The NASA technical book about grounding in spacecraft (NASA-HDBK-4001) (I believe the question may be relevant for any other terrestrial application) mentions this grounding scheme for subsystems:
Each subsystem enclosure is grounded (connected to the spacecraft chassis) and also each board has a single ground connection to the chassis as well (vertical wires.)
Why not simply connect each board ground to the enclosure as they are grounded? I know that multiple ground connections can create harmful ground loops, but that doesn't seem to be the case with a single connection to the enclosure.
Differences between space and earth electronics are:
- Radiation and charged particles, all systems are shielded with some thickness of aluminium, but if they are all electrically connected, the chassis voltage should be the same everywhere.
- Redundancy, things are typically made to tolerate one failure. Separate grounding for enclosure and board seems to agree with that, but there is no connection between board and enclosure (and I think there shouldn't be because that creates ground loops.)