Feature: Functional Safety
Keeping Safety as Standard Ian Curtis, safety consultant for Siemens Industry Automation advocates a ‘back to basics’ approach to ensure effective functional safety and says modern safety system tools can help reduce complexity, deliver value and drive risk reduction when implementing a Safety Instrumented System (SIS).
Things are getting ever more complex in the world of functional safety. Increasingly powerful programmable safety systems, the trend toward integration between control and safety, cyber security concerns and the increased use of Commercial Off The Shelf (COTS) technologies within safety systems are all contributing factors which ramp up the complexity factor.
The functional safety world has, despite its understandably conservative approach, changed considerably in recent years as it hangs on to the coat-tails of technological advancement. However the need to keep things as uncomplicated as possible from a human perspective is still very important. Even though programmable safety systems are much more capable and powerful than they once were the extra power and flexibility they bring needs to be controlled so that SIS are easy to understand and straightforward to apply throughout all stages of the safety lifecycle.
To help tame the complexity and thus ensure functional safety technology contributes effectively to risk reduction it is important to take a holistic approach. Effective use of existing and new standards and a structured approach to safety system implementation through effective functional safety management are part of the answer, but also important is the use of new technologies and tools which help simplify aspects of the safety lifecycle.
Standards show the way
When it comes to achieving best practise the accepted route is to follow the appropriate standards. The standards in question fall into distinct categories; application specific, sector specific or general. For certain applications, such as Burner Management Systems (BMS), one would typically follow an application standard whereas for more generic safety instrumented systems, such as an ESD system for a process plant, one would follow the process sector specific standard IEC61511. In the unlikely event that neither is applicable (i.e. when using a fully variable programming language), one would typically revert to using the basis standard for functional safety IEC61508.
Historically, many of the application oriented standards take a prescriptive approach whilst the newer standards, such as IEC61508 & IEC61511, promote a more performance-based approach. In an effort to get the best of both worlds there are recent guidance documents and standards advocating a combination of both approaches. Application standards typically describe, in detail, what must be done to implement the SIS whilst performance-based standards seek to ensure that the SIS will “perform” when there is a demand placed upon it, so, by using a combined approach, the standards not only describe “what to do” but also “how well to do it”.
Recent technical reports from the ISA such as “Guidance on the Identification of Safety Instrumented Functions (SIFs) in Burner Management Systems (BMS)” (ISA-TR84.00.05-2009) help show how this combined approach can work in practice.
Also, recent guidance on Fire and Gas systems such as "Guidance on the Evaluation of Fire, Combustible Gas and Toxic Gas System Effectiveness" (ISA-TR84.00.07-2010) follows a safety lifecycle similar to that of IEC61511, again marking a coming together of performance-based and prescriptive approaches.
Of course identifying the right standard or combination of standards to use is just the beginning. They have to be followed and this is not always easy. Performance-based standards are often open to interpretation, with areas of the standards which may be unclear on first reading, but help is at hand as there are various guides to implementing IEC61511 (“EEMUA Pub 222 Guide to the application of IEC61511 to Safety Instrumented Systems” is one such) and these provide useful clarifi- cation and sample documentation. Guides such as this are very useful in helping to interpret the standard as they effectively capture the experience of a wide range of practitioners and distil it into the form of useable advice. In a similar vein, the IEC61508 association provides role-based guidance in its toolbox talks, and specific guidance on key issues such as legacy systems from its website (
www.61508.org).
Functional Safety Management is key
The standards seek to address random hardware failures and systematic errors by having competent people develop, implement, operate and maintain a sound technical solution using good processes throughout.
The latest version of the standard, IEC61508 Ed.2 (2010), significantly increases the emphasis on functional safety management and makes competence a normative requirement. In essence companies must ensure that those involved in the safety lifecycle are competent to perform the activities required of them, and that they perform those duties following work processes that are in accordance with the requirements of the standard and provide documented evidence to demonstrate this.
If using sub-suppliers then it is incumbent on a company to ensure they too address issues of competence and FSM. A “joined up” approach between organisations is required to ensure nothing falls through the cracks. Roles & responsibilities need to be assigned and documented in a project safety plan.
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www.thepanelbuilder.co.uk May 2012
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