Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual - Manual (Page 29)

Publication 1746-UM003A-EN-P
Installation and Wiring 2-9
For a 3-wire configuration, the module can compensate for a maximum cable
length associated with an overall cable impedance of 25 ohms.
Three configurations of RTDs can be connected to the RTD module, namely:
•
2-wire RTD, which is composed of 2 RTD lead wires (RTD and Return)
•
3-wire RTD, which is composed of a Sense and 2 RTD lead wires (RTD and
Return)
•
4-wire RTD, which is composed of 2 Sense and 2 RTD lead wires (RTD
and Return). The second sense wire of a 4-wire RTD is left open. It does not
matter which sense wire is left open.
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•
To limit overall cable impedance, keep input cables as short as possible.
Locate your I/O chassis as near the RTD sensors as your application permits.
•
Ground the shield drain wire at one end only. The preferred location is at
the chassis mounting tab of the rack, under the RTD module. Refer to IEEE
Std. 518, Section 6.4.2.7 or contact your sensor manufacturer for additional
details.
•
Route RTD/resistance input wiring away from any high-voltage I/O wiring,
power lines, and load lines.
•
Tighten terminal screws using a flat-head screwdriver. Each screw should be
turned tight enough to immobilize the wire’s end. Excessive tightening can
strip the terminal screw. The torque applied to each screw should not exceed
0.25 Nm (2.25 in-lbs) for each terminal.
•
Follow system grounding and wiring guidelines found in your SLC 500
Installation and Operation Manual, publication 1747-6.2.
IMPORTANT
Details of cable specifications are shown on page A-5.
IMPORTANT
The RTD module requires three wires to compensate for
lead resistance error. We recommend that you do not use
2-wire RTDs if long cable runs are required, as it reduces
the accuracy of the system. However, if a 2-wire
configuration is required, reduce the effect of the lead wire
resistance by using a lower gauge wire for the cable (for
example, use AWG #16 instead of AWG #24). Also, use
cable that has a lower resistance per foot of wire. The
module’s terminal block accepts one AWG #14 gauge wire.