A major electrical short caused by an accidental spilt pail of water in an older 20-story building not only caused loss of power but also resulted in several fires and complete loss of the fire alarm system.
It seemed like any other normal day on the job. A cleaning crew began washing the 12th floor electrical room of a 20-story cooperative residential building. Normal, that is, until one of the workers accidentally kicked over a bucket of soapy water, which ran down an exposed chase serving a vertical aluminum 3000A bus duct. This contaminated water shorted out the bus duct and caused an explosion as well as several fires. As a result of the damage, a 3000A circuit breaker tripped-but not until the scenario for disaster unfolded. The co-op management's insurance company called in our forensic engineering firm to determine the cause of the electrical short and subsequent fires.
In the beginning Following original construction, inspectors deemed the building's electrical system code-compliant. The building used a vertical bus duct, which is one of the most effective methods of power distribution. However, no fire stops or protective barriers were installed to stop water from flowing down the vertical shaft containing the exposed, uninsulated bus duct. The spilt water caused a major phase-to-ground fault on the 3000A bus duct, resulting in the melting of aluminum conductors, steel enclosures, and major electrical equipment on three floors. The accident produced a series of subsequent fires in several of the electrical closets. This increased the amount of damage by melting fire alarm conductors, thus preventing the fire alarm protective system from working.
The accident also destroyed emergency-lighting branch circuit conductors, leaving the building completely without life safety protection. The loss of main power also caused the elevators to shut down.
Picking up the pieces. Although our forensic team's challenge involved finding cause of the disaster, we first had to get the building in a condition so that residents could return as soon as possible. The temporary repair had to proceed at quite an accelerated pace, however, it could not retard our efforts in finding the root cause of the fire and explosions. Included in our scope of responsibility was the development of a new electrical design to handle present and future loads while providing measures to prevent a similar situation from happening in the future.
We installed temporary feeders to restore the electrical system to operation; this allowed occupants to return to their units. As an older building, the present structure did not meet present Code requirements in a number of ways.
The downtime while temporary service was in place gave management the chance to upgrade pertinent systems. The client recognized the benefit of using an electrical system design that exceeded NEC requirements. In the real estate market, one could debate if the so-called "minimum" electrical code became in fact the defacto "maximum" code for this building upon construction. This investigation became a textbook case of learning from experience. It proves some truth to the old adage "judgment comes from experience, and experience comes from bad judgment."Getting to the heart of the matter To begin our investigation, we first studied electrical protection devices. We noted the circuit breaker scheme included ground fault detection. This puzzled us because the ground fault detector should have opened the main power circuit and prevented the meltdown of the bus and subsequent fires and explosion. Why did the ground fault detector fail to respond? When examining the detector, we found its adjustable setting had been "retarded" to delay operation.
We then asked many questions, finding out the facility frequently experienced loss of power due to nuisance tripping of the main breaker. Management instructed its electrical maintenance personnel to adjust the setting of the ground fault detector to prevent the problem. We contacted the manufacturer of the breaker that included the ground fault detector and obtained data on the different settings of the device. It was not difficult to recognize the long delay setting of the protection device, which allowed the phase-to-ground fault to last long enough to melt the vertical bus work and cause the catastrophic consequences. Danger always arises when a person takes certain action to solve a problem but is not fully cognizant of the consequences of such action until it's too late. This is especially true when the protective features of a device are changed.
During our forensic investigation, we spent a great amount of time inspecting the life safety systems to find out why they failed. To keep costs down during original construction of the building, the electrical contractor had both the emergency power conductors and fire alarm conductors installed close to the bus duct serving as the power feeder. All these conductors were in a vertical chase, and none had fire ratings. The fires and explosion put both the emergency power and fire alarm conductors out of service.
A plan for the future. We recommended alternate methods of installation that would assure a minimum 2-hr fire rating. The analysis of the fires and explosion required a new determination of the connected load and a reasonable projection of anticipated future growth. The results of the study caused us to recommend an increase of the power system's capacity.
Recognizing the inherent weakness of the original arrangement of the building's electrical and life safety systems, we redesigned them and incorporated enhanced safety modifications. In lieu of a single 3000A bus duct system, our design called for installation of two 2000A systems, each serving 50% of the building load while providing capacity for future load growth. Located in the electrical closets on each floor is an open chase containing the vertical bus work. Each opening now has a fire stop where the bus duct pierces the floor. Also, the curbs that go around the chase on each floor extend so they now have a height of 18 in. (originally they were 2 in. high).
We relocated the emergency power feeders so they are now separate from the bus duct distribution closets. We also installed fire alarm conductors, with 2-hr rated mineral wool insulation, in rigid conduit in a place that maintains separation from the conduit feeders providing the main power. Cleaning crews were properly trained on techniques of diminishing the risk of damage to electrical systems.
Lessons to learn At our completion, we wrote a manual for the client to use as a standard operating procedure in similar buildings it owns/manages. One of the major lessons we tried to teach was for the client's maintenance personnel to think analytically when responding to a problem. Our investigation uncovered a "fault" not only in the electrical sense but also in management's response to an electrical problem.
This investigation shows why you should not rush to judgment. When an installed system fails, it is trying to alert you of a problem that must be identified and resolved. Changing a setting to prevent a ground fault from nuisance tripping is not an adequate response. Grouping life-safety system conductors with main power feeders in a vertical shaft will compromise safety and increase the damage to a system that fails. All fire detection/alarm conductors should have a 2-hr fire rating to provide proper protection to inhabitants.
A modern state-of-the-art electrical system now serves the facility. Its design exceeds Code requirements in a number of ways.
Building systems depreciate Codes undergo periodic modifications in response to changes in technology. Systems, when they become obsolete, can benefit from an upgrade. Owners, plant engineers, consulting electrical engineers, and electrical contractors should arrange, from time to time, to take the "pulse" of building systems they design or operate. It shouldn't take a fire or explosion to gain our attention. Many older buildings have years of life left in them. It's up to all of us to make sure we "live" in the safest possible environment. In the long run, it's truly the cheaper choice.
A typical floor has an electrical closet containing a plug-in bus switch protecting dry-type transformers rated at 480/277V and 480/208-120V. Each closet contained a group of utility meter cabinets and sub-mains serving each of the cooperative apartment units in the building. Adjacent to that equipment are circuits for the emergency lighting and fire alarm systems.