Origin and Extent of Fire Damage
Examination of the area of the fire origin revealed severe heat damage to the left heat exchanger cooling fan relay, designated R2-53, as well as nearby relays R2-54 and others. The R2-53 relay exhibited unique features not seen on adjacent relays, including loose terminal studs and several holes burned through its housing. Wire bundles running below the left and right heat exchanger cooling fans and the ground service tie relays showed heat damage to the insulation, with the greatest damage located directly beneath the R2-53 relay.
Determination of Ignition Source
The distinctive damage pattern on the R2-53 relay and the concentrated wire damage below it indicated that the fire originated from an internal failure of that relay. Disassembly of the R2-53 relay revealed that it had been repaired, but the repair did not meet the manufacturer's standards. According to the relay manufacturer, the housing damage was consistent with a phase-to-phase arc between terminals A2 and B2.
Circuit Breaker Examination
During the on-scene investigation, three of the four circuit breakers associated with the left heat exchanger cooling fan were found in the tripped position. The fourth circuit breaker was submitted, along with the three tripped units, to the Materials Integrity Branch at Wright-Patterson Air Force Base, Dayton, Ohio, for further testing. The circuit breakers underwent visual inspection and a series of tests: insulation resistance, contact resistance, voltage drop, and a calibration test measuring minimum and maximum ultimate trip times. The circuit breaker that did not trip showed no anomalies that would prevent normal operation; it met all specifications and functioned properly during the calibration test. However, the lab report noted that, as a thermal device, a circuit breaker is designed to trip under sustained current overload. It is possible that intermittent arcing or a resistive short occurred, or that the circuit opened before the breaker reached a temperature sufficient to trip.