How to

How to do the commissioning and testing of a PLC?

The PLCs are inevitable in automation industries, it can be considered as an electronic system that is digitally operated. A PLC has a programmable memory for the internal storage of the user-developed programming and this program would have certain instructions. This instruction would guide the PLC to do functions such as logic, sequencing, timing, counting, and arithmetic controls. So with the help of these instructions the PLC can control various machines or processes.

PLC commissioning

The program which is loaded in the PLC is never final it can be edited according to our need, we can change the program as per the requirement of the new system. The PLC commissioning means bringing the PLC system to interact with other control equipment and also the process without any failure.

During the PLC commissioning sometimes program changes are required. In the PLC commissioning

  • All the cable connection between the PLC and the field devices will be checked
  • The incoming power supply must be checked to ensure that it matches the voltage setting of the PLC
  • All the protective devices are checked if it is set properly
  • The emergency stop will be checked for correct operation
  • The software loading and testing will be done
  • In the PLC commissioning all the input and output devices are checked to determine if the input and output devices are connected to the proper input-output points and also giving the correct signal.

Before doing the PLC commissioning we need to check certain factors for the safety of the controlled equipment and the related process. Equipment which is associated with PLC must be safe. So before the commissioning, the technicians must do certain things.

Validation of the safety requirements according to the safety analysis and the subsequent cause-effect diagram. The logic diagram must be verified and its conversion into the application software. Pre-installation simulation in the factory of the PLC with a complete function test at the I/O level. During the event of the complex system, a process simulator is required and certain characteristics must be tested such as the response time, PLC behavior during the power failure, emergency stop, and run mode change. While testing the software simulation of the expected error conditions must be done.

PLC testing before the commissioning

By using the installation report the testing of the installation must be done.

Field wiring

There should be separate installation for the redundant wiring and function survival of very essential cables.


Appliances must be properly earthed in order to get the proper PLC function, signal cable shield should be earthed where the maximum capacity is located.

The length of wiring should be checked, and there should be separate cables for the input, output, and power circuits. Mains live conductors must be checked, the field wiring must be separated from the internal I/O cabling and bus lines. Special care must be given where the mechanical contacts are in series with the inductive loads in DC circuits. We must also check the compliance with the current service and environmental conditions like the temperature, shock, vibration, etc.

PLC setups and check

In this, the interaction between the PLC system and the process periphery will be checked. Binary input checking, the binary, and digital input signals are checked in order to ensure that the physical states of sensors comply with the signal latches in the PLC. Analog input check, this checking is done to ensure the agreement of physical value and data received by the PLC. Binary outputs checking is done to check the ability to switch and in this, it will check if there are any forced binary and digital outputs are set.

The commissioning functionality test of PLC, process, and other control equipment must be performed according to a commissioning test plan. This test plan also includes the different operating modes of the PLC. Fault simulation is performed after the functional test, mostly the fault occurs at I/O and other interfaces to the PLC. So we need to check the failure modes of

  • Contactors, actuators, and sensors
  • Inputs and outputs
  • Field wiring
  • Fuses and circuit breaker
  • Interruption of mains
  • Interlocks

The PLC commissioning can be divided into four types and they are

  • Checking the hardware
  • Transferring and testing the software
  • Optimization of the software
  • Commissioning of the system

Checking the hardware

Hardware checking is the first stage of the commissioning process, the hardware checking can be done by ensuring that each sensor, switch, and button is connected to particular input and each actuator to the output. During the commissioning process, we must make sure that the address and cables should not be mixed. The sensors and actuators must be in the proper actuated system. Before starting the commissioning we must make sure that the output is in the test mode and the actuator must function properly. If there is any change in the register allocation and further documents like this, then it must be updated in order to run the commissioning process properly.

PLC software testing

The input devices such as limit switches can be used to give the open and closed contact conditions and thus we can check the corresponding LED on the input module. The LED will glow if the input is closed and it won’t glow when open. In output devices, they will be installed with PB so that we can check each output. Another way to test the input and output is by forcing and this method is used when the PLC is put in monitor or test mode. Forcing is done by using the software and it is used with keying in instructions from the program console to turn on or off the Inputs/outputs instead of mechanically manipulating them.

Program testing

If a program is written then it needs to be tested and debugged. This is done by simulating the field inputs with an input simulator. The program will be downloaded from the PC to the CPU. The selector switch will be placed in the RUN position. The simulator switches are operated and the resulting indication will be observed on the output. Most of the PLC consists of a software checking program, and this would check through the installed program for incorrect device address and it would display all the input/output points used and it would also show the counter and timer settings if any error is detected. So if there is any error then there would be a message, an example for this will be

  • Certain output address which is used as output more than once in a program
  • The timer or counter is used without a preset value
  • There is no reset in the counter which is being used

Most of the PLCs are loaded with a simulation unit and it is capable to read and write information directly into the input and output memory and thus it simulates the actions of inputs. The installed program can be run and the outputs and inputs will be simulated so that they and all the preset values can be checked. In order to do this kind of operation, the terminal should be monitor or test.

Software optimizing

After checking the software whether it runs properly the next step is to do the software optimizing. It is to ensure that the program can be improved after the first test run. We can make corrections or changes not only in the user program but also in the documentation to facilitate the next stage of the commissioning process.

PLC testing

Generally, the fault percentage of the PLC is reduced by checking the sensors, actuators, and wiring than the PLC itself. The faults in the PLC are mostly related to the input and output channels and the power supply than in the CPU. Many PLCs are composed of built-in fault analysis procedures and this would carry out the self-testing and display fault codes. Timing checks will be carried out by the PLC to determine the time take for certain functions. The PLC would also do the last output check, it involves the use of status lamps to indicate the last output which has been set during a process that has come to halt. The PLC also does the replication test in this test the system would repeat every operation twice and if the result is the same then there is no fault.


post-graduate in Electronics & communication.

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