Safety-related control systems reliability

In the context of safety-related control systems, the Essential Health and Safety Requirements of the Supply of Machinery (Safety) Regulations 1992 lay down generic requirements that must be considered by suppliers for the safety and reliability of control systems control devices starting and stopping devices mode selection failure of the power supply and the control circuit software and movable guards. 

A personal and relatively informal view of the current and to some extent expected roles of PLCs in safety related systems is presented. An answer to the question "Is there a role for PLCs in safety related systems " is sought. The systems referred to here are plant or equipment control or protection systems. To answer this question, PLCs are explained for those who are not familiar with them and the tenn safety related is explored. The opportunity is taken to set out a basis for discussing safety related control and protection equipment by clearly defining terms such as reliability. The p r then sets out to answer the question proper by comparing two candidate solutions for a typical application. The application is described and the merits of the two candidates, one relay based and one PLC based, are compared. The impact that future developments might have are discussed. 

It needs to be pointed out that ESP 1 is not an unsafe system even if the overall reliability is worse. The system offers the additional safety related function for electronically controlled car stabilization. This function is as important as the anti-lock braking and has a great potential to avoid accidents. Therefore, the worse (functional) reliability for anti-lock braking can be charged against the improved safely of the ESP. 

The plant control systems are non-safety-related. However, for high reliability, three channels are used in all control loops that are directly involved in the generation of power. The median of the three is used for control, and the other two are used to identify channel failures with no interruptions in the power generation process Two channels are used to control auxiliary systems that are needed frequently Single channels are used for auxiliary systems that see infrequent use... 

The System 80+ standard Design incorporates the second approach for assuring the reliability and adequate level of safety for the Class IE power sources and safety-related systems by the selective connection of non-safety-related equipment and strict control of the interface between the non-safety-related equipment and Class IE power system. 

Unresolved Safety Issue (USI) A-30 in NUREG-0933 (Reference 1), addresses the reliability of DC power supplies used in the control and actuation of safety-related components and systems. 

The demand rate is determined by the characteristics of the associated equipment under control (EUC) in combination with the reliability of the EUC control system. The hazard rate is determined by the risk considered to be tolerable in the particular application, taking into account the risk reduction provided by any other safety-related systems or risk reduction measures relevant to the hazard being considered. 

Reliability and system safety System safety analysis and control measure go hand in hand with system reliability. In fact, they con lement each other. Each of them gives a lot offeed to the other. However, one cannot be substituted for the other. It is somewhat like the relation between acid and base. When they operate in close coordination with each other the result is always better in system development... 

H. Bezecnyl, D. Inverso, V. Maggioli, G. Rabe, A. Weinert, Guidelines for the use of programmable logic controllers in safety-related systems, in European Workshop on Industrial Computer Systems Technical Committee 7 (Safety, Reliability and Security) Working Paper 6009 Version 13, October 1997. 

At the center of this is the EN 50126, which deals with reliability, availability, maintainability, and safety for railway systems. EN 50129 applies to safety-related electronic control and protection systems. And EN 50128 applies to safety-related software for railway control and protection systems. These CENELEC standards cova all railway design, operations, and certification in the EU though there are other related and applicable standards, these are the most important... 

Also, indication systems that show the status of safety-related systems (such as are used for monitoring the plant in accidents) need to have high integrity. Hence these systems often feature in special high-integrity displays in control rooms with SIL 2 or even SIL 3 reliabilities. 

Thus far the components that make up a control system have been described without considering in detail how they are integrated to ensure that the systems have sufficient robustness, or safety integrity, for the safety application in which they will be used. Safety integrity refers to the reliability with which the system will perform the safety-related tasks it has been designed to perform, as well as the extent to which it will continue to perform its safety functions in the event of faults occurring. We are therefore interested in system performance in terms of reliability and fault tolerance. 

The aim is to ensure that those parts of a control system that perform safety functions have adequate safety performance for the application. Safety performance, as previously noted for safety integrity, relates to the reliability and fault tolerance of the safety-related parts, and their ability to provide the required level of risk reduction. It is therefore axiomatic that the process of satisfying the requirements of the standard for any particular machine starts with a risk assessment of that machine. 

In software engineering only functional requirements can be directly implemented in software. The nonfunctional requirements are controlled by other aspects of the system. For example, in a computer system reliability is related to hardware failure rates, and performance is controlled by CPU and memory. Nonfunctional requirements can in some cases be decomposed into functional requirements for software. For example, a system-level nonfunctional safety... 

Economics and price of the final article often dictate a specific type of mbber that can be used. The expected usable life for the product is controlled by many factors including end customer awareness, competitive situation in the marketplace, safety, reliability, and other factors. Rubber is almost always used as a functional part of another system. For example tires, hoses, belts, O-hngs, and numerous mbber components are used in manufacturing automobiles and tmcks. The overall life of the vehicle as well as its performance level often control or direcdy relate to the service life or quality level of the mbber parts. 

CCPS defines reliability as the probability that an item is able to perform a required function under stated conditions for a stated period of time or for a stated demand. In addition to relying on QA/QC, companies use dependable data to conduct reliability analyses. Though related,- reliability is different from quality. While quality control is concerned with the performance of a product or process at one time, reliability is concerned with the performance of a product over its entire lifetime. Reliability engineering addresses all aspects of a product s life, from its conception, subsequent design, and production processes, through its practical use lifetime, with maintenance support and availability, and covers reliability, maintainability, and availability. Process safety metrics data can provide valuable input to the life data analyses used to estimate the probability and capability of parts, components, and systems to perform their required functions for desired periods without failure, in specified environments. 

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