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 circuit.  [Figure 9-144] 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 interaction of the currents  occurs. Mixing return currents from various sources should  be avoided because noise is 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 ground 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) system, 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.” 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 essentially constant. Ground return circuits  should have a current rating and voltage drop adequate  for satisfactory operation of the connected electrical and  electronic equipment. EMI problems that can be caused by a  system’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 insulated 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 structure must not be used for ground return  because they may create a fire hazard. Power ground connections for generators, transformer  rectifiers, batteries, external power receptacles, 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 accommodate normal and fault currents of the system  without creating excessive voltage drop or damage to the  structure. At least three fasteners, located in a triangular or  rectangular pattern, must be used to secure such brackets in  order to minimize susceptibility to loosening under vibration.  If the structure is fabricated of a material, such as carbon fiber  composite (CFC), that has a higher resistivity than aluminum  or copper, it is necessary to provide an alternative ground  path(s) for power return current. Special attention should be  considered for composite aircraft. Power return or fault current ground connections within  flammable vapor areas must be avoided. If they must be  made, make sure these connections do not arc, spark, or  overheat under all possible current flow or mechanical failure  conditions, including induced lightning currents. Criteria for  inspection and maintenance to ensure continued 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 localized 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 voltage  regulation problems. Likewise, repeated localized material  heating by current surges can cause material degradation.  Both problems may occur without warning and cause no  repeatable failures or anomalies. The use of common ground connections for more than one  circuit or function should be avoided except where it can be  shown that related malfunctions that could affect more than  one circuit do 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. 9-86 Figure 9-144.  Ground wires.