Rockwell Automation 1746-INT4 Thermocouple/mV Isolated Input Module Quick Start - Manual (Page 10)

10 Thermocouple/mV Isolated Input Module
Publication 1746-QS002B-EN-P - July 2002
Wiring Guidelines
Follow these guidelines when planning your system wiring.
•
To limit the pickup of electrical noise, keep thermocouple and millivolt
signal wires away from power and load lines.
•
For high immunity to electrical noise, use Alpha 5121 (shielded, twisted pair)
or equivalent wire for millivolt sensors; or use shielded, twisted pair
thermocouple extension lead wire specified by the thermocouple
manufacturer. Using the incorrect type of thermocouple extension wire or
not following the correct polarity may cause invalid readings. See IEEE Std.
518, Section 6.4.2.7 or contact your sensor manufacturer for additional
details.
•
When trimming cable leads, minimize the length of unshielded wires.
•
Ground the shield drain wire at only one end of the cable. The preferred
location is at the I/O chassis ground.
•
For maximum noise reduction, use 3/8 inch braid wire to connect cable
shields to the nearest I/O chassis mounting bolt. Then connect the I/O
chassis to earth ground. These connections are a requirement regardless of
cable type.
•
Tighten terminal screws to 6-8 inch-pounds. Excessive tightening can strip
the screw.
•
The open–circuit detector generates approximately 20 nano–amperes into
the thermocouple cable. A total lead resistance of 25 ohms (12.5 one–way)
will produce 0.5 mV of error.
•
Follow system grounding and wiring guidelines found in your SLC 500
Modular Hardware Style User Manual, publication 1747–UM011.