Lack of breaker maintenance escalates a destructive arcing fault and leaves an office high-rise building with major fire and smoke damage.

When a prolonged arcing fault in the 480V electrical system of an 18-story California office building destroyed some electrical equipment, the result was devastating. A maintenance worker was fatally injured, scores of tenants were displaced due to fire and smoke damage, the building was without power for more than two weeks, and the owner had to close the site for more than two months for repair and cleanup.

In the end, the owner faced a bill of $22 million in physical damages as well as another $10 million in business interruption claims. Needless to say, the building owner and tenants expected some fast answers. Who was ultimately responsible for this destruction? The owner filed damage claims through his insurance company against the building maintenance company and manufacturers of many of the electrical components involved in the failure.

Our task, as forensic engineers, was to represent one of the defendants in the investigation and ensuing trial. After arriving on the scene the day after the accident, we found that an early version of an aluminum bus duct system distributed electric power in the building to panels on each floor. The 5000A main bus duct ran from the basement to a switchboard on the seventh floor. From there, bus duct feeders extended from one feeder circuit breaker to the 18th floor, and a second one ran down to the basement. On each floor, the bus duct fed distribution panels (consisting of fused disconnect switches) accessible through non-interlocked doors.

We decided the problem started in a second-floor panel, when a maintenance person tried to replace 480V fuses to restore escalator service. The arcing fault fatally injured the worker and vaporized most of the interior panel. According to witnesses, neither the 2000A feeder circuit breaker nor the 5000A main circuit breaker tripped during the minute-long arc. The secondary-side fuses on the utility’s transformer (located in a vault under the street) finally cleared the fault.

To help our client, we had to establish the cause of the arcing fault. We also needed to find out why the feeder and main circuit breakers didn’t interrupt the fault current and limit the damages. Our investigation revealed the fault occurred on the line side of the disconnect switches (inside the panel). When the technician removed the fuse, this caused movement in one of the fuse clips. The movement, coupled with degraded insulation and close spacing of the bus bars, created a line-side short circuit. The resultant fault current should have tripped the breakers, but it didn’t. We had to learn why.

We inspected the damaged panel, fuses, and bus ducts to trace the course of the arcing fault. We reviewed engineering drawings of the panel and later learned from the manufacturer that these panels were designed specifically for the structure. We carried out fault-current calculations using the Kaufmann and Page (General Electric engineers) low-voltage arcing-fault model, and calculated arcing time based on our estimates of the weight of aluminum, copper, and steel vaporized by the arc. We also conducted arc-damage tests at the High-Current Laboratory of Pacific Gas and Electric Co.

The circuit breakers weren’t equipped with ground-fault protection. That’s why they didn’t trip on low-level line-to-ground arcing-fault currents.

After years of preparation, the trial lasted 10 months. During that time, experts from a variety of scientific and engineering disciplines argued about the cause of the arcing, the magnitude of the arcing-fault current, and whether the breakers should have tripped. To help the jury understand the technical testimony, the parties used hundreds of exhibit boards with photographs, photomicrographs, scanning electron micrographs, fuse curves, circuit-breaker tripping curves, time lines, and engineering drawings. The jury saw the damaged remains and mock-ups of the electrical equipment. We also showed a video that reconstructed the progress of the arc. The case settled at the close of evidence, just before it would have gone to the jury.

As a result of the accident, the owner renovated the building to bring all of the services up to code. Installers interlocked the fuse-compartment doors on the busway fuse panels with the switches. The obvious lesson from this accident is maintain your circuit breakers and other protective equipment—as the old saying goes, “an ounce of prevention is worth a pound of cure.”

Kusko is corporate vice president, Exponent Failure Analysis Assocs., Natick, Mass; Jacobs works for Harvey, Siskind, and Jacobs, LLP, San Francisco.