Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module/ Instructions - Manual (Page 11)

SLC 500 Thermocouple/mV Analog Input Module 11
Publication 1746-IN015C-EN-P - July 2001
Thermocouple Junctions
There are three types of thermocouple junctions:
•
Grounded Junction - The measuring junction is physically connected to the
protective metal sheath providing electrical continuity between junction and
sheath.
•
Ungrounded Junction - The measuring junction is electrically isolated from
the protective metal sheath. (Ungrounded Junction can also be called
Insulated Junction.)
•
Exposed Junction - Does not have a protective metal sheath so the
measuring junction is exposed.
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.
TIP
Refer to Appendix A of the SLC 500 Thermocouple/mV Analog
Input User Manual, publication 1746-6.22 for additional
information on wiring and using grounded junction,
ungrounded junction, and exposed junction thermocouple
types.