiff could not accommodate this request. As a result, my client moved for summary judgment against the plaintiff. The court granted my client's motion, and dismissed the case on the basis of spoliation of evidence.
Lessons learned. Based on my investigation in this case, I drew several conclusions. First of all, I believe the incident occurred as a result of outgassing of chlorine from the PVC cable, which, in turn, combined with moisture in the damp finishing building to produce hydrochloric acid that corroded the copper cable and splice connector. These conditions combined eventually caused the resistance of the splice to increase, causing the plastic tape covering the splice to deteriorate.
The electrical tape covering the splice would have exhibited signs of heat damage before the failure and the fault to the grounded enclosure housing occurred. Had regular inspections of the electrical equipment been conducted, deterioration of the tape would have been detected, and corrective action could have been taken before the incident occurred.
While this conclusion could not be proved without access to the cable section shown in Photo 2, it is my conclusion that the copper 3/0 cable, a typical product choice of this type of application, was improperly manufactured — in that the copper conductor strands of the cable were not protected against chemical attack by the chlorine present in the PVC insulation. In my opinion, this was the proximate cause of the failure of the splice connector and the resultant loss of power in the finishing building at the hog farm.
An expected selection for the electrical cable in the environment of the finishing building would be THW cable, which is thermoplastic insulation designed for use in areas of heat and moisture (wet). Study of the compression splice that I was able to examine indicated that the compression splice was properly assembled and was suitable for the application.
If the 3/0 electrical cable involved in the incident was improperly manufactured and allowed chlorine to outgas, the electrical contractor would not have been aware of such a defect and would have no way to test for such a condition. The electrical contractor would make his cable selection based on the application and rating of the cable. Therefore, he would have no reason not to trust that the labeling of the cable by the manufacturer was correct.
Peserik is founder of James E. Peserik Associate, Inc., a consulting forensic engineering and fire and accident investigation firm based in Coopersburg, Pa.
The Case of the Self-Closing Switch
Several readers wrote in with similar comments regarding the “Forensic Casebook” article on page 14 of the October 2006 issue — all questioning how the accident could have occurred had the switch been truly “racked out.”
In response to these comments, the author writes, “The foreman on the site had claimed the switch was ‘racked out.’ He was not familiar with the method of de-energizing the switch and thought that once they had pulled it a few inches out, it was ‘racked out.’ What was really happening is that about ¼ inch of the blades were still in the gear. When the unit closed, the power flowed through the ‘racked out’ stabs!”
I apologize for any confusion this may have caused. — John Nicholas, JNLV Consulting, Las Vegas