Ground loops, Personal computers and ground loops, Ground loops 18 – Watlow CAS200 User Manual
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Chapter 2: Installation
CAS200 User’s Guide
18
Watlow Anafaze
Ground Loops
Ground loops occur when current passes from the process
through the scanner 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)
The CAS200 has 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 precautions:
•
Use all ungrounded or all well-grounded
thermocouples, not a mix.
•
If using a mixture of thermocouples or low-voltage
inputs (<500 mV) 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 scanner’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 scanner, this can provide a path to
ground for current from the process that can enter the
scanner through a sensor (such as a thermocouple). This
creates a ground loop that can affect communications and
other scanner 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 scanner are isolated and not conducting current into
the unit.