Top 5 PLC Failure Causes

Lewys Gammond Phillips

Lewys Gammond Phillips

Marketing Officer, Kontroltek Ltd

There are many types of PLC failure causes — some which are expensive, some which are cheaper to fix. But, they all have a detrimental impact on a business, usually causing downtime.

A programmable logic controller (more commonly known as a PLC) is an industrial computer. It monitors inputs and outputs, and makes logic-based decisions for automated processes or machines.

PLCs were introduced in the 1960s. They are still commonly found in production facilities today. They are robust, and can survive harsh conditions including severe heat, cold, dust, and extreme moisture. Their programming language is easily understood, so they can be programmed without much difficulty. PLCs are also modular, so they are fairly adaptable and can be plugged into various setups.

Despite their resilience and rugged design, PLC-based control systems can still break down. PLC failure causes costly downtime usually. This article discusses the top causes of PLC control system failure.


80 percent of PLC failures can be grouped into one of the below factors:

module failure of the input/output (I/O)
problems with a field device
problems with the power supply
You’ll recognise this in your facility usually by a sudden halt in a production process, or by an irregularity in a programme sequence. This stops a PLC system receiving the signal it needs to enact a certain sequence.

When these incidents occur, an engineer will be called out. He or she will examine the system, and be able to locate the problem to a specific I/O module and input or output point.

Once this has been identified, the engineer can trace the problem to its root cause. This could be a range of issues: a PLC configuration error, tripped circuit breaker, loose terminal block, failure of a VDC supply, or wiring problems. It could be that the I/O module itself needs replacing. This relies on having a readily available supply of replacement parts, something that is becoming increasingly difficult for legacy systems.


Correct grounding of the PLC is essential to protecting the safety of the component – and its maintenance crew. Furthermore, a well-grounded enclosure serves as a sound barrier for electrical white noise, so ground integrity is always important.

Each time a maintenance engineer inspects a PLC, the ground wiring should be checked to see if any problems are present, such as a partially damaged ground wire, as these are liable to have restricted capacity, even if it still works to some degree. Wires must be fully intact and grounded to send electricity to a PLC system.

Engineers will test the integrity of the ground with a multimeter. By checking the resistance of the PLC ground terminal to a main earth bonding point in the equipment enclosure, the root of the problem can be discovered.


PLCs are certainly durable and reliable – but this consistency is also reliant on having an uninterrupted power source. Power supply issues can result from a range of causes – the most common sources being loose or corroded cables, or power supply failure.

Many manufacturing plants will have backup power systems in place in the event of a power failure. When this happens, a redundant power source will keep vital operations running. Some facilities will even use an uninterrupted power supply (a UPS) as a secondary power source.

Even if an industrial plant considers a UPS nonessential and a complete process stop is manageable in the event of a power outage, it’s important to remember that a PLC’s memory can be lost when the power fails. This will result in process data shortfall, as well as the complete loss of operational programs. To prevent this, a PLC sometimes employs its own backup battery to ensure the device restarts correctly when power is restored.

Failure to maintain and replace the batteries in a PLC or UPS can lead to a major system failure in the event of a power outage.

It is vital to back up the PLC software regularly and store it securely. If an industrial plant fails to back up the system, it makes it incredibly difficult to resume normal processes in the event of PLC memory loss. In many cases, the outage will be minor and only last for a half-hour or less, yet the damage will be permanent. With backups of your PLC software, facilities can easily overcome such incidents with little or no loss to a PLC system.

4. HEAT:

Although resilient, no electronic component is invincible. A PLC system surrounded by heat-emitting equipment could be at risk of failure if not properly safeguarded. As a consensus, all equipment should be maintained at a temperatures far lower than the maximum threshold specified by the manufacturer.

It’s important to note that a humid environments can also have a damaging effect on a PLC. If condensation goes undetected and accumulates within the controller, a PLC is at risk of suddenly ceasing to function. Consequently, your facility could wind up temporarily shut down pending costly repairs due to heat-related problems. Industrial plants can mitigate by using panel-cooling systems or by considering where the control panel will be located during installation design.


No PLC works alone in an industrial facility. They’ll mostly need to communicate with periphery devices, such as HMIs (Human Machine Interfaces). Typically, this will happen over an industrial network – commonly based around an Ethernet connection. A loss of communication between these devices will, more often than not, result in immediate downtime.

Engineers can mitigate against communications failures by ensuring the physical network infrastructure is correctly installed and terminated, that network devices are suitable for purpose — especially when more and more devices are added — and firmware patches are regularly installed to maintain reliable and secure operation.


Although it has been around for a long time, the PLC is not invincible. However, with proactive maintenance, environment control and contingency plans should a failure be experienced, your PLC control system will be there to keep your operations up and running every day of the year.

In some cases, however, even a well-maintained electronic component can fail. When sudden downtime strikes, the costs can mount quickly; but you could have things up and running again quickly if you know where to turn. At Kontroltek, we repair and service a vast range of industrial electronics, quickly and efficiently. Get in touch to find out more today.

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