Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 86)

Publication 1790-UM003A-EN-P
D-2 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, since there is no
isolation between signal and sheath (sheaths are tied together). It should
be noted that the isolation is removed even if the sheaths are connected
to chassis ground at a location other than the module, since the module is
connected to chassis ground.
Rockwell Automation recommends 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.
Using an Ungrounded
(Isolated) Junction
Thermocouple An ungrounded (isolated) junction thermocouple uses a measuring
junction that is electrically isolated from the protective metal sheath. This
junction type is often used in situations when noise will affect readings, as
well as situations using frequent or rapid temperature cycling. For this
type of thermocouple junction, the response time is longer than for the
grounded junction.
+
-
IN 0
+
-
IN 3
Module
Metal sheath with Electrical Continuity
to Thermocouple Signal Wires
Grounded Junction with Shielded Cable
Multiplexer
CH 0
CH 1