Grounding & Power Source Separation under FAA 25.1707(d)
FAA 25.1707(d) requires that independent airplane power sources — engines, APU generators, batteries — and their associated EWIS be designed and installed so a fault in one cannot adversely affect another. Independent sources must not share ground terminations, and static grounds must not share locations with independent power source grounds.
“Each EWIS associated with independent airplane power sources or power sources connected in combination must be designed and installed to ensure adequate physical separation and electrical isolation so that a fault in any one airplane power source EWIS will not adversely affect any other independent power sources. (1) Airplane independent electrical power sources must not share a common ground terminating location. (2) Airplane system static grounds must not share a common ground terminating location with any of the airplane's independent electrical power sources.”
Per AC 25.1701-1, the term 'independent airplane power source' covers the general sources of electrical power for the airplane: engines, the APU-driven generators, and batteries. Generating-system EWIS must be assessed against the same standards as the rest of the EWIS, with particular attention to power routing and physical proximity. Components that require multiple input power sources for redundancy demand additional treatment.
Isolation must be considered physically and functionally. Physically, common cause analysis identifies whether close routing creates damage propagation paths and, where it does, drives a damage assessment. Functionally, the failure of one power source must not adversely affect any other.
Ground termination separation is the simplest of the requirements but is frequently the source of finding letters. Splitting the termination of independent power systems means that a single grounding-point failure cannot disable multiple sources, and physically separating grounds reduces or eliminates introduced interference. SAE AS50881 and ARP1870A (latest revision 2012) provide aerospace-specific grounding and bonding guidance.
Common-ground findings are some of the most preventable items in a 25.1707 audit. Treating the ground stud as a physical layout decision — not just an electrical one — is what closes them out. Schematic-level review is not enough; the assessment has to walk the airframe and look at the actual hardware.
Frequently asked
What does 'independent airplane power source' mean in FAA 25.1707(d)?
Per AC 25.1701-1, it covers the general sources of electrical power for the whole airplane: engines, APU-driven generators, and batteries. Generating-system EWIS associated with each is in scope.
Can independent power sources share a ground stud under 25.1707(d)?
No. Subparts (d)(1) and (d)(2) explicitly prohibit independent power sources from sharing a common ground terminating location, and prohibit airplane system static grounds from sharing a terminating location with independent power source grounds.