Instrumented safety systems are not new. It has long been the practice to fit protective systems to industrial process plant where there is a potential threat to life or the environment should something go wrong.
These systems are independent of the normal process control, and take some action to render the plant safe in the event of a malfunction. Until recently, such systems have generally been designed according to established practice within the company concerned, or in accordance with local codes of practice. Some of the necessary equipment, for example shut-down control systems, was available designed and certified for safety applications, but much of the peripheral equipment was not. Designers and installers had to do the best they could with the equipment that was available.
This situation is now changing rapidly. The broad acceptance of the IEC 61508 standard* is giving safety equipment suppliers and users a common framework on which to design products and systems for safety-related applications. The benefits to users are: a more scientific, numeric approach to specifying and designing safety systems is possible; the nature of the risk can be quantified and a protective system appropriate to the risk can be designed. Because the protective system is assessed against a widely accepted standard, its suitability can be clearly demonstrated to all. Under- or over-specifying of protective systems is less likely and, in many cases, a less expensive solution may be shown to provide adequate protection.
Equipment suppliers, such as MTL, are now providing products certified to IEC 61508 for use in functional safety systems. The data provided with these products allow the user to integrate them into safety systems, to the IEC 61508 standard, and then state with confidence that the system meets the safety requirements.
This application note is intended to provide a brief introduction to the IEC 61508 standard, and to illustrate how it is applied. It does not claim to be a complete interpretation of the standard; that would be impossible in so few pages. However, we hope you will find it helpful, especially if the subject is new to you.