NASA-LLIS-0005

Abstract

<strong>Description of Driving Event:</strong> <br>A pair of shuttle orbiter teleprinter 28 VDC power cable wires shorted together. A short section of wire and insulation ignited and burned releasing a small amount of contaminants within the crew cabin. The burn through produced an open circuit, but was of insufficient duration (150 milliseconds at 50 amps) to open the circuit breaker. The crew manually opened the breaker and removed the cable. An investigation determined the cause of the failure was inproperly designed strain relief at one end of the cable where mated. The resultant flexure of the cable caused 360 degree annular breaks in the insulation of the power wires adjacent to each other. The cable was redesigned to utilize a more effective 90 degree bend strain relief device. The wire insulation material was changed from kapton to the more flexible teflon. Other orbiter cables of similar design were inspected and designs reviewed to preclude problems in similar configurations. This incident involved a non-mission essential component and the crew was potentially exposed to voltages below human shock hazard levels. However, the failure produced an ignition source.