Mar 22, 2011

Troubleshooting PLC Problems at Site

PLC Troubleshooting is the part which every Automation/PLC engineer has to master to become a good PLC Engineer. Identifying the PLC problems and proceeding for correct solution is a must to ensure proper running of the system. PLC installed for Critical applications and process need to be trouble free and should have high response time. Here in this article we will discuss about the common troubleshooting tips which will be advantage when PLC engineer goes to site for troubleshooting PLC.

How to Troubleshoot PLC at Site :-

Many a time it happens that after the Installation of PLC panel at customer site and commissioning it properly , a problem may arise after some time. The problem can be in Inputs/Outputs connected to PLC or in field wiring or lose connections or even due to malfunctioning of PLC. But for all the above problems customer always calls PLC engineer to solve it out , as client always thinks that there is some issue with PLC.

So its the core responsibility of PLC engineer to identify the problem and solve it out. Troubleshooting with PLC related issues is the thing which we are going to discuss today.

The first thing need to be checked is that first we should identify the source of problem that whether it is due to inputs/outputs or PLC internal hardware. Connect the Laptop and check each and every Input first in the PLC Program. If any input is not coming then check the reason for it. Similarly repeat the above procedure for all outputs. The reason for any Input signal not showing in PLC Program can be due to loose connection or malfunctioning of particular Input , so rectify the same.

Also if the output command is on from the PLC program but if it is not operating then again the loose connections can be the issue or malfunctioning of field device , so check it one oby one for each output device connected to PLC. If all the input and output devices are OK but still the system is not operating then we have to proceed further. The problem can be in the PLC I/O module or it can be even the processor issue. Experience has shown that more than 80% of the problems are due to malfunctioning of PLC CPU/Processor.Subscribe to Automation-Talk by Email

Now let us look out for the possible methods to check out the PLC Internal problems. First of all visually look out for the integrity of PLC power and ground.If ground is not connected than connect it and using the multimeter measure the voltage between the PLC ground and some other known ground , both AC and DC voltage should be zero between these two ground points.

After this check out the PLC Power supply , make sure that it is between the permissible range for PLC input range. If the PLC is operating by DC Power supply then also look out for AC ripples in DC Supply , it can be done using a digital meter . Due to AC ripples in DC supply have drastic effects on the operation of the microprocessors and memory devices typically found in PLC processors.

Finding Out PLC CPU/Processor Internal Issues
The final thing to be checked is the voltage of battery in the PLC , battery power is required to hold and maintain the PLC data words and Internal Memory. Other reasons for the abnormal behavior can be the Electro-Magnetic Interference or Radio Frequency Interference. Check out that is there anf Larger rating motor start up in the nearby environment , arc welding , lighting strikes or any use of radio transmitters. Try to correlate the abnormal behavior of PLC CPU with these issues.

Although these things may seem harmless to PLC but they can sometimes a big problem for PLC CPU to operate it properly .To solve out these types of problem the only thing which can be done is power wiring conditioning, grounding, and shielding.

We will be discussing more about the PLC realted Troubleshooting Tips and Tricks , till then you can subscribe to get all updates in yours Inbox.Subscribe to Automation-Talk by Email.

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