The Case of the Elevator Arc Flash

While servicing an elevator at an older San Francisco high-rise hotel, an elevator technician accidentally created an electrical arc flash at the fused switch. This accident left him permanently disabled. The arc flash lasted until the fire department arrived and turned off all of the power to the hotel—a period of more than 15 min. During the incident, not a single upstream fuse or circuit breaker opened. The arcing continued until nothing was left of the fuses or switch. Was the technician, hotel, or hotel’s elevator consultant responsible for this accident?

The hotel had hired the technician to upgrade its elevator system, which used relays and stepper switches for its control logic. He was to eliminate the thousands of moving mechanical parts—thereby reducing future maintenance—and install feeders from each of the six fused disconnect switches to each new electronic controller. Suing the hotel and hotel’s elevator consultant, the technician claimed they had pressured him to work on live circuits during the conversion. One of the technician’s lead attorneys hired my forensic engineering firm to investigate the cause of the incident and report on the findings.

After investigating, we learned that the hotel approached the electrician on the job site and gave him two options: He could either work with the 400A master fused disconnect switch in a closed position, thus maintaining power to five of the six elevators; or he could open the main disconnect switch, thereby leaving the six elevators without power and causing the hotel to close. He decided to keep the main disconnect switch closed.

He turned off the switch to one of the elevator’s 100A fused disconnect assemblies, then unlocked and opened the door to the switch. He believed this was safe because he was working downstream from the switch mechanism. However, the upstream side of the fused switch was still live. For extra protection, he placed cardboard and rags over the live input terminals.

Then, he successfully removed the old downstream conductors along with the electromechanical elevator controller, and installed the new phase conductors to the replacement electronic controller. However, he realized it would be best if there were also a separate ground conductor between the switch and new controller. Unfortunately, his fish tape got stuck in the conduit. As he and his helper pulled at the tape, the cardboard and rags shifted and the steel tape brushed a live part of the disconnect switch—while also being in contact with the conduit. The result was the disabling arc flash.

During the arc flash, the helper’s hand was on a union on the electrical conduit between the 100A disconnect switch and its associated elevator controller. As the current went through the conduit via the fish tape, it heated and caused a thermal burn on the helper’s hand. The fault current never reached a magnitude great enough to blow the 400A fuses powering all six elevators. No fuse blew during the incident, because the fault-current paths did not meet rules outlined in the NEC.

After further investigation, we learned that a few weeks before this incident the hotel had discovered the 400A fused disconnect switch (powering the six elevator 100A disconnect switches) was defective. However, the hotel neglected to mention this to the technician. The technician should have de-energized the 480V, 400A main fused disconnect switch, which served each of the elevators’ 100A fused disconnect switches, before proceeding with his work.