Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 124)

Publication 1762-UM002A-EN-P - July 2002
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
1762-IT4
Metal sheath with Electrical Continuity
to Thermocouple Signal Wires
Grounded Junction with Shielded Cable
Multiplexer