Watlow CLS200 - Manual (Page 40)

Chapter 2: Installation CLS200 Series User’s Guide
24 Watlow Anafaze Doc.# 0600-3050-2000
Ground Loops
Ground loops occur when current passes from the process
through the controller to ground. This can cause instru-
ment errors or malfunctions.
A ground loop may follow one of these paths, among others:
• From one sensor to another.
• From a sensor to the communications port.
• From a sensor to the dc power supply.
The best way to avoid ground loops is to minimize unneces-
sary connections to ground. Do not connect any of the fol-
lowing terminals to each other or to earth ground:
• Power supply dc common
• TB1, terminals 5, 6, 11, 12 (analog common)
• TB1, terminal 17 (reference voltage common)
• TB1, terminals 23, 24 (communications common)
• TB2, terminal 2 (dc power common)
Special Precautions for the CLS216
The CLS216 has single-ended inputs. All the negative sen-
sor leads are tied to the analog common. That means there
is no sensor-to-sensor isolation. Proper grounding is critical
for this unit. Take these additional precautions with a
CLS216:
• Use all ungrounded or all well-grounded thermocou-
ples, not a mix.
• If using a mixture of thermocouples or low-voltage in-
puts (<500 mV) and current inputs, connect the nega-
tive leads of the current transmitters to terminal 17
(Ref Com) on TB1.
• If using voltage transmitters, use only sourcing mod-
els or configuration. Sinking configurations will not
work.
• Isolate the controller’s communication port (if used) by
using an optically isolated 232-to-485 converter.
Personal Computers and Ground Loops
Many PC communications ports connect the communica-
tions common to chassis ground. When such a PC is con-
nected to the controller, this can provide a path to ground
for current from the process that can enter the controller
through a sensor (such as a thermocouple). This creates a
ground loop that can affect communications and other con-
troller functions. To eliminate a ground loop, either use an
optically isolated communications adapter or take mea-
sures to ensure that sensors and all other connections to
the controller are isolated and not conducting current into
the unit.