Watlow CLS200 Bedienungsanleitung PDF herunterladen (Seite 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.

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