Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module/ Instructions - Manual (Page 12)

12 SLC 500 Thermocouple/mV Analog Input Module
Publication 1746-IN015C-EN-P - July 2001
Grounded Junction Thermocouples
As shown in the wiring diagram above, it is recommended that grounded junction
thermocouples have either protective sheaths made of electrically insulated material
(e.g. ceramic), or that 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.
Exposed Junction Thermocouples
Recommended wiring for exposed junction thermocouples is shown in the
following illustration. Using exposed junction thermocouples may result in removal
of channel-to-channel isolation. This may occur if multiple exposed thermocouples
are in direct contact with electrically conductive process materials. To prevent
violation of channel-to-channel isolation:
•
For multiple exposed thermocouples, do not allow the measuring junction
of the thermocouple to make direct contact with electrically conductive
process material.
•
Use all ungrounded junction thermocouples instead of the exposed junction
type.
+
-
+
-
1746-NT8
Grounded junction with nonconductive
protective sheath
CH0
MUXES
CH3
Metal sheath with electrical continuity to
thermocouple signal wires.
(floating ground connection)