As usual, I'm sure I'll screw something up so additions and corrections are welcomed.

When hooking up multiple units to a multi-loop patch panel it is possible to inadvertently short something out because a connected device has one side of the line grounded.

Typically the minus side of the loop supply will be grounded and that is a return for all loops. Each loop is fed by the positive side of the supply through a current limiting resistor and then through the LOOPING jacks and then the SET jacks (TTY printer, TD, reperf, etc.) and then to the LOOP DEVICE (keyer/converter/etc.) and then back to minus/ground. This arrangement works if NO LINE TO A TTY IS GROUNDED

A note from hard-won experience - some TTY sets are polarity sensitive - M28 RO, KSR, and ASR may have a LESU with a line relay or selector magnet driver - you must apply proper loop polarity to the line inputs. Western Union devices might have one line grounded and you'll need to isolate that.

Some RTTY converters and modems have isolated loop inputs and outputs but many do not. Here's a quick list of what I have found -

FSK Receive Converters (with internal loop supply)

Dovetron - negative side of loop is ground

FSK Receive Converters (no internal loop supply)

AN/URA-8A, 8B (CV-89) - switches loop (pin A) to ground (pin B)

AN/URA-17 (CV-483) - switches loop (pin A) to ground (pin B)

AN/URA-17A (CV-483A) - switches loop (pin A) to ground (pin B)

AN/URA-17B (CV-483B) - switches loop (pin A) to ground (pin B)

AN/URA-17C (CV-483C) - switches loop (pin A) to ground (pin B)

AN/URA-17D (CV-483D) - switches loop (J8/A) to ground (J8/B)

                     (or for a negative loop switches J8/C to J8/B)

FSK TU receive/transmit (no internal loop supply)

AN/SGC-1 -

     SEND/RCV loop isolated (TTY+, TTY-)

AN/URA-17E (CV-3510A)
     SEND loop isolated (+loop J5/A, -loop J5/B)
     RCV loop isolated (+loop J8/A, -loop J8/B)

AN/URA-17F (CV-3510B)

     SEND loop isolated (+loop J9/A, -loop J9/B)
     RCV loop isolated (+loop J6/A, -loop J6/B)

CV-2460

     SEND loop isolated (+loop TB1/6, -loop TB1/7)
     RCV loop isolated (+loop TB2/5, -loop TB2/6)

AN/URC-32 transmit keyer (+loop TBD/1, ground TBD/16

AN/URT-23 transmit keyer isolated (+loop J7/B, -loop J7/C)

Note - I believe that all low level devices have one side grounded and the other is signal with +/-6v signaling.