Watlow CAS - Manual (Page 24)

14 CAS User’s Guide
Installation
Grounding
Connect the CAS power supply common to an external ground at only
one point, to avoid ground loops that can cause instrument errors or
malfunctions. Since the CAS uses a non-isolated measurement system,
it has the following connections to power supply common:
• Analog Common TB1 pins 5, 6, 11, & 12.
• Reference Common, TB1 pin 17.
• Communications ground (TB1 pins 23 & 24) if using RS-232.
• Power Supply Ground, TB2 pin 2.
Watlow Anafaze strongly recommends that you:
• Do not connect to earth ground unless recommended by the factory
for a specific application.
• Isolate outputs through solid state relays, where possible.
• Isolate digital inputs from ground through solid state relays. If you
can't do that, then make sure the digital input is the only place that
one of the above pins connects to ground.
• If you are using RS-232 from an un-isolated host, don't connect any
other power common point to earth ground.
Thermocouple Wiring
Use 18 or 20 AWG thermocouple (T/C) extension wire for all
T/C inputs.
NOTE
Most thermocouple wire is solid unshielded wire. Use shielded wire
if required at your installation; ground one end only.
Since the CAS has single ended inputs, it has little protection from
common mode voltage sources. Therefore, Watlow Anafaze highly
recommends that you use ungrounded thermocouples with the external
thermocouple sheath electrically connected to earth ground.
You can use 400 to 500 feet of thermocouple extension wire, depending
on wire type and size, and keep to accuracy and source impedance
specifications. Always install T/C wiring in a separate conduit away
from AC power (the 120 VAC control supply) and high power (240
VAC or higher) wiring.
Input Wiring
Use multicolor stranded shielded cable for analog inputs. We
recommend that you use #20 AWG wire. (If the sensor manufacturer
requires it, you can also use #22 or #24 AWG wiring.) Most inputs use a
shielded twisted pair; some require a 3 wire input.