Pinched Power Cord is Latent Defect Causing Fire When Appliance Is Not in Use

Abstract

After a fatal residential fire, witness statements and burn patterns pointed investigators toward an electrically powered upholstered reclining chair as the origin. A search for exemplar recliners identified slightly different designs of the power supply, which converts house current to low-voltage direct current for driving the motor. Although the fire left no direct evidence of its cause, analysis of unburned exemplars uncovered a design defect in the power supply electrical enclosure design, causing damage to the power cord during assembly. The transformer was found to press against the two-conductor power cord, in a location inside the unit that was concealed after assembly. The newer units did not have this design defect. Investigators developed the hypothesis that over time, the sustained force of the transformer against the cord enabled the insulation to deform such that a short circuit occurred in the power cord and caused the fire — even when the recliner was not in use and if the house wiring circuit had been protected by a circuit breaker. This paper details the investigation, testing, and findings, including dissenting expert opinions. More importantly, it shows how forensic engineers conduct detective work and apply scientific principles to achieve useful results.