1、Lessons Learned Entry: 1188Lesson Info:a71 Lesson Number: 1188a71 Lesson Date: 2002-03-19a71 Submitting Organization: JSCa71 Submitted by: Maria L. MurphySubject: Fire Hazard Associated With Battery Backup For Phone System Description of Driving Event: During routine maintenance, employees discovere
2、d 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 ra
3、ised 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 t
4、he 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 cur
5、rent 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-02-01Lesson(s) Learned: Damage to equipment plus design flaw res
6、ults in excessive electric power to system componentsRecommendation(s): As the interim corrective action, the manufacturer and investigative team have determined that the risk of fire can be reduced significantly if the current available in a short circuit situation can be limited. Since the booster
7、 power supplies are wired in parallel with the main power supplies, disconnecting the booster connected to power supplies with the flawed boards have the effect of reducing the shorted current from over 10 amps to near 5 amps. As indicated previously, over 6 amps Provided by IHSNot for ResaleNo repr
8、oduction or networking permitted without license from IHS-,-,-are required to initiate a fire. The booster supplies remained disconnected until the permanent corrective action was complete. As the permanent corrective action, the flawed boards were replaced with the most recent revision level from t
9、he manufacturer.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 Sciencea71 Space Operationsa71 Aeronautics ResearchAdditional Key Phrase(s): a71 Communication Systemsa71 Configuration Managementa71 Energya71 Faciliti
10、esa71 Fire Protectiona71 Flight Equipmenta71 Ground Equipmenta71 Hardwarea71 Industrial Operationsa71 Information Technology/Systemsa71 Payloadsa71 Spacecrafta71 Test & Verificationa71 Test Articlea71 Test FacilityAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2002-05-06a71 Approval Name: Ronald A. Montaguea71 Approval Organization: JSCa71 Approval Phone Number: 281-483-8576Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
