scope:

Description of Driving Event:

During various shuttle orbiter missions, several anomalies occurred which were subsequently attributed to particle contamination within limit switches. In three such instances the particles were metallic substances and of a sufficient size (greater than 0.015 inches) to create a premature state change. In a fourth instance, the particle contamination was non-metallic which resulted in a high contact resistance and the loss of a "closed" indication. These switch failures were attributed to contamination from metallic (weld expulsion, less than 0.010 inches), glass from headers, ceramic from insulators, rubber from back-filling equipment, polymers, and polyesters. Limit switches are often used to: (1) prevent systems or components from reaching damaging stress levels and, (2) inhibiting unwanted events or conditions. The primary corrective action to prevent recurrence of these failures was to impose Particle Impact Noise Detection (PIND) testing on all future limit switch manufacturing. additional techniques included improved cleaning processes and the addition of filters to purging and drying fluids to preclude particle introduction into the components.