Watlow Series D8 - Manual (Page 40)

Chapter 2: Installation Series D8 User’s Guide
22 Watlow Anafaze Doc. 0600-3120-2000
• Connect the D8 case to earth ground. The D8 system in-
cludes noise suppression circuitry. This circuitry requires
proper grounding.
• Separate the 120 Vac and higher power leads from the
low-level input and output leads connected to the D8 se-
ries controller. Do not run the digital I/O or control out-
put leads in bundles with ac wires.
• Where possible, use solid state relays (SSRs) instead of
electromechanical relays. If you must use electrome-
chanical relays, avoid mounting them in the same panel
as the D8 series equipment.
• If you must use electromechanical relays and you must
place them in a panel with D8 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 normal-
ly-open 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
We strongly recommended the following:
•
Isolate outputs through solid state relays, where possible.
• Isolate RTDs or “bridge” type inputs from ground.
• Isolate digital inputs from ground through solid state re-
lays. If this is not possible, then make sure the digital in-
put is the only connection to earth ground other than the
chassis ground.
Ground Loops
Ground loops occur when current passes from the process
through the controller to ground. This can cause instrument
errors or malfunctions.
A ground loop may follow one of these paths, among others:
•
From one sensor to another.
• From a sensor to the dc power supply.