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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:

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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.)

I'm confused.

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This totally depends on the purpose of the bond you are making. The bad term "grounding" mixes all of these things and creates confusion.

Shielding:

The more bonds, the better

Preventing a part from floating:

One bond is enough at DC, more are necessary for RF attachment.

Returning signal/power current:

Only one logical connection should be used (can be still several conductors). For RF return currents this happens implicitly, selected by the loop of least inductance. But for DC return currents, you must lay out those return conductors intentionally, with a single link, or your return current will flow everywhere.

So it appears the module chassis is bonded to the mainframe for shielding purposes. But the module board is connected to the mainframe maybe for power and signal transfer, so needs a common ground level hence the link to the mainframe. Also if several modules communicate to each other, they also must have a GND conductor in their mutual links. Otherwise return currents would be sent all the way through the main frame creating copious EMI.

I know that multiple ground connections can create harmful ground loops

As explained above, in the majority of scenarios, ground loops are a good thing! Only if you're not careful and mix different purposes of a grounded conductor, e.g. passing signal return currents through the chassis, ground loops can lead to interference.

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  • \$\begingroup\$ > Also if several modules communicate to each other, they also must have a GND conductor in their mutual links. Otherwise return currents would be sent all the way through the main frame creating copious EMI. Not really because signal currents are mA or less, but yes, most of the interfaces are either differential or include GND. \$\endgroup\$
    – Joan
    Commented May 12, 2022 at 11:31
  • \$\begingroup\$ @Joan a transient mA signal current is a strong EMI source in a radio-quiet environment. It is not so much the amps that are important here, but the loop area. And if you provide no return current line, even tiny transient currents can be nuissances. And is it really mA ? Most signal links are something like 100 Ohm impedance and a few Volts, so transients are usually several 10s or up to 100 mA. \$\endgroup\$
    – tobalt
    Commented May 12, 2022 at 11:39
  • \$\begingroup\$ obviously if you put an RF environment, mA may be a problem, but I didn't say that is the case. The 100 ohm impedances are typically for differential signals and the differential voltage across is in the 100s of mV, otherwise it would be a waste of power. for single ended interfaces, like UART, with CMOS or even TTL inputs, currents are almost 0 (CMOS) or 2-3 mA (TTL) \$\endgroup\$
    – Joan
    Commented May 12, 2022 at 11:44

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