Rockwell Automation 1790P-4R0 CompactBlock LDX I/O RTD/Resistance Input Module - Manual (Page 24)

Publication 1790-UM002A-EN-P
2-12 Installation and Wiring
RTD Wiring Considerations
Because the operating principle of the RTD module is based on the
measurement of resistance, take special care when selecting your input
cable. For 2-wire or 3-wire configurations, select a cable that has a
consistent impedance throughout its entire length. Cable specifications are
noted below.
When using a 3-wire configuration, the module compensates for
resistance error due to lead wire length. For example, in a 3-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.
Table 2.5 Cable Specifications
Description Belden #9501 Belden #9533 Belden #83503
When used? For 2-wire RTDs and
potentiometers For 3-wire RTDs and
potentiometers.
Short runs less than
100 feet and normal
humidity levels. For 3-wire RTDs and
potentiometers. Long
runs greater than 100
feet or high humidity
levels.
Conductors 2, #24 AWG tinned
copper (7 x 32) 3, #24 AWG tinned
copper (7 x 32) 3, #24 AWG tinned
copper (7 x 32)
Shield Beldfoil aluminum
polyester shield with
copper drain wire Beldfoil aluminum
polyester shield with
copper drain wire Beldfoil aluminum
polyester shield with
tinned drain wire
Insulation PVC S-R PVC Teflon
Jacket Chrome PVC Chrome PVC Red Teflon
Agency Approvals NEC Type CM NEC Type CM NEC Art-800, Type CMP
Temperature Rating 80°C 80°C 200°C
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
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 AWG #16 instead of AWG
#24). The module’s terminal block accepts two AWG #14
gauge wires.
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 .