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Application Note

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Troubleshooting and theneed for speed

Instrument and automationtechnicians are constantly chal-lenged to keep instrumentationloops and I/O working at peakefficiency while using the leastpossible time to do it.

When Fluke first releasedthe 771 mA Clamp Meter in

Better ways totroubleshoot automation

and process control loops

2007, technicians found thatmeasuring loop current withoutbreaking the circuit saved agreat deal of time. Now, the new772 and 773 models can saveeven more time.

By incorporating the functions

of a loop calibrator, thesemore advanced tools allowtechnicians to troubleshoot onthe spot.

Tracing control loopproblems

Often the first indication ofa control loop problem comesfrom the operator: I think wehave a bad valve or this loopisnt responding the way itused to. In either case, its thetechnicians signal to begintroubleshooting.

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2 Fluke Corporation Better ways to troubleshoot automation and process control loops

The first step is to measurethe 4-20 mA signal, either bybreaking the loop connecting inseries with a DMM, or by usinga mA clamp meter like the Fluke

771 and verifying the loop cur-rent value. If the loop currentmeasured is not as expected,there are three likely causes:broken/disconnected/shortedwires, a bad loop power supply,or faulty instrumentation.

If no problem is found in thewires, use a DMM (or the 773clamp meter) to check the looppower supply. If the powersupply shows no output, use the24 V loop power function of themeter to substitute for it; if the

loop then works properly thesource of the problem is obvious.If the wiring and the power

supply both check out, its timeto check the transmitter. If youhave a loop calibrator, processcalibrator or multi-functionclamp-on meter, use its mAsimulate mode to substitute forthe transmitter. If the loop per-forms as requested, the problemlies with the transmitter, if not,it is elsewhere.

If a final control element

(valve positioner, etc.) is sus-pected, use the mA source/simulate mode on the Fluke772/3 to feed a signal into itwhile watching the local indica-tor for a response.

Loop malfunctions

If the problem is not a dead loopbut an inaccurate one, likelypossibilities include a bad I/Ocard on the PLC or DCS, or a badfinal control element (I/P on avalve positioner, etc.). Its usu-

ally best to start by doing a fieldcheck of the transmitter, local orremote indicator or final controlelement.

For a final control element,use a clamp-on meter to mea-sure loop current and comparethe value to the local positionindicator on the valve or otherfinal control element. Relay thatinformation to the operator toverify findings.

In the case of a measure-ment loop, use the clamp meterto measure loop current, thencheck with the operator to seehow well the value indicated on

the control panel agrees withthe actual loop current. This willgive a quick check on the PLC orDCS I/O card that handles thatparticular loop. Its also possibleto use the meters mA source/simulate mode to send a knownsignal to the control room; asbefore, compare the value asread by the operator to theactual current in the loop.

Some loops show randomfluctuations or intermittent faultsthat tend not to happen while

a technician is watching. Thesolution here is to use a clampmeter with a scaled mA output.In this mode the meter measuresthe current in the loop withoutbreaking the circuit, and pro-duces an identical and isolatedmA output. Feed that output toa DMM with a logging function;by allowing the DMM to recordover time, any disturbance willbe recorded.

Field checks and plant

commissioningStart by using a clamp-on loopcurrent meter like the Fluke 771to check each loop for currentin a matter of seconds, withoutdisconnecting anything. If aloop is not working, a multifunc-tion clamp meter can also makequick work of diagnostics. If cur-rent is not present on some loopsgo on to classic troubleshooting:check the wiring, the powersupply, and the control systemsI/O cards (by using the meter to

inject a signal into the I/O, thencontacting the operator to askwhat he sees. If the operatoragrees with what is being sent,then there may be somethingamiss with the transmittereither the transmitter itself or,if this is a new installation,perhaps miswiring, the sensorsinput to the transmitter.

