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9/27/01 AC 43.13-1B CHG 1 SECTION 15. GROUNDING AND BONDING 11-185. GENERAL. One of the more im portant factors in the design and maintenance of aircraft electrical systems is proper bonding and grounding. Inadequate bonding or grounding can lead to unreliable operation of systems, e.g., EMI, electrostatic discharge damage to sensitive electronics, personnel shock hazard, or damage from lightning strike. This section provides an overview of the prin ciples involved in the design and maintenance of electrical bonding and grounding. SAE ARP-1870 provides for more complete detailed information on grounding and bond ing, and the application of related hardware. 11-186. GROUNDING. Grounding is the process of electrically connecting conductive objects to either a conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault cir cuit. a. Types of Grounding. If wires carrying return currents from different types of sources, such as signals of DC and AC generators, are connected to the same ground point or have a common connection in the return paths, an in teraction of the currents will occur. Mixing return currents from various sources should be avoided because noise will be coupled from one source to another and can be a major problem for digital systems. To minimize the interaction between various return currents, different types of grounds should be identified and used. As a minimum, the design should use three ground types: (1) ac returns, (2) dc returns, and (3)all others. For distributed power systems, the power return point for an alternative power source would be separated. For example, in a two-ac generator (one on the right side and the other on the left side) sys tem, if the right ac generator were supplying backup power to equipment located in the left side, (left equipment rack) the backup ac ground return should be labeled “ac Right”. The return currents for the left generator should be connected to a ground point labeled “ac Left” b. Current Return Paths. The design of the ground return circuit should be given as much attention as the other leads of a circuit. A requirement for proper ground connections is that they maintain an impedance that is es sentially constant. Ground return circuits should have a current rating and voltage drop adequate for satisfactory operation of the con nected electrical and electronic equipment. EMI problems, that can be caused by a sys tem’s power wire, can be reduced substantially by locating the associated ground return near the origin of the power wiring (e.g. circuit breaker panel) and routing the power wire and its ground return in a twisted pair. Special care should be exercised to ensure replacement on ground return leads. The use of numbered in sulated wire leads instead of bare grounding jumpers may aid in this respect. In general, equipment items should have an external ground connection, even when internally grounded. Direct connections to a magnesium (which may create a fire hazard) structure must not be used for ground return. c. Heavy-Current Grounds. Power ground connections, for generators, trans former rectifiers, batteries, external power re ceptacles, and other heavy-current, loads must be attached to individual grounding brackets that are attached to aircraft structure with a proper metal-to-metal bonding attachment. This attachment and the surrounding structure must provide adequate conductivity to ac commodate normal and fault currents of the system without creating excessive voltage drop or damage to the structure. At least three fas teners, located in a triangular or rectangular pattern, must be used to secure such brackets Par 11-185 Page 11-71PDF Image | AFS-640
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