SLC 500 Thermocouple/mV Analog Input Module
15
Publication 1746-5.23
Grounded Junction Thermocouples
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.
As shown in the above wiring diagram, 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.
+
-
+
-
1746-NT8
Grounded junction with nonconductive
protective sheath
CH0
MUXES
CH3
Metal sheath with electrical continuity to
thermocouple signal wires.
(floating ground connection)
Spare Allen-Bradley Parts