Checking DCS and PLCI/O cards

The mA process clamp metercan be used as an accuratesignal source to check the oper-ation of input/output cards onprogrammable logic controllers(PLCs) and distributed processcontrol systems (DCSs). For4-20 mA input cards, disconnectthe process loop and use themeters mA source mode to feedin a known signal value (4.0 mAfor zero, 12 mA for 50 %usingthe meters 25 % step function,and 20.0 mA for 100 %) andcompare it to the value shownon the operators readout.

Voltage input cards (1 V to 5 Vor 0 V to 10 V) are checked ina similar way, using the metersvoltage source function.

Checking a valvepositioner

Milliamp clamp meters can beused for periodic in-field checksof electronic valve positionersas part of preventive mainte-nance programs. Accounting formanufacturer-specific instruc-tions, perform quick operational

checks using the Fluke 772/3 asa signal source while observ-ing the valve stem position,mechanical position indica-tors, or flow indicators as inputchanges are made.

Mitch Stewart, Field ServiceManager, L2 Systems, tells ofusing the 4-20 sourcing outputof a mA process clamp meter todrive a control valve open andclosed when the process outputfrom the PLC wasnt working.We disconnected the PLCs

output at the control valve andconnected the [meter] up to thecontrol valve and ran it openand closed to verify that the I/Pon the valve worked correctly,he explains.

The general method is toset the meter to the 4-20 mAsource/simulate mode andconnect it to the input termi-nals of the valve positioner. Setthe meter to output 4 mA andwait for the positioner to settle;then vary the current in small

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3 Fluke Corporation Better ways to troubleshoot automation and process control loops

Fluke CorporationPO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa +31 (0) 40 2675 200 orFax +31 (0) 40 2675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com

2009 Fluke Corporation.Specifications subject to change without notice.Printed in U.S.A. 6/2009 3497464A A-EN-N

Modification of this document is not permittedwithout written permission from Fluke Corporation.

Fluke. Keeping your worldup and running.

increments between 4.0 mAand ~3.9 mA, while feeling thevalve stem with your free handto check for any sign of move-ment. Adjust for zero movement

between these two currentsettings by using the zeroadjustment on the positioner.

Next increase and decreasecurrent from 4 mA to ~4.1 mA.Insure that the valve stem justbegins movement above the~4.1 mA setting and fully closedat 4 mA.

Span can be checked simi-larly, by setting the meter at20 mA, ~19.9 mA and ~20.1 mA,and linearity can be checkedby using the meters 25 % step

function.

Checking loop isolators

To check a loop isolator, applya mA input signal to the deviceand measure its 4-20 mA outputusing the clamp-on currentmeasuring function. This twochannel simultaneous source/measure function in the 773can also be used for valvesthat report their position using4-20 mA.

Checking VFDs

Variable frequency drives (VFDs)are used to power motors,blowers and fans in processapplications as well as conveyorsystems and machine tools.Control inputs are generally volt-age (1 V to 5 V or 0 V to 10 V) orcurrent (4 mA to 20 mA). A mAprocess clamp meter can feedin a signal to simulate a normalinput while the technicianobserves the result.

Quick calibration

While not classified as loopcalibrators, todays mA processclamp meters boast accura-

cies of 0.2 %, and can be usedfor quick calibration checks,while cutting down on thenumber of instruments needed.For example, checking a pro-cess transmitter on the benchnormally requires (aside froma pump and separate pressurestandard) a loop power supplyand an instrument for read-ing the transmitters 4-20 mAoutput. But with todays mA pro-cess clamp meters its possible to

both power the transmitter andread the output. This tiny littlething, says Paul Jusak, Main-tenance Engineer, Puget SoundEnergy, allows you to do that

function without having to dragout a separate power supply.

Summary

Todays mA process clampmeters can save instrumenta-tion and automation techniciansa great deal of time in troubleshooting, because they canreplace a number of separateinstruments. The technician nolonger has to spend 15 min-utes going back to the shop toget an instrument, because the

one instrument he takes withhim will do all the necessaryfunctions. And, adds Jusak,instead of having two tools inyour pouch you now have onetool in your pouch for doing all4-20 mA loop calibrations andtroubleshooting. That to me ispretty doggone convenient.