After touching a fuse in an open 12kV fused switch cubicle, an electrician (contracted by a California water company for annual preventive maintenance) electrocuted himself and died. The switch's sole load, a 12kV/480V transformer, was secondary grounded with high-capacity cables.
California's worker's compensation laws protected the electrician's employer (an electrical contractor) from lawsuits. However, the water company had a potential wrongful death civil suit on its hands for “maintaining an unsafe workplace.” Several weeks after the accident, the company's attorney hired me to find out how the accident happened and if the water company had any liability. If so, the attorney wanted to know if the company could pass that liability to a third party who might have had more direct responsibility for any errors that caused the incident.
After reviewing the facts, I decided to investigate the scene of the accident with the attorney and another expert. We familiarized ourselves with the electrical system at the pumping plant by scanning the one-line drawings for any anomalies in design and by observing the equipment's connections. It was a straightforward system with a single-source feed from the utility, a large backup generator, and a pair of transfer switches.
The normal preventive maintenance routine called for the utility to perform a “hot” disconnect of the 12kV power at the pole-top by unclamping a set of jumpers. However, the utility wasn't available to perform the prescribed disconnection on the day of the accident. Therefore, the contractor apparently chose to shut it down himself by opening the plant's 12kV unit-fused switch — thereby bypassing its inclusion in the maintenance cycle. When the electrician threw the switch to the OFF position, the facility's main transformer would have gone silent. The electrician only tested its 480V secondary with a voltmeter, which showed zero potential. Then, for the safety of the other electricians on the job, the foreman electrician installed a set of high-capacity grounding jumpers between the secondary and a good ground connection.
As we dug deeper into the condition of the equipment, it appeared someone had serviced and tightened various drawout breakers and bus bar connections in the main control room during maintenance operations. However, none of this work could have energized the switch. Even though protocol prohibited the foreman from servicing any live equipment, he chose to use a large shop vacuum to clean the concrete bottom of the 12kV fuse compartment of the unit-fused switch. No witnesses saw what happened next, but we found evidence of arcing at the bottom of one of the fuses and along the side of the cabinet where the foreman placed his hand.
At this point, we were still mystified as to the cause of the electrocution. At the time of the accident, the plant's backup generator had kicked in during the maintenance shutdown. To properly investigate the circumstances leading up to the tragic accident, we asked the local utility to disconnect its 12kV pole-top connection so we could inspect the 12kV fused switch with the system safely de-energized. First, we checked and eliminated the possibility of a back feed from the on-site generator.
After using personal protective equipment and a high-voltage meter to assure the system was de-energized, we made some continuity checks of the wiring that entered the bottom of the switch cubicle through PVC conduits. We were shocked to find the “hot” supply (line) wires from the power company's feed were connected to the exposed load side of the fuses in the easily accessible fuse compartment!
The load wires left the line terminals at the top of the heavily guarded switch blades, passed over a steel barrier, and continued down the heavily isolated back side of the switch cubicle to the conduit that led to the facility's 12kV/480V transformer. The fused switch was wired backward!
We came to the conclustion that the author of the shop drawings or the installation contractor must have reversed the conduits to the switch's line-side input and load-side output compartments. The wire installer merely followed the conduits and installed the cables in the obvious and logical way. Unfortunately, the installing electrical contractor failed to test the finished system.
Regardless of the fact that someone incorrectly installed the system, the deceased electrician was responsible for violating the first rule of electrical work: Always assume the circuit you're working on is hot until tested and proven de-energized. You must also remember to verify that your meter is operational before and after testing the circuit you're servicing. Most of all, this unfortunate situation illustrates another important industry lesson — never assume all installations are correctly wired (even when installed by the best contractors).
Crawford is a forensic electrical engineer and electrical contractor in Palo Alto, Calif.