Common Causes of Programmable Logic Controller (PLC) Failure

Reasons Why PLC Control Systems Fail

At industrial facilities, numerous electronically controlled machines are programmed to move in various directions. This is made possible by a programmable logic controller (PLC), which translates commands into a language machines can understand.

 

When a user presses a button on a machine, the machine cannot comprehend the action without a programmable code. Therefore, a code is necessary to prompt the machine to act. This PLC code resides in a central processing unit that serves as an intermediary. The user pushes a button, the PLC translates this into a command the machine understands, and the command is executed.

 

Since their inception in the late 1960s, PLCs have become more advanced. Today, they are programmed for a variety of complex tasks and run continuously to translate constant streams of commands for machinery. Despite their sophistication, PLCs can encounter problems.

 

Common causes of PLC control system failures include module failure, power outages, and poor network connections. Issues can also arise from overheating, moisture, and electromagnetic interference. To address these problems, factory engineers must regularly inspect their systems to prevent issues from escalating.

 

Although PLCs are designed to operate in harsh environments, even the most advanced codes can fail if exposed to power disruptions or physical damage to the circuitry. Therefore, it is essential to understand the causes of PLC failure and how to prevent and address them effectively.

Input/Output Modules and Field Devices

In four out of five instances where a PLC fails, the issue typically stems from one of these three factors:

 

- Module failure of the input/output (I/O)

- Problems with a field device

- Issues with the power supply

 

When one of these components fails, the problem becomes evident through disruptions in the process, sometimes causing it to stop abruptly.

 

Each of these problems prevents the PLC system from receiving the necessary signal to execute a sequence. To resolve such issues, an engineer will examine the system software to identify the root cause, which is usually linked to a specific I/O point.

 

Once the engineer identifies the problematic I/O module, they can trace the problem's extent from one end to the other. Various issues may be involved, such as:

 

- An error in the PLC configuration

- A loose terminal block

- A failed VCD supply

- Wiring problems

- A tripped circuit breaker

 

In some cases, the I/O module may need to be replaced. For older systems, this can be challenging since newer modules might not be compatible with the existing components.

When one or more inputs behave erratically or fail completely, it usually indicates an issue with either the PLC or the power source. The first step is to determine if the problem originates from the I/O module. Next, check if the wires or power source are causing the issue. If these components are not the source of the failure, the field devices should be inspected.

 

The components of the system that are physically separate from the I/O module should have their configurations checked for errors. With any field device, a problem could also stem from damage to the circuitry, which may sometimes be caused by moisture exposure.

Ground Integrity

For the safety of both the PLC and its maintenance crew, correct grounding is essential. Ground integrity also acts as a shield against electrical white noise. Inspectors should always examine the ground wiring during PLC inspections to identify any potential issues.

 

For instance, a ground wire that has been partially damaged by environmental factors may have reduced capacity, even if it still functions to some extent. A loose grounding connection would also render the grounding ineffective. It is crucial for wires to be fully intact and properly grounded to ensure reliable electricity flow to a PLC system.

 

Common issues to look for in ground wiring include damage to the wires and weak connections. During inspections, engineers can use a multimeter to test the wiring. They should check if the PLC ground terminal shows resistance to the equipment's connecting point. Understanding this can help identify the root cause of grounding issues.

Power Supply Failure

For a PLC to execute programming code between an input and output, it requires a steady, uninterrupted power supply. When power is disrupted, it could be due to various causes. Excluding local or regional blackouts, the most common causes of power failure include grid failures, loose connections, and worn cables.

 

To mitigate the risk of power failure, most modern industrial facilities have backup power sources. In the event of a main power outage, these secondary power sources maintain the operation of critical functions, either continuously or at least long enough for a proper shutdown. Some facilities employ an uninterruptible power supply (UPS), which serves as a redundant power source.

 

However, not all factories deem this process essential. As a result, these facilities could suffer significant system damage during a primary power source outage. The damage occurs due to the electrical shocks system components receive during brownouts or blackouts. A power surge could severely damage various system components, and some may not function as effectively afterward. Additionally, process data can be lost during a blackout if operators do not save the data before the outage.

 

Electrical Noise Interference

Foreign signal interference from electrical noise can significantly affect the performance of a PLC. The most common cause of electrical noise is electromagnetic interference, which typically occurs when a large motor is activated or when lightning strikes nearby. Another source of electrical noise is radio frequency interference, which can result from nearby antennas and handheld transmitters.

 

The impact of electrical noise can range from minor disruptions to complete PLC failure. Therefore, it is crucial for facilities to minimize the potential for such interference. Failing to do so can lead to prolonged downtime and expensive damage.

 

In an industrial setting, handheld devices that might cause interference should be prohibited on the factory floor. Additionally, any machinery that could contribute to interference should be kept separate from the PLC. If necessary, consult a service engineer to identify the most effective ways to isolate or shield potentially interfering components.

