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
requires that NO LINE TO A TTY SET IS GROUNDED
A note from hard-won experience - some TTY sets are polarity sensitive -
Teletype Model 28 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.
Patch panel jacks are arranged with + on tip and - on sleeve.....TTY Sets must
be connected to the patch panel appropriately.
Some RATT converters and modems have loop connections that are isolated from
ground. but many do not.
Here's a quick list of what I have found -
AFSK Receive Converters (no internal loop supply)
AN/URA-8 (CV-60) + loop (J1206/A), ground (J1206/B)
AN/URA-8A, 8B (CV-89) +loop (J509/A), ground (J509/B)
AN/URA-17 (CV-483) + loop (J6/A), ground (J6/B)
AN/URA-17A (CV-483A) + loop (J6/A), ground (J6/B)
AN/URA-17B (CV-483B) + loop (J6/A), ground (J6/B)
AN/URA-17C (CV-483C) + loop (J6/A), ground (J6/B)
AN/URA-17D (CV-483D) +loop (J6/A), ground (J6/B)
AN/URA-17D (CV-483D) (negative loop) -loop (J6/C), ground (J6/B)
AFSK TU receive/transmit (no internal loop supply)
CV-2460/SGC
SEND loop isolated (+loop TB1/6, -loop TB1/7)
RCV loop isolated (+loop TB2/5, -loop TB2/6)
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)
Transmitter Keying Connections
AN/URC-32 transmit input (+loop TBD/1, ground TBD/16)
AN/URT-23 transmit input isolated (+loop J7/B, -loop J7/C)