The Case of the Recurring 911 Call

Oct 1, 2000 12:00 PM, By Howard F. Prosser

A downed overhead power line sets off a series of fires in a junkyard.

Just as water always seeks its own level, electricity always flows in a circuit back to its source. This became ever so apparent in an unusual set of fires I recently investigated for an insurance company. The fires began from an unknown source, when a phase conductor from a 12kV overhead feeder fell on a pile of cars waiting for recycling in a junkyard.

Although witnesses called the fire department, the heat from the car fire also damaged a barn-type garage, located on the adjacent property (separated by a 6-ft-tall corrugated steel fence). A short time after leaving the scene, the fire department got a second call to return to the area. This time, the fire struck a wooden machinery shed, which was 150 ft away (but adjacent to another portion of the same corrugated steel fence) from the initial fire.

I quickly realized the importance of the fence. The corrugated sheet steel fence ran north/south - almost directly under the parallel overhead lateral line. Then, it turned west at the machinery shed. A separate phase conductor on the same feeder fed the transformer serving the machinery shed. Unknown to the firefighters, the downed power line had electrified the steel fence! Luckily, no one was electrocuted.

As the fault current from the downed conductor tried to flow back to the grounded neutral of the overhead distribution feeder, it found a path through the steel fence and earth. Arc marks on every joint between fence panels confirmed the current path. The currents flowing through the fence heated ends up to incandescence. Since the end of the fence abutted the machinery shed, it also caught fire.

After the firefighters returned to the station a second time, another alarm called them back to a fire in a nearby restaurant/apartment building. This two-story property was also suspiciously located next to the junkyard, separated by the same steel fence. By the time the firefighters answered the third call, the fire (originating from a gas meter in the basement) had burned through to the first floor. This fire occurred because the fault current now flowed in the underground water pipe, which had a lower resistance than the surrounding earth.

As in this case, gas pipe lines inside buildings are grounded by connection to water pipes. Through this connection, the fault current returned back to the grounded neutral of the secondary windings of the wye-connected pole top transformer via the water pipes.

Since a potential difference now existed in the gas line going to the restaurant, the high-voltage fault current jumped across and arced through the dielectric union between the metal gas pipe and the meter - burning the insulating gasket and releasing natural gas that the arc ignited. Fortunately, there was no explosion, and the building was not a total loss.

While extinguishing the restaurant fire, the fire department received yet another call from a house located next to the restaurant. Again, a gas meter was the culprit. Since the meter was outside, there was no damage to the house except for some scorched brick. Once again, the fire stemmed from a high-voltage fault current that destroyed the dielectric union at the gas meter.

In the beginning, I was somewhat unsure of the story behind this bizarre chain of events until I saw the fire department's photos, including the melted and arced union at the gas meters as well as the melted conductors from the feeder that fell on the junk cars. Although no one ever completely determined the cause of the first fire, the high current flowing through the car bodies probably ignited the interiors.

Since there were so many claims left to resolve, my biggest challenge was to explain the connection between the four fires. I presume the matter settled out of court because I was only asked to issue a report.

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