Common GND Line? 2

Is there any circumstance where the GND line should be separated?

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In general, yes

> in your specific example, there might be a reason to not have your motor drive board's ground arranged serially with the other circuits' grounds, since even digital circuits might be unhappy with motor drive ground noise. Since you say the grounds "branch", that's probably OK, since the ground noise would go back to the PSU first.
> analog circuits might not be tolerant of the ground noise from digital circuits; again running everything back to a common ground at the PSU might make that OK, but it's sometimes a black art.
> some particular circuits, mostly analog, depend on floating grounds to allow for much higher voltage or current generation; high pulse current (>100A) diode drivers might have that kind of setup, but these tend to have transformer isolation.

TTFN (ta ta for now)
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In general, it is acceptable to connect all of the consumers to a common ground (GND) line in a low-voltage DC system as you have described, especially when the system is functioning properly. However, depending on the specific application and the nature of the loads, there may be cases where separating the ground is recommended to prevent issues such as ground loops or noise interference.

For example, if the system includes sensitive electronic components that are prone to noise interference or if there are long cable runs involved, it may be advisable to isolate the grounds of certain loads or to use a separate ground reference for specific components. Additionally, some safety considerations may require a separate ground for certain parts of the system.