Two electricians receive burns while using a poorly designed test instrument with inadequate leads.
Just before the arc flash severely burned the two electricians, they were searching the textile mill for a 480V feed to power the facility's new sewing machine. When they found an available fused disconnect switch, one of the electricians opened the panel door with the trip device - which allowed the downstream end to remain hot. With both hands on the tester, he applied the small leads to the hot terminals in the electrical panel to identify any present voltage. Almost immediately, the insulation in the tester failed - causing a 3-phase flashover arc that resulted in severe burns to the electrician's hands and face. It also burned the other electrician standing close behind.
After the accident, the electricians filed a personal-injury suit against the switch-panel manufacturer, whose counsel retained me to study the case. The principal allegation was that the separator bar (made of a synthetic material offering a very high electrical resistance) in the panel was designed to become conductive. Unfortunately, the arc destroyed the tester. We could not identify any remains in the residual at the vicinity of the incident. Fire residual coated the entire inside of the switch panel and the separator bar of the switch disconnect mechanism.
Because the mill had changed hands several times since the incident occurred 13 years before, we couldn't find any eyewitnesses. The few photographs taken at the time of the accident did not give a clear picture of the situation. Based on these facts, we finally persuaded counsel to arrange a visit to the site. We were surprised to find the mounting of the adjacent switch boxes were unchanged after all those years. Some of the fire's residual were still present, but we found only one switch changed. We took measurements and photographs of the scene.
A detailed study of the fused disconnect switch revealed no evidence it had caused the flashover. It was very clear that the switch itself was a victim of an intrusion of some sort. The flashover had badly melted the fuses and brackets at the lower end of the panel and burned the finish on the inside and outside of the box. There were no holes in the box - which could have indicated a direct short to the box itself. Because the separator bar had to be in permanent and direct mechanical contact with all three lines, it had to be mechanically strong and a very good electrical insulator. The arcing left it undamaged, except for some surface discoloration.
We bought a tester of the same make and model the electrician used during this incident. Designed to indicate potentials of 120V, 240V, 480V, and 600V, the unit was about 5-in. long, 5/8-in. wide, and 3/8-in. thick - with the test leads entering at one end. The leads were only 3-in. long - including the probes! After dissecting the unit, we found that less than 1/8-in. separated the lead terminals. Neon bulbs indicated measured voltage levels. The very close unprotected terminals for the leads offered easy access to any intruding pollution, which could easily cause a short.
Both electricians carried one of these instruments in a "head-down" position in tool pouches on their belts, with the leads extending upward. As they worked that day in the hot, humid climate, it was likely that perspiration dropped into the tool pouches and into the tester terminals - thus creating highly conductive "shorts" in the unit. However, no one attempted to duplicate the incident.
For the trial, the construction of an imitation electrical panel reflected the same configuration as the switch box in question. The construction of another electrical panel, similar to a switch box close to the panel where the incident occurred, was also used as a visual aid at the trial. The imitation panels were in the same relative positions as they were during the incident. Recreating the setting showed the switch box in question was so close to its left-hand neighbor that its door would only open about 45. This made it very inconvenient for the electrician to perform his work. Because he had large hands, he also found it difficult to apply the very short leads of the tester.
The manufacturer's representative tested the actual separator bar with a 10kV source in the courtroom, demonstrating to the jury that the unit was indeed designed as a good insulator (not as designed to fail - as the plaintiff alleged).
It seemed very clear that something - probably perspiration - had intruded into the terminals of the tester leads. When the electrician touched the 480V conductors, the leakage current escalated and caused the flashover. When using the rms values of voltage, the peak values are substantially greater. In the case of 480V subscript rms, the peak value is 678V. Ohm's law stipulates that the higher the voltage the more the current flow.
Looking at the scene and the switch itself, it became clear what had happened. Although the tight placement of the switch panels was a serious aggravating factor, the electricians should have used better judgement by choosing a better designed test instrument with leads of a reasonable length. The jury agreed with me and denied the plaintiff's claim.
Note: The manufacturer of this tester was an offshore firm - and thus unassailable in court.