Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module User Manual - Manual (Page 95)

Publication 1746-UM022B-EN-P - January 2005
Using Grounded Junction, Ungrounded Junction, and Exposed Junction Thermocouples B-3
Grounded Junction
As shown in the following illustration, the shield input terminals are
internally connected together, which are then connected to chassis
ground. Using grounded junction thermocouples with electrically
conductive sheaths removes the thermocouple signal to chassis
ground isolation of the module. This is inherent to the thermocouple
construction. In addition, if multiple grounded junction
thermocouples are used, the module’s channel-to-channel isolation is
removed since there is no isolation between signal and sheath and the
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.
For grounded junction thermocouples, it is recommended that they
have protective sheaths made of electrically insulated material (e.g.
ceramic), or the metal protective sheaths be floated. The metal sheaths
would need to be floated with respect to any path to chassis ground
or to another thermocouple metal sheath. This means the metal sheath
must be insulated from electrically conductive process material and
have all connections to chassis ground broken. It should be noted that
a floated sheath may result in a less noise immune thermocouple
signal.
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1746-NT8
CH0
MUXES
CH3
Grounded junction with
nonconductive protective sheath
Metal sheath with electrical continuity
to thermocouple signal wires.
(floating ground connection)