Fail Safe PLC

B
While there are PLC that are "designed" to be fail safe, there is no such thing as a 100% fail safe PLC. That being said, Siemens appears to use the term "fail safe PLC" for its safety PLCs (i.e. Simatic S7 300, 400F, S5-95F, etc) and there is a safety PLC made by Honeywell which has the name "Fail Safe Controller" or FSC. My thought is that this is just wordsmithing by vendors to distinguish their products. The generic name for such a PLC is a safety PLC and they distinguish themselves from regular PLC (commonly called general purpose PLC) by things like increased diagnostics, redundancy, high reliability designs, fail safe design, security features, utilization of safety lifecycle concepts in design and manufacture, and usually third party certification(typically by TUV) to IEC 61508 or other suitable standards for a particular industry.

Common brands of safety PLC, in no particular order, Triconex, HIMA, ICS Tiplex, ABB August, Siemens Quadlog(formerly Moore), Siemens, and AB

An article that discusses safety PLC which can be found at:

http://www.controldesign.com/Web_First/CD.nsf/ArticleID/JFEY-5L8QRG

Bill Mostia
=====================================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
[email protected] (b) [email protected] (h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.
 
In one word diagnostics, a normal PLC might have 30 to 40 % diagnostic coverage and some number of unrevealed failure modes. A fail safe PLC such as the PRO-SAFE form Yokogawa, has close to 95% diagnostic coverage or higher and an extremely low number of unrevealed failure mode. Normally a fail-safe PLC is used for Emergency Shutdown applications, and carries a much higher price tag then a normal PLC.
If you have such an application that requires the use of a fail safe PLC then you might need to conceder the use of a Triple Modular Redundant – TMR - system such as Triconex.
 
I think you may be talking about a Safety PLC. There is a big difference between a Safety PLC and a regular PLC. The Safety PLC is designed with redundant circuitry and cross checking to insure that it always fails to a safe condition. Use of a Safety PLC alone does not insure a safe system, it is only one component in the overall system design.

For more information see:
http://www.controldesign.com/Web_First/CD.nsf/ArticleID/JFEY-5L8QRG
 
In continuation with the matter posted above by one of our gentlemen, we can also consider ICS Triplex TMR PLC (Trusted) meeting AL6. SIL3 level, meeting the IEC standards. Recently I have done a job for Petronet LNG in Gujarat (India) and worked with the Trusted TMR and believe me, its quite much more advanced than Triconex!
 
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