In today's article, I cover some of the most common causes of PLC failures, as well as solutions to avoid them.

Input/Output Modules and Field Devices

Image by Emmanuel Okih

I/O failures can be caused by:

  • Error in the PLC configuration
  • Slacked (loose) terminal blocks
  • Damaged wires
  • Incompatible modules (old vs new models)
  • Faulty Intrinsically Safe (I.S.) barriers
  • Faulty Field Terminal Assemblies (FTA)

Typically any of these problems will prevent the PLC from functioning properly, as they either interrupt the PLC program execution or stop it abruptly.

To resolve these, use the system diagnostics to determine the root cause of the problem and after which, carry out a total system check to determine the extent of the problem.

If the fault doesn’t originate at the PLC, inspect the field devices wired into them. Problems with field devices are usually due to damaged circuitry caused by exposure to adverse conditions; moisture, vibration, heat, electromagnetic interference, chemicals, etc.

Power Outages

PLC failures can often be caused by frequency interference and unplanned power outages.

These can result in the backup of the PLC program failing, as well as the scrambling of memory that renders the PLC program unreadable by its central processing unit.

Solutions to consider to protect against these failures include:

  • Ensure PLC programs are backed up regularly
  • Change PLC batteries that back-up Volatile Memory routinely
  • Insure backups are stored safely, preferably on a redundant storage solution kept in a dry cool location free from any form of Electromagnetic influence.

Power Supply and Earth Integrity

Power failures obviously disrupt proper functionality of a PLC, and are typically caused by overloaded or worn power cables, slack connections, grid failure, faulty power supply modules, etc.

Consequences of power failure to a PC include:

  • System damage due to electrical shocks received by system components
  • Burn-out components due to power surges
  • Loss of process data due to power surges

Power failures can be avoided using:

  • Backup power source to insure a constant flow of power to the PLC
  • Uninterrupted power supply or redundant power source

Earth integrity failures are typically due to damage ground wires and slack connections. It should be noted that earth failures also present an unsafe condition for maintenance crew.

For proper earth integrity, always look out for any damage to wires and slack connections by testing the wiring with a multimeter to ensure the PC Earth terminals is secured to the connecting point of the equipment.

Network and Communication Issues

Network and Communication between PLCs, peripheral devices and Human Machine Interfaces, DCS and SCADA systems is typically established via wired communications cables.

And when Network and Communication fail, it usually prevents the connected devices from carrying out their intended functions.

Causes of Network and Communication failure can include:

  • Mis-configuration of devices when installed or replaced
  • Obsolete equipment which doesn't support newer devices
  • Incompatible changes to network settings
  • Hardware failures of network equipment
  • Power supply failure of network equipment

Network and Communication failure are prevented by:

  • Regularly checking to ensure connection points are solid
  • Ensuring Firmware is up to date and regularly install security patches

Overheating

When the manufacturer’s approved distance and maximum temperature threshold of equipment installed with PLC hardware is not maintained, PLCs and/or peripheral parts could malfunction as result of overheating.

To avert this, be sure to follow manufacturer’s recommended distance for equipment around the PLC, especially when adding new equipment into existing panels.

Conclusion

The best way to avoid self inflicted PLC issues and failures is to be sure you and your team are following the manufacturer's installation and maintenance procedures.

This includes insuring that your PLC has adequate cooling, as well as reliable and appropriately protected power supply. That new I/O modules and field devices are wired and grounded correctly, and new communication cables are routed according to their specs. And that any changes to network setting take into account all of the existing devices on the network, old and new alike.

If these steps are taken, along with regular PLC program backups and replacement of PLC batteries within their rated lifespans, then most if not all of the above PLC failures can avoided.

Written by Emmanuel Okih
Automation and Control Systems Engineer and Freelance Writer
Edited for content and clarity by Shawn Tierney

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Emmanuel Okih

Emmanuel Okih is a freelance writer for The Automation Blog and entrepreneur who spends his days working as an Automation and Control Systems Engineer in the Oil and Gas Industry
Emmanuel Okih

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