The so-called Safety Life Cycle (SLC) came into being as a set of best engineering practices. Adopting it reduces development and design times and increases reliability levels. The main aim of all international functional plant safety standards is to address the causes of accidents by implementing a safety management system. This system has to ensure that staff have adequate technical management skills. In engineering terms, this system is known as the Safety Life Cycle (SLC). It is a model designed to optimize design, safety and integrity, minimizing the probability of introducing systematic failures or errors into the system. These errors and failures can be caused by specification, design, construction, installation and commissioning issues, by failed or partial implementation of the requirements, by incomplete processes and procedures or inadequate use of the methods and techniques envisaged for carrying out the various tasks.
The safety life cycle concept has been incorporated into international standards such as ANSI/ISA84, IEC 61508 and IEC 61511. These standards are now widely accepted as a basis for compliance with local, national and international laws and regulations as well as “best engineering practices”. Ultimately, understanding the entire process of an SLC should be a prerequisite for choosing a SIL (Safety Integrity Level) for any safety system.
The safety life-cycle approach requires that first of all a problem be identified and evaluated, and then that a project be developed to solve the problem. Finally, the project is verified and validated.
The need for a formally-defined SLC process has emerged forcefully over the past two decades, as the inevitable need for better-performing processes has pushed control systems to a higher level of complexity where electronics and programmable systems have become central to safety strategies.
Safety Life Cycle according to IEC 61508
The safety life cycle (SLC) defined by the IEC 61508 standard is a closed-loop process, which continues throughout the lifetime of the system. Control and analysis tasks are continuously performed as long as the system is in operation and whenever the system is modified or updated. A generic SLC can be divided into three main task areas: system analysis (focused on identifying hazards and on making potential countermeasures available), system implementation (centred around system design and construction), system usage (commissioning, operation, maintenance, modifications and close-down).
The overall safety life cycle proposed by IEC 61508 covers the whole system life span and is made up of 16 phases connected to the system safety function. For each single phase, the standard prescribes the tasks and steps to perform, establishes what information is needed to complete each step, what documentation needs to be produced, and lastly indicates what methods and techniques to use.
Safety Life Cycle for system validation
Generally speaking, standards regarding functional safety such as IEC 61508, IEC 61511, EN 50126 (specific for the railway sector), in addition to the overall safety life cycle, also define one safety life cycle for system development and another for software development. In such cases, the life cycle starts off with planning design tasks and ends with system validation. Again, in such cases, the life cycle can be represented as a cyclical process to be followed each time a change is made. The cycle can be retraced either partially or entirely depending on the consistency of the changes and based on the results of the impact analysis.