Loss of Network Communication

Most PLC control systems need to communicate with surrounding equipment to function properly. This includes peripherals and human-machine interfaces, which are connected via Ethernet cables that transmit commands between points.

 

If a connection between two devices fails, they cannot execute their programmed functions. Consequently, communication loss often results in downtime at an industrial facility. System diagnostics will usually reveal the root cause of the disconnect, which is sometimes located in the central processing unit.

 

To prevent such occurrences, engineers must regularly inspect the connections between different system components. During these inspections, technicians should check the physical infrastructure of the network at each point to ensure commands can be activated and terminated as intended.

 

When additional devices are added to the system, further inspections are necessary to ensure the new connections are secure and that older connections remain unaffected. To maintain safe operations, firmware updates and patches should be installed with each new device to protect against security vulnerabilities.

Thermal Management in Electronic Systems

Excessive thermal buildup poses a significant hazard to electronic systems. Particularly, a Programmable Logic Controller (PLC) in close proximity to high-temperature devices is vulnerable to malfunction without adequate protective measures. It’s essential to regulate the ambient temperature to remain substantially below the manufacturer’s maximum permissible limit. Failing to do so might lead to the overheating of the PLC or its associated components, compromising their functionality.

Moreover, high humidity can introduce moisture, further jeopardizing PLC integrity. Unnoticed condensation within the PLC housing can cause abrupt operational cessation. This can lead to an unplanned halt in operations, necessitating expensive remedial actions due to thermal issues.

In settings equipped with Heating, Ventilation, and Air Conditioning (HVAC) systems, it’s crucial to adjust the cooling to maintain a cool environment around the PLC. Heat sources should be positioned at a prudent distance to prevent thermal interference. Ensuring that the PLC functions within the safe temperature range prescribed by its manufacturers is key to mitigating risks associated with thermal stress.

Environmental discrepancies

Environmental discrepancies can precipitate malfunctions or outright failures in Programmable Logic Controllers (PLCs). Such issues arise when the PLC’s internal code is at odds with external changes. For instance, alterations in the connected peripherals or updates that the PLC’s code cannot reconcile could disrupt operations. A PLC’s operational scope is confined to its programming parameters.

Additionally, the integration of an input device without thorough compatibility assessment can lead to communication breakdowns. The PLC may fail to interpret and execute the commands from such devices. To prevent these issues, comprehensive testing of any system modifications is imperative to ensure seamless operation.

Risk Management

To minimize the risk of issues with Programmable Logic Controllers (PLC), it’s crucial for facility engineers to adhere to strict procedural guidelines. This includes ensuring that all components are securely interconnected and that the PLC’s operational environment is free from electrical disturbances, moisture, and excessive heat.

Historically, some manufacturing plants have neglected adequate maintenance protocols, leading to frequent system failures and component breakdowns, which are expensive to rectify. A common and significant oversight is the lack of monitoring for overheating, which can severely damage any equipment operated by a PLC. Additionally, the heat can lead to moisture accumulation, potentially causing controller malfunctions.

Electrical noise presents a grave concern as well, as it can interfere with the PLC’s command processing. If a system is instructed to perform a specific action, the PLC may fail to execute the command due to signal interference. Consequently, it’s imperative for facilities to implement stringent controls to prevent any sources of electromagnetic or radio frequency interference from being in proximity to the PLC.

 

Q&A on common causes of PLC failure:

Q: What often leads to PLC module breakdowns? A: Module malfunctions in PLCs can stem from defects, aging, or adverse environmental conditions, which disrupt signal flow and affect functionality.

Q: How do power supply problems impact PLCs? A: Inconsistent voltage, spikes in power, and blackouts can lead to unstable operations or even complete failures in PLC systems.

Q: Why is maintaining proper grounding crucial for PLCs? A: Grounding stabilizes the PLC and shields it from disruptive electrical noise, ensuring smooth operation.

Q: What issues with field devices can cause PLC disruptions? A: Malfunctions in input/output units or field devices can prevent PLCs from receiving essential signals, leading to operational failures.

Q: In what way does electrical noise affect PLCs? A: Electrical noise can cause confusion in the PLC’s command code, leading to misinterpretation or non-execution of commands.

Q: What are the effects of losing network communication on PLCs? A: Network disconnections can isolate PLCs, hindering their ability to exchange critical operational data with other system parts.

Q: Can environmental conditions lead to PLC breakdowns? A: Indeed, extreme temperatures and damp conditions can cause PLC systems to malfunction or cease functioning.

Q: How do compatibility issues with PLCs arise? A: Compatibility problems may occur when PLCs fail to recognize updates or changes in the peripherals they control.

Q: What happens when a PLC’s memory gets corrupted? A: Corruption of memory can erase program data, causing incorrect operations or a total system shutdown.

Q: Why should PLC systems be secured against cyber threats? A: Cyber threats can lead to unauthorized access or attacks, compromising the security and operation of PLC systems.