Redundant PLC - Yes or No?
Article#: 00115
Date: 2026-02-01
Author: Radim
In industrial automation, there are many ways of improving machine availability.
One idea that appears quite often is to use a redundant PLC.
At first glance, the reasoning seems simple: if a PLC failure can stop the machine, then using two PLCs in a redundant configuration should reduce the risk of downtime.
But as with many technical decisions, the right question is not whether this solution works, but whether it addresses the real causes of production stops.
From practical experience, machines usually do not stop because of PLC hardware failures
(software-related issues are a different topic and often a significant source of downtime, but they are not addressed by PLC hardware redundancy).
Machines stop because a sensor is dirty, misaligned, or damaged.
A valve does not switch.
A drive reports a fault.
A power supply fails.
A cable or connector causes an intermittent problem.
Or a mechanical part is worn or broken.
And sometimes the reason is simply incorrect operation or maintenance.
Compared to all these causes, the PLC CPU itself is usually one of the most reliable parts of the whole machine.
If only the PLC is made redundant, then only one specific failure case is covered: a failure of the PLC hardware itself.
At the same time, redundancy always increases complexity.
There is more hardware, more software, more configuration, and more things that have to be tested, commissioned, diagnosed, and maintained.
This also means higher costs, not only during the project phase, but over the whole lifetime of the machine.
A more complex system also means more ways in which things can go wrong and more situations that are difficult to understand and troubleshoot.
This does not mean that redundancy is wrong or useless.
There are applications where redundancy is not only useful, but absolutely necessary.
Typical examples are continuous or semi-continuous processes, 24/7 operations, or plants where a restart takes many hours and one hour of downtime causes very high losses.
In such cases, redundancy is never just about using two PLCs in a redundant configuration.
The entire system architecture is designed with availability in mind: power supplies, networks, I/O, controllers, and often also field devices (such as sensors, actuators, remote I/O, and valve terminals).
If we ask what actually causes our machine to stop and what leads to long downtimes, the answer is rarely "PLC hardware failure".
Very often, the answer is things like poorly designed and insufficiently tested control software, unclear diagnostics, missing or confusing messages on the HMI, poor and obsolete documentation, difficult access to machine components for inspection or replacement, and critical dependencies on other or higher-level systems.
A machine is always only as reliable as its weakest part.
And machine availability is always the result of the overall system design: mechanics, electrics, pneumatics, proper architectures and modularization, software quality, documentation, available know-how, diagnostics, service concept, maintenance, and spare parts strategy.
© Radim-Automation, 2020–2026. All rights reserved.
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