Rockwell Automation 1746-NR4 SLC RTD/Resistance Input Module User Manual - Manual (Page 43)

Publication 1746-UM008B-EN-P - December 2006
Install and Wire the Module 43
When using a three-wire configuration, the module compensates for
resistance error due to lead wire length. For example, in a three-wire
configuration, the module reads the resistance due to the length of
one of the wires and assumes that the resistance of the other wire is
equal. If the resistances of the individual lead wires are much
different, an error may exist. The closer the resistance values are to
each other, the greater the amount of error that is eliminated.
There are several ways to insure that the lead values match as closely
as possible.
• Keep lead resistance as small as possible and less than 25 Ω.
• Use quality cable that has a small tolerance impedance rating.
• Use a heavy-gauge lead wire which has less resistance per foot.
Wire the Resistance Devices (Potentiometers) to the NR4 Module
Potentiometer wiring requires the same type of cable as that for the
RTD described in the previous subsection. Potentiometers can be
connected to the RTD module as a two-wire interconnection or a
three-wire interconnection.
See Two-wire Potentiometer Connections to Terminal Block, on
page 44, for 2-wire connection and Three-wire Potentiometer
Connections To Terminal Block, on page 45, for 3-wire connection.
IMPORTANT
To ensure temperature or resistance value accuracy, the
resistance difference of the cable lead wires must be equal to
or less than 0.01
Ω.