Electrical Case Studies
Case Study: The Importance of Visual Aids & Modeling
A 100+ year old structure located in Western Maryland was being renovated and converted into luxury apartments.The overall structure was sound, and it was completely
gutted and retrofitted with new, modern utilities, including an elevator. Three new
service entry cables, both 3 phase and single phase, were routed through and under
ground floor 2″ X 12″ interior ceiling joists and connected to panel boxes at the rear of
the structure. The joists were solid oak and actually measured 2″ X 12″.
Early on a Sunday morning, a fire occurred inside the building. The local Fire Marshal
placed the source at one of the service entry cables that ran through the interior of the
structure. These cables were fastened to the undersides of the ground floor ceiling joists
at the front of the building, and they then transitioned to holes drilled in the center of
the joists for the remainder of the run to the service panels located at the rear of the
structure. The source of the fire was placed over six feet from the transition points of
the three service entry cables. The building owner brought suit against the electrical
contractor who had installed the cables and rewired the structure. Plaintiff’s experts
opined that a bend in the cables at the transition point caused one of them (the one
identified as the source of the fire by the local Fire Marshall) to short circuit, and that
moisture on the surface of this cable allowed surface arc tracking to extend over six
feet along the cable from the transition point to the point of origin and ignite the oak
floor joist at this point. Plaintiff’s experts offered weather data collected at an airport
several miles from the fire scene that showed that dew point temperature occurred at
the airport several hours before the fire. Trident was asked by the defendant, the electrical
contractor, to help investigate the claims.
Our initial investigation revealed that the particular service entry cable identified by the
Fire Marshall as the source of the fire was not energized at the time of the fire. Plaintiff
quickly changed his position and opined that the source of the fire was an adjacent
service entry cable that was energized. Fog began to shroud the case.
In an effort to help clear the fog, Trident then built a replica of the ceiling joist cable
transition using exemplar cable and oak lumber specially cut by a local sawmill to duplicate
the ceiling joists. The replica showed that the radius of the bend in the service
entry cable was within the 5X cable diameter specifications of the NEC. We then energized
the cable and subjected it to months of temperature and humidity cycling patterned
after weather data (from the nearby airport) referenced by plaintiff. Megger
readings were taken at regular intervals throughout the test cycle, and at the end of the
test period, the integrity of the cable insulation was found to be the same as it was at
the beginning of the testing. There was no insulation breakdown what-so-ever.
Trident investigated the weather data referenced by plaintiff and determined that the
readings were taken at a weather station located outside of the “nearby” airport,
approximately three feet above grass covered earth. Trident then established through
testing and research that dew point and relative humidity readings taken inside a closed
structure such as the loss site, which was located within the limits of a small city, varied
considerably from readings taken outdoors, in an open area, several feet above bare
ground level. We concluded that data offered by plaintiff did not accurately depict
conditions at the loss site, that dew point temperature was not reached within the
closed interior of the fire site building on the morning of the fire, and that the subject
service entry cables were not covered with moisture as opined by plaintiff’s experts.
Finally, we demonstrated that arc tracking across the cable surface, even if it had
occurred, would have not been able to extend six feet over a cable surface fastened
with plastic clamps to floor joists spaced at one foot intervals. Our findings showed that
the intervening ceiling joists and clamps would have interrupted the surface tracking
and prevented it from traveling six feet or more to the point of origin of the fire.
Trident’s findings were shared with plaintiff.
Nevertheless, plaintiff elected to proceed with trying the case in court. Trident transported
the ceiling joist cable transition model to the court room so that it could be
viewed by the jury. We received the court’s permission to megger test the model in the
court room. We also prepared visual displays of our other test results as well as visual
displays of the weather data involved. The various displays conveyed our findings in a
straight forward, simplistic manner that was easily understood by the jury.
Near the end of the trial, just before jury instruction, plaintiff accepted a settlement
offer that had been made earlier by the defendant, and the matter was concluded.