Safety engineering fail safe designs

Fail-safe is a system characteristic whereby any malfunction affecting safety will cause the system to revert to a state that is known to be within acceptable risk parameters. Fail-safe provides the ability to sustain a failure and retain safe control of the system or operation, or revert to a state which will not cause a mishap. A fail-safe design should be provided in those areas where failure can cause catastrophic damage to equipment, injury to personnel, or inadvertent operation of critical equipment (source MIL-STD-1472E Human Engineering, Design Criteria Standard, 1998). 

Reliability and safety may also conflict in engineering design when a choice has to be made between retreating to a fail-safe state (and protecting people and property) versus attempting to continue to achieve the system objectives but with increased risk of an accident. 

A safety instrumented system (SIS) may simply be described as a system composed of an engineered set of hardware and software controls that are especially used on critical process systems. More clearly, an SIS is designed to carry out "specific control functions" to fail-safe or maintaining safe operation of a process in the event when unacceptable or dangerous conditions take place. In the oil and gas industrial sector, SISs are used quite frequently on critical process systems. 

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