CLS200 Series User’s Guide
Chapter 2: Installation
28
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 communications port.
• From a sensor to the dc power supply.
The best way to avoid ground loops is to minimize unnecessary connections to ground. Do not
connect any of the following 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 Sixteen-Loop Models
Sixteen-loop models have single-ended inputs. All the negative sensor 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 sixteen-loop controller:
• Use all ungrounded or all well-grounded thermocouples, not a mix.
• If using a mixture of thermocouples or low-voltage inputs (<500mV) and current inputs,
connect the negative leads of the current transmitters to terminal 17 (Ref Com) on TB1.
• If using voltage transmitters, use only sourcing models 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 communications common to chassis ground. When
such a PC is connected 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 controller functions. To eliminate a ground
loop, either use an optically isolated communications adapter or take measures to ensure that
sensors and all other connections to the controller are isolated and not conducting current into
the unit.
