Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 136)

136 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix D Using Thermocouple Junctions
The shield input terminals for a grounded junction thermocouple are connected
together and then connected to chassis ground. Use of this thermocouple with an
electrically conductive sheath removes the thermocouple signal to chassis ground
isolation of the module. In addition, if multiple grounded junction
thermocouples are used, the module channel-to-channel isolation is removed,
because there is no isolation between signal and sheath (sheaths are tied
together). Note that the isolation is removed even if the sheaths are connected to
chassis ground at a location other than the module, because the module is
connected to chassis ground.
Figure 51 - Shield Input Terminals for a Grounded Junction Thermocouple
We recommend that a grounded junction thermocouple have a protective sheath
made of electrically insulated material (for example, ceramic). An alternative is to
float the metal sheath with respect to any path to chassis ground or to another
thermocouple metal sheath. Thus, the metal sheath must be insulated from
electrically conductive process material, and have all connections to chassis
ground broken. Note that a floated sheath can result in a less noise-immune
thermocouple signal.
+
-
IN 0
+
-
IN 3
1769-IT6
Metal Sheath with Electrical Continuity
to Thermocouple Signal Wires
Grounded Junction with Shielded Cable
Multiplexer