POWER LINES AND TREES
Charles C. Roberts, Jr., Ph.D., P.E.
An interesting fire scene presents itself as depicted in the following photographs. Figure 1 shows a badly burned microwave oven that was not in use at the time of the fire. The control circuit boards and wiring were badly burned from a fire inside the unit. Burn patterns on the cabinet above and ceiling tile suggest a fire origin in the microwave oven control circuitry.
Figure 2 shows an electric clock in the same room, which has become detached from the wall as a result of internal ignition of electrical components. Burn patterns suggest a fire origin in the clock. Several other appliances in the kitchen suffered similar damage and appeared to be fire origins.
Attic wiring (Figure 3) sustained electrical insulation breakdown, causing spot fires in the attic. Several random fire origins were found at various locations where wiring had been installed.
Figures 4 and 5 show typical random burn areas on the exterior aluminum siding. There was no evidence of lightning in the area at the time of the fire. Apparently, the aluminum siding acted as an electrical conductor with burned areas found at gaps between sections of siding, as a result of electrical energy dissipation.
Figure 6 yields a clue as to the probable cause of the fire. According to the home owner, several explosive sounds were heard in the vicinity of the transformer. In Figure 6, the single phase overhead transformer supplying electrical power to the home was nearly surrounded by a large pine tree. Several burned branches were found at the base of the power pole.
Apparently, branches from the pine tree formed a conductive path from high voltage conductors (electrical utility distribution system) to the low voltage (120V) conductors connected to the home electrical system. Tree branches and tree sap can form a conductive path to home wiring and apply damaging voltages because of the high distribution voltage associated with power lines.
Multiple fire origins at electrical equipment in the home attest to the over voltage condition that exceeded the breakdown strength of various electrical appliances and wiring. Electrocution can occur from such an incident, although, luckily, none of the occupants were injured from the event. No evidence of a defect in the transformer was found. Inadequate power line tree maintenance is the probable cause of the loss.