Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 31)

Publication 1746-UM614B-EN-P - September 2007
Install and Wire the Module 31
Wiring Considerations
Thermocouple inputs are highly susceptible to electrical noise due to
the small signal amplitudes (microvolt/°C). Most applications require
that the processor and I/O chassis be installed in an industrial
enclosure to reduce the effects of electrical interference. Consider the
following conditions when selecting a slot location for the module.
Position the module away from other modules that:
•
connect to sources of electrical noise, such as relays and ac
motor drives.
•
generate significant heat, such as 32-point I/O modules.
Follow these guidelines to wire your input signal cables.
•
To limit the pickup of electrical noise, keep thermocouple and
millivolt signal wires as far from power and load lines as
possible.
•
For high immunity to electrical noise, use Alpha 5121 (shielded,
twisted pair) or equivalent wire for millivolt sensors; or use
shielded, twisted-pair thermocouple extension lead wire
specified by the thermocouple manufacturer. Using the incorrect
type of thermocouple extension wire or not following the
correct polarity may cause invalid readings.
•
Ground the shield drain wire at only one end of the cable. The
preferred location is at the I/O chassis ground (see page 31).
(Refer to IEEE Std. 518, Section 6.4.2.7, or contact your sensor
manufacturer for additional details.)
•
Keep all unshielded wires as short as possible.
•
Tighten screw terminals with care. Excessive tightening can strip
a screw.
•
The open-circuit detector generates approximately 20
nano-amperes into the thermocouple cable. A total lead
resistance of 25 ohms (12.5 one-way) will produce 0.5 microvolt
of error.
•
Follow system grounding and wiring guidelines found in your
SLC 500 Installation and Operation Manual, publication
1747-UM011.