The Well-Designed System

Current functional safety standards, lEC 61508 and ANSl/lSA-84.00.01-2004 (lEC 61511 Mod), (Ref. 1 and 2) state that probabilistic evaluation using failure rate data be done only for random failures. To reduce the chance of systematic failures, the standards include a series of design rules in the form of specific requirements. These requirements state that the safety instrumented system designer must check a wide range of things in order to detect and ehcninate systematic failures. 

Examples of these requirements include the need to check that an instrument is being used within the stated environmental conditions and application restrictions of the manufacturer. Specific requirements are made to check for compatible materials whenever any part in a safety instrumented function has process wetted parts. A planned comprehensive testing program with written test results is required. A periodic inspection and test plan must be created and verified. 

A process safety review is done before startup that includes a second check that aU other requirements have been met. Systems designed to meet aU these requirements should have a sufficiently low level of 

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The concept of the "well designed system" was also presented in Chapter 3. A simplistic definition of such a system would be one where aU the techniques and measures presented in our functional safety standards to prevent systematic failures are followed. These techniques and measures are planned to significantly reduce the chance of a systematic fault to a tolerable level. Therefore, systematic failure rates caused by human error including failures due to installation errors, failures due to calibration errors and failures due to choosing equipment not suited for purpose are not included in the calculation. 

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