[sdiy] Question about earthing on power-one psus

[Julian]

Hello there,

Using dual output power-ones, like the HBB-15 or HAA-15 etc. you get seven terminals,  which read across as:

-out
-s

+s
com
-s

+s
+out

Now on those that ive bought secondhand, the terminals have allways been jumpered into 3 groups, as ive kinda demonstrated above.

Now the +/- out and +/-sense terminals i understand, but the middle group?  Why do i not just run the ground back to my mains lead
ground?  Why is it necisary to run to this central group, and, excluding grounding the chassis for saftey reasons, why is there not
a ground back from the psu?

Thankyou,

Julian - looking to improve the grounding of his modular.