BY GARY B. “SKIP” TINAGERO
On Thursday, January 12, 2012, 12 Albuquer-que (NM) Fire Department (AFD) companies responded to a two-story multiresidential structure fire. The caller stated that she could see the interior wall of her apartment turning black and there was a light smell of smoke. The individual evacuated the apartment with her children and notified the neighbors of the possible structure fire.
The first-in AFD unit arrived within four minutes and ensured that all occupants were out of the structure. Light smoke was inside the reported apartment of origin, and there was black staining on the surface of a wall. A 1¾-inch charged handline was stretched into the apartment from the A side of the structure. AFD units began removing the drywall of the blackened wall and located the fire in the wall’s void space.
The fire was quickly extinguished, and additional drywall was removed from the walls and ceilings to search for fire extension. The fire had consumed one of the studs in the exterior wall, burned through the top plate, and extended into the second floor parallel chord floor trusses. The apartment sustained minor smoke damage and moderate damage to the living room wall and ceiling areas in the search for fire extension. The floor trusses of the second floor were heavily charred and severely weakened as a result of the fire extension.
ANALYSIS
The origin and cause investigation revealed that the fire started inside the exterior wall of the apartment. The exterior wall finish was traditional stucco (insulating board, a tar paper vapor barrier, metal lath or chicken wire, a base coat, scratch coat, and finish coat). There was no apparent heat source in the area of origin that could have caused the fire. An electrical conduit can be seen in the wall (photo 1), but it is clearly below the charred area. The only other building utility was a cable TV box mounted on the exterior of the building. The box was connected to the structure with toggle bolts (photo 2), which penetrated the stucco surface and the exterior wall sheathing.
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| (1) The electrical conduit. (Photos by author.) |
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| (2) The cable box connected to the structure. |
The cause of the fire was not immediately determined. It was hypothesized that there may have been an unknown electrical source searching for and finding a ground potential causing heating or arcing inside the wall. If that were true, it could be further theorized that the heating/arcing was responsible for causing pyrolysis of the wooded structural members until they reached an environment suitable for sustained burning.
Having eliminated all other possible sources of ignition, electricity was the only heat source that could not be eliminated as an ignition source. This electrical source must have been caused by a short circuit somewhere in or on the structure. It was thought that a “hot” lead may have come in contact with the wire mesh on the building (which is used to hold the stucco exterior in place) and energized it. This could be possible through an exposed hot wire touching a conduit or metal mounting box for the exterior lights.
The only ground potential in the charred fire area was the cable box attached to the wall. A ground of this type could complete the electrical circuit and be responsible for the heat source. The metal cable box was grounded, as is the normal practice when installing cable drops to buildings. The box was in contact with the wire mesh through the use of the toggle bolts that were used to secure the cable box (photo 3). The photo shows the connection of the toggle bolt spreaders in the wall touching the wall’s wire mesh.
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| (3) Toggle bolt touching the wire mesh. |
This theory was put to the test with the use of a continuity circuit tester. The utility provider removed electrical power to the building during fire suppression activities. The protective cover to the building’s power panel was removed. This was the panel that supplied exterior lighting at night; it was controlled through the use of a photocell. As photo 4 illustrates, one end of the continuity tester was attached to the ground wire inside the cable box and the tip of the tester was touched to the buss bar in the electrical panel. Obviously, there should never be continuity between these two items. However, as photo 5 shows, the tester light is illuminated, indicating there is a completed circuit between ground and the “hot” side of the electrical panel.
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| (4) The continuity test from the cable box to the electrical panel. |
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| (5) The test light is illuminated. |
This test revealed that the wire mesh, which supports the traditional stucco wall finish on the building’s exterior, was energized at night when the photocell turned on the power. The electrical current came in contact with the wire mesh and eventually found a ground from the cable television box. This may have taken many weeks, months, or years. The heating or arcing resulted in the long-term pyrolysis of the combustible wall components until all the conditions were right for sustained combustion.
This electrical short circuit theory was later confirmed by a licensed electrician who found the exposed electrical wires. They were found during removal of the old electrical wires in the building’s electrical conduit. The building was brought back up to code and is once again occupied.
This real-life hazard can be a very troubling situation for firefighters. As we conduct fire suppression activities, we need to be certain that all electrical sources have been accounted for and isolated. There may be unknown electrical potentials in buildings that can kill or injure unsuspecting firefighters. Be aware of the hidden dangers.
GARY B. “SKIP” TINAGERO is a battalion commander and 21-year veteran of the Albuquerque (NM) Fire Department where he is assigned to Battalion 2. He has a master’s degree in executive fire service leadership and is a student in the National Fire Academy’s Executive Fire Officer Program. He is a former fire investigator and is an adjunct instructor for the New Mexico State Firefighters Training Academy.
