April 2019

n INDUSTRIAL CONTROLS The top causes of PLC system failures T he programmable logic controller (PLC) is usually considered to have made its debut in 1968. Ever since, these industrial workhorses have been used to make logic-based decisions in automated processes. Despite their resilience and rugged designs, PLC-based control systems can still break down and their failure can lead to costly downtime. When a PLC system fails, identifying the cause can be tricky. Often, a copy of the PLC software, a laptop, programming lead and a multimeter, are the only tools needed to diagnose 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% of PLC failures are the result of issues with field devices, I/O modules or power supplies. Typically, these defects manifest themselves as a process stopping suddenly, or irregularities in performance, because the PLC system is waiting for a signal to allow it to step through its program sequence. In this situation, an engineer can usually determine 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, you can trace the problem back to its root cause. This could be a PLC configuration error, a tripped circuit breaker, a loose terminal block, a failure of a 24V DC supply or wiring issues. It may be that the I/O module itself needs to be replaced. To do this you need a readily available supply of replacements – something that is becoming increasingly difficult for legacy systems. Ground integrity Proper earthing is important for protecting both the PLC and maintenance personnel. During maintenance or diagnosis, engineers can perform visual checks of earth wiring to establish if there has been any damage or whether there are any loose connections. You can test the integrity of the ground PLCs may have been around for more than 50 years, but that doesn’t mean they are invincible. Failures can occur for many reasons, and it is important to be aware of what can go wrong with PLCs to minimise downtime and maximise their bene ts. James Davey, service manager at the systems integrator Boulting Technology, discusses the main causes of PLC system failures and how to mitigate against them. “By following a best practice routine, companies can minimise the chance of PLC system failures.”