Watlow CAS200 - Manual (Page 29)

CAS200 User’s Guide Chapter 2: Installation
Watlow Anafaze 17
Avoiding RFI/EMI
To avoid or eliminate most RFI/EMI noise problems:
• Connect the CAS200 case to earth ground. The
CAS200 system includes noise suppression circuitry.
This circuitry requires proper grounding.
• Separate the 120 or 240 Vac power leads from the low-
level input and output leads connected to the CAS200
series scanner. Do not run the digital I/O or alarm
output leads in bundles with ac wires.
• 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 the CAS200 series equipment.
• If you must use EM relays and you must place them in
a panel with CAS200 series equipment, use a 0.01
microfarad capacitor rated at 1000 Vac (or higher) in
series with a 47
Ω, 0.5 watt resistor across the N.O.
contacts of the relay load. This is known as a snubber
network and can reduce the amount of electrical noise.
• 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 (Watlow
Anafaze part number 26-130210-00). You can also
place a transorb (back-to-back zener diodes) across the
digital output, which limits the digital output voltage.
Additional Recommendations for a Noise Immune System
It is strongly recommended that you:
• Isolate outputs through solid state relays, where
possible.
• Isolate RTDs or “bridge” type inputs from ground.
• Isolate digital inputs from ground through solid state
relays. If this is not possible, then make sure the
digital input is the only connection to earth ground
other than the chassis ground.
• If you are using EIA/TIA-232 from a non-isolated host,
either (1) do not connect any other power common
point to earth ground, or (2) use an optical isolator in
the communications line.