Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module/ Instructions - Manual (Page 13)

SLC 500™ RTD/Resistance Input Module 13
Publication 1746-IN007C-EN-P - December 2002
•
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 chassis, 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 small flathead 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-lb.) for each terminal.
•
Follow system grounding and wiring guidelines found in your SLC 500
Installation and Operation Manual, publication 1747-UM011.
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.
There are several ways to insure that the lead values match as closely as possible.
They are as follows:
•
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.
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.
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
.