Behind the arc blast that left factory technician with third-degree burns
When he arrived at work that day, an experienced machinist/technician quickly learned that one of the presses was not working properly. One of 50 employees in the factory performing various jobs on heavy machines, such as lathes and milling machines, he took it upon himself to investigate the problem. Not long after, he found that one of the circuit breaker’s attachment screws was not tightened fully. Making a conscious decision to replace the circuit breaker in the “hot” panel, he accidentally caused a short circuit when he dropped the screwdriver he was using to tighten the screws holding one of the conductors. Although he didn’t receive an electrical shock — no current actually flowed through him — he suffered burns over 90% of his body due to the plasma spray that resulted from the arc blast.
Power was fed to the factory’s main panel through a 3-phase transformer on the street. There was a circuit breaker (rated at 2,000A) and shutoff at the main panel. Power was further distributed to various machines via subpanels located throughout the factory. Upon leaving the main panel for the subpanels, there was no shutoff. Voltage delivered to the heavy machinery was nominal 240V.
The press was drawing excessive current, and every so often it tripped the circuit breaker in the subpanel that fed the press. Upon inspecting the equipment, the victim discovered that one of the circuit breaker’s attachment screws was not tightened properly. This produced a small gap (less than a millimeter) between the electric lead of the breaker and the conductor, which gave rise to arcing. Although the arcing was localized and harmless, over time it had built up a deposit around the circuit breaker’s contact (Photo 1).
After investigating other places in the machine’s circuit, the technician concluded that this loose screw was the cause for the power fluctuation and power losses in the machine. Therefore, he decided to replace the faulty circuit breaker.
Because there was no breaker between the main panel and the subpanel, the technician could not turn off the power to the subpanel without shutting down the entire plant. A plant-wide outage would require the approval of the plant president, who had already approved shutting off the power on that coming Saturday.
Rather than waiting until the end of the work week for the plant-wide outage, the technician chose to replace the circuit breaker “hot,” because he felt the machine needed attention immediately. There were three bus connections at the breaker for each phase — 240V (nominal) and 125A.
Putting on gloves rated at 600V, the victim worked under physical tension so as to make no mistakes while the circuit remained hot. He carefully removed the bad circuit breaker. Then he attached one side of the replacement unit to the three output wires and proceeded to connect the three standoff bars. While he was screwing down one of the screws, he dropped the screwdriver. A large discharge occurred, which blew out the transformers located on the power poles outside the plant, eliminating power to the entire plant.
Miraculously, the technician did not receive even the slightest jolt of electricity during the accident — no shock nor hint of electrocution. However, the incident caused severe arcing at the subpanel, resulting in the technician receiving second- and third-degree burns that kept him in the intensive care unit for six months. The fact that his burns had nothing to do with touching an electric source demonstrates that you can still receive serious burns even when electricity does not enter your body.
Investigation and Analysis
I was hired by the insurance company as a forensic engineer to represent the machine shop, after the injured technician sued the company and a host of others, including the electrical contractor.
During the initial installation performed by the electrical contractor, which occurred about a year before the accident, the screws of the problematic circuit breaker had not been tightened fully. This was the fault of the original electrical contractor who had installed all of the wiring. This left a paper thin gap between the electric lead of the subpanel and an attachment standoff, which gave rise to localized arcing. Although the arcing was harmless, over time, a layer of oxide built up on one of the tabs (Photo 1). This layer of oxide gradually reached a point (on the day of the accident) where it began to impede the flow of current. For this reason, the machine that depended on this breaker was not receiving power in a normal fashion. This original circuit breaker was later tested and found to be faulty, as alleged by the injured technician.
The dropped screwdriver caused a phase-to-ground fault and severe arcing, which was confirmed by the great level of soot found on the bus bars that served this breaker. The source of the soot was due to vaporized plastic. The fact that the technician dropped the screwdriver meant that he was not in contact with the electric circuit. This saved him from suffering an electrical shock, which could have been fatal, considering the available currents at this location in the electrical system.
The arcing was so strong that the blast vaporized the surrounding copper and plastic components. Photo 2 (click here to see Photo 2) shows the front panel (or the face plate) of the subpanel after the accident. Notice the semi-circular hole in the metal in addition to the rectangular hole on the left. This semi-circular hole was not present before the accident. The severe arcing vaporized 1/8 in. of cold rolled steel, creating this hole on the face plate. Photo 3 shows the plaster wall facing the power panel. Orange colors are due to copper vaporization. Black colors are due to plastic insulation being vaporized and possibly steel. Just to be clear — the copper and steel did not melt; they vaporized. No cooled off puddle melt was found nearby. Not only did the vapor coat the wall surrounding the panel, but it also coated the technician. The hot gases laced with vaporized metal caused second and third degree burns on the upper half of his body. Photo 4 shows where the technician stood between the wall and the subpanel — the location where he was sprayed over the top half of his body with hot plasma and vaporized metal.
The circuit breaker at the main panel did not react fast enough to break the circuit. It was a “slow-blow” surge arrester. The current quickly rose until it reached 4,000A. This blew out the transformers on the pole outside the factory. Arcing continued at the subpanel until the current blew out the transformers. In other words, the power transformers became fuses for this accident.
Clearly, this accident was the result of the original electrical system design. With the existing setup, the supply to the machine could only be turned off by shutting off the power to the entire facility. The electrical contractor who initially wired the electrical system did not put in a secondary disconnect between the main panel and the subpanel. This configuration was a clear violation of the requirement of NEC Articles 210 and 215, and 240. This is also reflected in OSHA. For example, see 1926.404(e)(1)(iv), which specifies that a shutoff for heavy machinery should be approximately 25 ft (or less) away from the machine, and all clusters of machines or components on a particular circuit should have their own subpanel with its own shutoff.
Independent shutoffs for each of the subpanels would have helped in making easy repairs to the machine and also prevented the secondary circuit breaker from tripping. In addition, the machine involved in the accident was well over 100 ft from the main panel and not in a straight line that could be visible. Furthermore, by leaving behind a spare circuit breaker after wiring the plant, the electric contractor enabled an unlicensed person without proper training to install a component that needed proper training and licensing. This component’s availability facilitated the opportunity for the technician to take it upon himself to repair the defective breaker.
The case settled before trial with the burned technician receiving an estimated $1.5 million, the cost of his medical care plus a financial settlement for the pain he had suffered. The contractor who installed the panels and wiring bore the majority of the costs involved in this settlement.
Hmurcik is a consultant at Lawrence V. Hmurcik, LLC. He can be reached at: firstname.lastname@example.org. Patel is a PhD candidate at the University of Bridgeport, Bridgeport, Conn. He can be reached at: email@example.com.