Failsafe PLC

  • Thread starter RANA EHTASHAM UL HASSAN
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Thread Starter


This terminology has always baffled me as I couldn't comprehend what does it mean. Some say that in case of power failures a failsafe PLC would take process control values to pre-configured values which are safe positions as far as the operation is concerned... while others argue that since a process instrumentation is already configured to lock to failsafe position in case of failure, then what remains as the purpose of going for such an expensive option, i.e. 'failsafe PLC'?

Fred Gjoertz

Sounds like an expensive add-on to me. I would rather go for a redundant solution with inexpensive PC-based controllers. We have been using this for years for mission critical systems which require high level of safety, security and resiliency. Using our Control Design Platform, we and our clients have been able to make double, triple and even quadruple redundant solutions using standard industrial PCs, which may even run different OSs like Linux, RTOS-32 or Windows XP(e). Running different OSs ensures that a software fault bringing the controller to a stop will not affect the other controllers in the redundant solution. For more information check out our web-pages:



Abhijit Goswami, Haldia

Any failure may be classified as safe or dangerous. If the device has inbuilt diagnostics, it can be further classified as safe-detected, safe-undetected, dangerous-detected and dangerous-undetected.

A fail-safe PLC has high level of diagnostic subroutines handled by firmware to ensure that the PLC delivers desired response under all conditions. Any exceptions may lead to PLC shutdown either through firmware routine or using application program to meet specific user requirement.

Ideally a fail-safe PLC should have no dangerous-undetected failure, where the PLC continues run and manage the process with one or many internal functions not in order!!!

Fail-safe PLCs are normally certified by TUV.