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AC 43.13-1B CHG 1 9/27/01 in order to minimize susceptibility to loosen ing under vibration. If the structure is fabri cated of a material such as carbon fiber com posite (CFC), which has a higher resistivity than aluminum or copper, it will be necessary to provide an alternative ground path(s) for power return current. Special attention should be considered for composite aircraft. d. Current Return Paths for Internally Grounded Equipment. Power return or fault current ground connections within flammable vapor areas must be avoided. If they must be made, make sure these connections will not arc, spark, or overheat under all possible cur rent flow or mechanical failure conditions, in cluding induced lightning currents. Criteria for inspection and maintenance to ensure con tinued airworthiness throughout the expected life of the aircraft should be established. Power return fault currents are normally the highest currents flowing in a structure. These can be the full generator current capacity. If full generator fault current flows through a lo calized region of the carbon fiber structure, major heating and failure can occur. CFC and other similar low-resistive materials must not be used in power return paths. Additional voltage drops in the return path can cause volt age regulation problems. Likewise, repeated localized material heating by current surges can cause material degradation. Both prob lems may occur without warning and cause nonrepeatable failures or anomalies. e. Common Ground Connections. The use of common ground connections for more than one circuit or function should be avoided except where it can be shown that related mal functions that could affect more than one cir cuit will not result in a hazardous condition. Even when the loss of multiple systems does not, in itself, create a hazard, the effect of such failure can be quite distracting to the crew. (1) Redundant systems are normally provided with the objective of assuring contin ued safe operation in the event of failure of a single channel and must therefore be grounded at well separated points. To avoid construction or maintenance errors that result in connecting such ground at a single point, wires that ground one channel of a redundant system should be incapable of reaching the ground attachment of the other channel. (2) The use of loop type grounding systems (several ground leads connected in se ries with a ground to structure at each end) must be avoided on redundant systems, be cause the loss of either ground path will re main undetected, leaving both systems, with a potential single-point failure. (3) Electrical power sources must be grounded at separate locations on the aircraft structure. The loss of multiple sources of electrical power, as the result of corrosion of a ground connection or failure of the related fasteners, may result in the loss of multiple systems and should be avoided by making the ground attachments at separate locations. (4) Bonds to thermally or vibration- isolated structure require special consideration to avoid single ground return to primary structure. (5) The effect of the interconnection of the circuits when ungrounded should be con sidered whenever a common ground connec tion is used. This is particularly important when employing terminal junction grounding modules or other types of gang grounds that have a single attachment point. Page 11-72 Par 11-186PDF Image | AFS-640
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