Juniper SRX5400 Hardware Manual page 410

416
The marked input voltage of -48 VDC for a DC-powered services gateway is the nominal voltage
associated with the battery circuit, and any higher voltages are only to be associated with float voltages
for the charging function.
Because the services gateway is a positive ground system, you must connect the positive lead to the
terminal labeled RETURN, the negative lead to the terminal labeled -48V, and the earth ground to the
chassis grounding points.