Rockwell Automation 1762-IR4 RTD/Resistance Input Module - Manual (Page 27)

Publication 1762-UM003A-EN-P - February 2003
Installation and Wiring 2-9
•
Keep shield connection to ground as short as possible.
•
If noise persists for a device, try grounding the opposite end of
the cable. (You can only ground one end at a time.)
•
Refer to Industrial Automation Wiring and Grounding
Guidelines, Allen-Bradley publication 1770-4.1, for additional
information.
RTD Wiring Considerations
Since 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. See Cable
Specifications on page A-4.
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.
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.
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 .