NASA-LLIS-1188

Abstract

<strong>Description of Driving Event:</strong> <br>During routine maintenance, employees discovered a telephone junction box under a raised flooring area that was sparking and smoking. An investigative team has determined that the root cause of the event is a design flaw in some telephone system power supplies, which can result in a negative ground fault. During previous maintenance under the raised floor, a telephone cord was partially severed and the positive feed was shorted to ground. When combined with the negative ground fault associated with the power supply design flaw, the shorted cord resulted in a complete circuit with highest resistance at a small junction box located between the power supply and the telephone. Tests have indicated that at least 7.5 amps must be sustained for several minutes in order to burn the junction box. Although the power supplies have 5 amp fuses, most are wired in parallel with booster power supplies also containing 5 amps fuses resulting in a current potential of over 10 amps under the right circumstances. The design flaw has been traced to a specific version of circuit boards from a specific manufacturer. See Government - Industry Data Exchange Program (GIDEP) SAFE ALERT H9-S-02-01.