Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 31)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 31
Installation and Wiring Chapter 3
• Routing field wiring in a grounded conduit can reduce electrical noise.
• If field wiring must cross AC or power cables, be sure that they cross at
right angles.
• If multiple power supplies are used with analog millivolt inputs, the power
supply commons must be connected.
Terminal Block Guidelines
• Do not use the module’s NC terminals as connection points.
• Do not tamper with or remove the CJC sensors on the terminal block.
Removal of one or both sensors will reduce accuracy.
• For millivolt sensors, use Belden 8761 shielded, twisted-pair wire (or
equivalent) to be sure of proper operation and high immunity to electrical
noise.
• For a thermocouple, use the shielded, twisted-pair thermocouple extension
lead wires specified by the thermocouple manufacturer. Using the incorrect
type of thermocouple extension wire or not following the correct polarity
will cause invalid readings.
• To be sure of optimum accuracy, limit overall cable impedance by keeping a
cable as short as possible. Locate the module as close to input devices as the
application permits.
Grounding Guidelines
• This product is intended to be mounted to a well-grounded mounting
surface such as a metal panel. Additional grounding connections from the
module’s mounting tabs or DIN rail (if used) are not required unless the
mounting surface cannot be grounded.
• Keep cable shield connections to ground as short as possible.
• Ground the shield drain wire at one end only. The typical location is as
follows:
–
For grounded thermocouples or millivolt sensors, this is at the sensor
end.
– For insulated/ungrounded thermocouples, this is at the module end.
Contact your sensor manufacturer for additional details.
• If it is necessary to connect the shield drain wire at the module end,
connect it to earth ground using a panel or DIN rail mounting screw.
• Refer to Industrial Automation Wiring and Grounding Guidelines,
Allen-Bradley publication 1770-4.1 , for additional information.
ATTENTION: The possibility exists that a grounded or exposed
thermocouple can become shorted to a potential greater than that of the
thermocouple itself. Due to possible shock hazard, take care when wiring
grounded or exposed thermocouples. See
Appendix D , Using
Thermocouple Junctions.