Inside the Top Causes of PLC Control System Failure

By James Davey, Service Manager, Boulting Technology 

On New Year’s Day 1968, the programmable logic controller (PLC) was first designed. It has been used ever since to make logic-based decisions in automated industrial processes. Despite their resilience and rugged design, PLC-based control systems can still break down and their failure can lead to costly downtime. This article discusses the top causes of PLC control system failure.

When a PLC control system breaks down, identifying the cause can be tricky. Often, a copy of the PLC software, a laptop, programming lead and a multimeter are the only tools necessary for diagnosing the fault, along with some knowledge of the processes. Sounds straightforward? In many cases it is, but the trap of complacency has a habit of biting. Below is a list of common reasons why PLC control systems fail.

I/O modules and Field Devices

About 80 per cent of PLC failures are a result of field devices, Input/Output (I/O) module failure or power supply issues. Typically, these defects manifest as a sudden process stop or irregularity of performance, because the PLC control system is waiting for a signal to allow it to step through its program sequence. In this situation, the engineer usually determines where the sequence has stopped by interrogating the software ‘on-line’, with the aim of tracing the problem to a specific I/O module and input or output point.

By identifying the I/O point, the engineer can trace the problem to its root cause. This could be a PLC configuration error, tripped circuit breaker, loose terminal block, failure of a 24 VDC supply or issues with wiring. It may be that the I/O module itself needs replacing. This relies on having a readily available supply of replacements, something that is becoming increasingly difficult for legacy systems.

Ground Integrity

Proper grounding is important in protecting both the PLC and maintenance personnel. During maintenance or diagnosis, the engineer can perform a visual check of ground wiring to establish if there has been any damage or if there are any loose connections.

The engineer can 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, we can establish if this is the root of the problem.

Dealing with Interference

Electromagnetic interference (EMI) and radio frequency interference (RFI) are common in industrial environments that contain a variety of electrical equipment. Companies need to control electrical noise, because it can lead to intermittent faults or unusual behaviour and even PLC failure.

There are many ways to mitigate the risk of downtime caused by electrical noise. A service engineer can recommend ways to minimise noise by relocating sensitive equipment, segregating systems with high power components and adding barriers, grounding, or shielding cable between sensitive equipment.

Network and Communications

Most PLC control systems need to communicate with periphery devices such as Human Machine Interfaces (HMIs). A typical communication medium will consist of an industrial network, which industrial plants increasingly base around industrial Ethernet. A loss of communication between devices will often result in immediate plant 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 and firmware patches are regularly installed to maintain reliable and secure operation.

Managing the Risks

By following a best practice routine, companies can minimise the chance of PLC control system failure. Engineers should ensure the environment in which the control system operates is sound. Through systematic inspections, engineers can identify any overheating or electrical noise problems.

Obsolescence management is also important because PLC manufacturers regularly cycle their product ranges. If a component is several years old, it is important to have a readily available replacement. Businesses can manage this internally or by using a third party, such as Boulting Technology, who is able to indicate which areas of a control system are more likely to lead to failure and put a contingency plan in place to mitigate that risk, including sourcing legacy components.

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

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