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i understand that in a SIL-3 PLC components like I/O modules, processor & power supply has to be redundant. But can a configuration with redundant processors & power supply and non-redundant I/O modules be certified as a SIL-3 PLC or a PLC of this configuration can be regarded as a SIL-3 PLC?
Your appreciation of the typical conventions in industry related to safety systems are generally correct.

The answer to your question really depends upon the manufacturer of the PLC and the standard that you are designing to.

For example the requirements for minimum hardware architecture are slightly more restrictive in IEC61511 than in IEC61508, also some manufacturers incorporate a certain amount of redundancy within processor modules and therefore may be suitable for SIL3 applications with a single proceesor module.

The decision to incorporate further levels of redundancy can therefore be based upon the required plant/system availability targets.

Remember just buying a PLC certified for use in SIL3 applications does not guarantee that you have met the required SIL performance target. The SIL applies to all the equipment implementing the safety function from process sensor to final actuator. It is the instruments and actuators that usually dominate the functions ability to achieve the target.

Hope this helps

For your application why dont you think about Triconex. with IEC61511 now available. It has a capability to ensure you the safety level by SIL2, SIL 3.

There are any number of safety systems out there that can meet this SIL target. Technology has moved on and the idea that 2 out of 3 is safer than other architectures is simply wrong. As someone who works with safety everyday I can confidently tell you that the PES is the least of your worries. PFD and hardware fault tolerance requirements may result in sensor and final element architectures you did not envisage! You will also have to demonstrate a proof testing and operational regime that maintains the SIL of all parts of your SIF.