Loss of normal neutral conductor forced current to seek another return path causing a transformer ground conductor to overheat and start a fire.
A delivery truck makes its way to its scheduled stop. After presenting the Bill of Lading to the guard at the entrance gate, the truck driver proceeds south along a driveway through the employee parking lot. The truck passes beneath an overhead telephone line, catches the cable, and pulls it to the ground. Soon after, a fire breaks out in a nearby apartment building. How did the fire start?
When the telephone cable caught on the truck, it caused one of the two attached utility poles to fall and the other to bend. The latter pole supports several electric service drops and three pole-top transformers. When this pole bent, a break occurred in the neutral conductor of the utility distribution feeder. The neutral return current found an alternate return path via the electrical system ground wire, the pipes supplying the building's water service, the earth, and the building transformer's grounding system. Then, the service ground wire in the building overheated, igniting a joist and main wooden support beams in the basement. Who's to blame?
The electric utility drop is at the rear of the east side of the building. By the time I arrived, someone had removed the electric meters, and an installer connected a new telephone. I found the five electrical service panels for the building in the basement at the rear of the building.
Someone had nailed the bare ground wire for all five of the building's electrical services to one of the basement ceiling joists. The ground wire ran along the main wooden support beam of the building and connected to the water supply pipe at the front of the basement. The basement ceiling area, above where the groundwire was nailed to the joist, and the main beam were badly burned. The fire created a hole through the basement ceiling, extending into the kitchens of the first floor apartments. An inspection of the building's service entrance components and the meter base panel showed no electrical activity. However, the heat partially melted the copper groundwire inside the building.
Next, the investigation turned to the delivery truck and overhead telephone and power lines. According to the State of Wisconsin Motor Vehicle Laws (where the truck was registered) and the State of Illinois Vehicle Code (where the truck was operating), the vehicle's height was well under the 13.5 ft maximum height limit that both states impose. The overhead clearance of the new telephone cable spanning the driveway (a new cable was installed quickly after the incident) was less than 15.5 ft, which is the minimum distance the Illinois Commerce Commission (ICC) specifies. This is the minimum vertical clearance required for conductors over parking lots. To my surprise, the new telephone cable attachment point on the new utility pole was higher than the old cable position at the time of the incident!
Based on site inspections, truck measurements, examination of the electrical system and associated items, and a review of the accident report and applicable codes, I filed a report to the trucking company's insurance carrier with the following conclusions:
1. Installers improperly hung the telephone cable and its supporting messenger with inadequate vertical clearance.
2. The truck height complied with applicable codes.
3. The truck struck the low hanging telephone cable and messenger wire causing the pole on the plant property to break.
4. When the truck caught the cable and its messenger wire, the utility pole at the rear of the building pulled to the east, which broke the utility's neutral conductor leading into the building.
5. The neutral return current from the five separate services found an alternate return path through the electrical system ground wire and the building's water service, and back to the transformer supplying the building.
Since the service ground wire shouldn't carry current (per the NEC), it overheated, melted in several places, and ignited the joist and main wooden beam in the basement causing the fire.
In my opinion, losses relating to this incident were due to improper installation of telephone cables. These lines did not meet the vertical ground clearance, as specified by the NESC. Had installers given the telephone line proper Code clearance, the accident would not have occurred.
Prosser is Vice President and Director of Electrical Engineering, Engineering Systems Inc., a professional engineering firm in Aurora, Ill.