Current loop tester for loop checking.

How to select a current loop tester for loop checking.

What is the purpose of loop checking.

Picture of a floating industrial plant When industrial factory's are build many different parts and systems are connected to create the factory. Before production can start all parts of the systems have to be verified, and made sure the intended functionality is established. The PCS systems is the central control system of the factory, the PCS gathers its information from measurement transmitters mounted on various locations in the factory. The complete measurement system including control system based on the measurement is called a loop.

Before commissioning of the plant can start each loop has to be checked.

Loop testing will verify:

  • The hardware components of the loop are connected and functional.

  • The input and output cards of the PCS.

  • The wiring to the sensor is connected to the correct terminals on the PCS.

  • The loop is powered.

  • The software in the PCS, face-plates, alarms and graphics are present, configured correct and scaling of the 4-20 mA signal is correct.

Loop checking procedure

Before loop checking can start

Loop checking is done at the end of the construction period and before commissioning. Because loop checking involves several systems these systems must be available during loop checking:

  • Mechanical construction is completed.

  • Cabling has to be completed.

  • The PCS, SIS and other systems involved should be installed and functional. SAT is completed.

  • Documentation for all loop components is available. (loop-drawing, instrument calibration data, PCS configuration, SMART data)

Loop test procedure.

Industrial pressure transmitter indicating 7.7 barManipulating the signal of a transmitter may be difficult. For example to simulate the full range of a -30 to 200 degr Celsius temperature sensor one has to create a temperature of -30 to 200 degr Celsius in 25% increments. It is much easier to manipulate the 4-20 mA signal from the temperature transmitter. This is done with a current loop tester.

With this current loop tester the 4-20 mA signal from the transmitter is simulated. This signal is forwarded to the PCS and evaluated.

During the functional check of a loop with the current loop tester, checks shall be made of the fail conditions of the loop components. Does the component’s action on failure meet the specified fail safe action. Also the complete loop shall be checked for fail safe operation, does a component failure bring the loop to it’s specified fail safe action? If transmitters are specified according the NAMUR NE43 recommendation, this shall be verified as well.

Loop test steps:

For each loop, the following steps will be checked:

  1. Check the documentation: loop-drawing, instrument calibration data, PCS configuration, SMART data.

  2. Locate the instrument. Check the P&Id.

  3. Visual Inspection: Check all components installed and tagged as indicated on drawing, Electrical connections tight, covers in place. Local readout, easily and safely readable?

  4. Check the transmitter is powered, and if a display is present on the transmitter the indicated value is correct.

  5. Disconnect the wiring from the transmitter

  6. Check the PCS indicates a wire break failure.

  7. Connect the wires to the current loop tester.

  8. If required, set the current on the current loop tester to 3,5 mA, check the PCS indicates the correct failure.

  9. Set the current on the current loop tester to 4 mA. Check the readout value on the PCS, is correct and the lowest value from the range. (0%)

  10. Set the current on the current loop tester to 8 mA and check the readout is 25%

  11. Set the current on the current loop tester to 12 mA and check the readout is 50%

  12. Set the current on the current loop tester to 16 mA and check the readout is 75%

  13. Set the current on the current loop tester to 20 mA and check the readout is 100% of the range.

  14. If required set the current on the current loop tester to 21,5 mA and check the PCS displays the correct error.

  15. Remove the current loop tester and reconnect the transmitter. Check the signal is valid, so for a temperature transmitter in a 20 degrees environment the automation system shall indicate 20 degrees.

  16. Complete the documentation, fill out the loop check sheet, on the checked loop.

  17. Done, Next.

Loop check results

The results of the loop check have to be recorded on the loop check sheet. These shall be marked:

P Pass

F Fail

PR Pass after repair.

When the loop check fails, various failures can be identified which need to be passed on to the responsible department:

Installation failures.

The specified hardware or installation is not as per specification, incorrect installation incorrect instruments etc.

Configuration failures.

Software is not configured as per functional specification.

Engineering failures.

Despite installation of the designated instruments in accordance with the applicable drawings and specifications the desired functionality can’t be realized.

Choosing the best current loop tester.

Special attention the current loop tester used for this task has to be connected and disconnected for each loop which will be tested. So easy connection and disconnection of the wires is important. Also the testing is done with steps 0-25%-50%-75%-100% for efficient testing selecting these steps has to be quick and accurate.

List of abbreviations

PCS Process Control System

SIS Safety Instrumented System

FAT Factory acceptance test

SAT Site acceptance test

SIT Site Integration Test

PLC Programmable Logic Controller

DCS Distributed Control System

HMI Human Machine Interface

P&ID Process (Piping) and Instrument Diagram

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