Watlow CAS - Manual (Page 26)

16 CAS User’s Guide
Installation
Wiring: Noise Suppression
If the outputs control dry contact electromechanical relays with
inductive loads
, like alarm horns , you may get Radio-Frequency
Interference (RFI, or "noise"). This section explains how to avoid noise
problems; read it before you wire the CAS.
Symptoms of RFI
• If your system displays the following symptoms, suspect RFI.
• The CAS display blanks out and then reenergizes as if power had
been turned off for a moment.
• The process value does not display correctly.
RFI may also damage the digital output circuit, so digital outputs will
not energize. If the digital output circuit is damaged, return the system
to Watlow Anafaze for repair.
Avoiding Noise Problems
Use these techniques to avoid noise problems.
Where possible, use solid state relays (SSRs) instead of
electromechanical (EM) relays. If you must use EM relays, try to avoid
mounting them in the same panel as Watlow Anafaze equipment.
Separate the 120 Vac power leads from the low level input and output
leads connected to the CAS. Don't run the digital output or PID control
output leads in bundles with 120 Vac wires. (Never run input leads in
bundles with high power leads
— see the General Wiring section.)
If you must use EM relays and you must place them in a panel with
Watlow Anafaze equipment, use a .01 mF capacitor rated at 1000 Vac
(or higher) in series with a 47 ohm, ½ watt resistor across the normally
open (NO) contacts of the relay load. This network is known as an arc
suppressor or snubber network.
You can use other voltage suppression devices, but they are not usually
required. For instance, you can place a metal oxide varistor (MOV)
rated at 130 Vac for 120 Vac control circuits across the load, which
limits the peak AC voltage to about 180 Vac. You can also place a
transorb (back to back zener diodes) across the digital output, which
limits the digital output loop to 5 Vdc. (These parts are available from
Watlow Anafaze.)
These steps will eliminate most noise problems. If you have further
problems or questions, please contact Watlow Anafaze.