• Northern IndustrialSeit 1978
  • Northern IndustrialTechnische Beratung
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Programmable Logic Controllers (PLCs) are like any other piece of machinery or electronic equipment - they can break. When a PLC does fail, it can be very difficult to identify the reasons why and the cost of downtime adds additional stress. Here we look at the reasons why a PLC may break down and what we can do to avoid it.

The most common reasons PLCs fail

Although it can be difficult to narrow the fault down, the most common reasons are:

- I/O modules and field devices
- Power supply issues
- Interference
- Networks and communications
- Heat

So why do these elements in particular cause problems? Let’s look into each one in more detail.

I/O modules and field devices

Did you know 80% of PLC failure is caused by I/O modules, field devices or power supply issues? When one of these faults occur, they show by either stopping suddenly or cause the PLC to be irregular in performance.

When this happens, an engineer is required to diagnose where the sequence has stopped. They can do this by interrogating the software ‘On-line’, this is so that they can trace the problem to a specific I/O module and input or output point. By tracing the fault to a specific fault, the engineer can find the root cause which could be down to anything such as PLC configuration failure, loose terminal block, tripped circuit breaker, issues with wiring or even a 24VDC power supply failure.

Power supply issues

Loose or corroded connections is another big reason why PLCs can fail. Many companies have redundant power systems or they will install uninterruptible power supplies (UPS) in order to keep things up and running should a mains power failure occur, keeping control of items to maintain safe operation.

If a power failure occurs and the PLC isn’t kept online by a backup battery the PLC can lose its memory which will affect all operational programs. Ensuring you have a backup battery will protect your PLC from losing its memory and make sure it restarts correctly.

We always recommend backing up and storing your PLC programs regularly, as a lost program is often harder and more time consuming to replace than a piece of hardware, increasing the length of downtime. ​​​​​​​


An easy way of diagnosing faults in a PLC is to use a copy of the PLC software, a laptop, a multimeter and a programming lead.

Electromagnetic interference (EMI) and radio frequency interference (RFI) are not uncommon in industrial environments due to the variety of electrical equipment installed. Anything from small radio transmitters to large motors can cause interference. However, you can take measures to control electrical noise and avoid the unusual behaviour, intermittent faults and PLC failure, such as:

- Relocating sensitive equipment
- Shielding cables between sensitive equipment
- Grounding
- Segregation of systems with high power components and add barriers
- Ensuring the physical network infrastructure is correctly installed and terminated correctly


Installing a control system in the wrong environment and without sufficient airflow causes components to overheat and then fail, resulting in downtime. Never allow the equipment to go over the manufacturer’s recommended temperature as it is likely to fail. High humidity also causes condensation on electrical products, which leads to PLC failure. 

To prevent overheating and associated risks, think carefully about where a control panel is located when installing it, or install panel-cooling systems to keep temperatures down and air flowing.

Network and communications

The very nature of PLCs means they must communicate with other devices such as HMIs and other equipment. When communication is lost between devices it results in immediate downtime. Communication failures can be reduced by ensuring that the physical network infrastructure is installed and terminated correctly. 

If you’re adding more devices to a network, it is important to ensure they are suitable for the purpose and that firmware patches are installed regularly to maintain reliable and secure operation.

Managing the risks of PLC failure

It can be easy and simple for PLC users to manage the chance of control system failure. Engineers can prevent risks by following a few simple steps. Ensure your engineers or maintenance team perform regular visual inspections to identify overheating or electrical noise problems in the early stages. Test backup batteries and UPS systems frequently to ensure reliable operation should a power fault occur. 

We also recommend checking the following things regularly:

- Wiring integrity
- Grounding
- Terminals
- Field devices
- Ethernet and industrial networks
- Software backup and firmware patch installation

Obsolescence management

Finally, a major factor is obsolescence management. PLC manufacturers often update and change their range. It is important that you have replacements for devices and equipment that are several years old. Companies can manage this by themselves or by hiring someone to identify which areas are most likely to fail and put a plan together to manage the risks.

Although all these factors do help reduce the risk of downtime, it can still occur. This is why it is vital that PLC software is regularly backed up, so that normal functions can resume quickly if downtime does occur. Another way of making the system more secure is upgrading firmware.

Here at Northern Industrial, we have a wide range of PLCs available from manufacturers including, Siemens, Beckhoff and Allen Bradley. For more information please contact our friendly sales team info@nicontrols.com. We can also help you with obsolescence management with our free report service, find out more at nicontrols.com/obsolescence