Rockwell Automation 1746-NR4 SLC RTD/Resistance Input Module User Manual - Manual (Page 41)

Publication 1746-UM008B-EN-P - December 2006
Install and Wire the Module 41
For a three-wire configuration, the module can compensate for a
maximum cable length associated with an overall cable impedance of
25 ohms.
As shown in RTD Connections to Terminal Block on page 42, three
configurations of RTDs can be connected to the RTD module, namely:
• two-wire RTD, which is composed of two RTD lead wires (RTD
and Return).
• three-wire RTD, which is composed of a Sense and two RTD
lead wires (RTD and Return).
• four-wire RTD, which is composed of two Sense and two RTD
lead wires (RTD and Return). The second sense wire of a
four-wire RTD is left open. It does not matter which sense wire
is left open.
• 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 will permit.
• Ground the shield drain wire at one end only. The preferred
location is at the RTD module. Refer to IEEE Std. 518, Section
6.4.2.7 or contact your sensor manufacturer for additional
details.
• Each input channel has a shield connection screw terminal that
provides a connection to chassis ground. All shields are
internally connected, so any shield terminal can be used with
channels 0…3.
• Route RTD/resistance input wiring away from any high-voltage
I/O wiring, power lines, and load lines.
IMPORTANT
Details of cable specifications are shown on page 122.
IMPORTANT
The RTD module requires three wires to compensate for lead
resistance error. It is recommended that you do not use
two-wire RTDs if long cable runs are required, as it will reduce
the accuracy of the system. However, if a two-wire
configuration is required, reduce the effect of the lead wire
resistance by using a lower gauge wire for the cable (for
example, use 1.291 mm (16 AWG) instead of 0.511 mm
(24 AWG)). Also, use cable that has a lower resistance per foot
of wire. The module’s terminal block accepts two 2.5 mm
2
(14 AWG) gauge wires.