Are We Ready for Digital Safety?

Safety Applications in Conjunction With Networks Will Be the Future in the Automation World

By Andrew Eberhard

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10th AnniversaryEnjoy this 10th anniversary look back at the content published since we started in 2002. Here's a column as it originally appeared in the Winter 2003 issue that outlines the benefits of a digital safety network, and foresees an inevitable and large acceptance of the fast emerging technology.

 

Programmable safety applications are entering automation fields that had been reserved for conventional electromechanical technology. Are all industries ready to accept safety systems that rely on bits and bytes? The answer: Not yet. But that time should not be far off.

For many products and systems, a failure to function can expose people and the surrounding environment to hazards or contribute to production losses. A standard safety assessment evaluates how much "safety" has to be incorporated into a device or system to achieve the appropriate safety level. Products such as safety PLCs, fire-detection systems, light curtains, or safety bus systems are considered safety-relevant devices.

Although standard fieldbus applications grew dramatically in the past few years, safety functions had to be realized in a second layer. A second layer usually contains safety relays or is implemented within a special safety network. Additional wiring costs for the diagnosis of safety functions often is necessary, and flexibility is limited due to heterogeneous engineering.

The status of safety-oriented parts or layers can be made available to the control system by coupling with the operative fieldbus. Safety applications in conjunction with networks, regardless of their structure, will be the future in the automation world. Openness and interoperability are key factors to expedite the safety automation process. It won't be too far in the future that Bluetooth or Wi-Fi-enabled control pendants will be a part of an overall safety network.

The current experience gathered in safety networking in the operative area of plant and machine controlling paves the road in the right direction. Benefits include reduced wiring, comprehensive diagnostic possibilities, increased flexibility, and a higher level of safety.

When engineering a safety system, its safety integrity must be built in. In other words, the safety integrity of the intended system architecture has to be predicted and evaluated. IEC 61508 is the major functional safety standard that introduces the concept of the safety integrity level (SIL). The SIL represents the probability that a safety system will not satisfactorily perform the required safety functions under all the stated conditions, within a stated period of time. IEC 61508 also is the basis for the certification of programmable electronic safety systems.

IEC 61508, Functional Safety of Electrical/ Electronic/Programmable Electronic Safety-Related Systems, is a seven-part international standard. It is generic, and applies to safety-related control systems, PLCs, devices and components (including sensors, actuators and operator interface). The four main areas covered by the standard:

●    Measures and techniques for avoiding or controlling faults (hardware and operating system software) during design and development.

●    Hardware fault tolerance of systems/sub-systems (structure) in combination with "safe failure fraction" and diagnostic coverage.

●    Probability of "failure to danger" of the subsystem by reliability modeling techniques.

●    Measures and techniques for avoiding or controlling faults during the design and development of application software.

The concept of  SIL introduced in IEC 61508 is a concept of classes of safety requirements for components, modules, subsystems or functions. The SIL indicates target failure measures for the safety function of an E/E/PES system. This method obtains Markov models for probabilistic calculations that make it possible to determine the accurate SIL level.

IEC 61508 is a powerful blueprint for the future. TÜV Rheinland, which has offices in the U.S. and other countries, currently is the only organization authorized to provide an assessment of safety networks around the globe.

In North America, the U.S. Occupational Safety and Health Administration (OSHA) has endorsed ANSI/ISA-S84.01-1996, Application of Safety Instrumented Systems for the Process Industries, as a "national consensus standard" for the application of safety instrumented systems (SIS) for the process industries. S84.01 covers electrical, electronic and programmable electronic technology, and follows the safety lifecycle, similar to IEC 61508.

Acceptance is growing all over the world. It's only a matter of time before this concept of programmable safety is embraced universally.

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