Programmable electronics

Being excellent at discrete logic, PLCs are a potential candidate for implementing interlocks. Process interlocks are clearlv acceptable for implementation within a PLC. Implementation of safety interlocks in programmable electronic systems (such as a PLC) is not universally accepted. Many organizations continue to require that all safety interlocks be hard-wired, but implementing safety interlocks in a PLC that is dedicated to safety functions is accepted by some as being equivalent to the hard-wired approach. 

Software In some programmable electronic systems (PES), errors are much easier to detect and correct than in others. Using the term software, in the wider sense, to cover all procedures, as distinct from hardware or equipment, some software is much friendlier than others. Training and instructions are obvious examples. As another example, if many types of gaskets or nuts and bolts are stocked, sooner or later the wrong type will be installed. It is better, and cheaper in the long run, to keep the number of types stocked to a minimum, even though more expensive types than are strictly necessaiy are used for some applications. 

HSE 1987. Programmable Electronic Systems in Safety Related Applications. Part I, An Introductory Guide. Part II, General Technical Guidelines. Health Safety Executive, Her Majesty s Stationery Office, London. 

Maggioli, V. J., and G. FI. Stike, Jr. 1990. Programmable Electronic Systems and Safety in Today s Chemical Plants. 24th Annual Loss Prevention Symposium. August 19-22, San Diego, California. American Institute of Chemical Engineers, New York. 

Programmable Electronic System (PES) A system based on a computer connected to sensors and/or actuators in a plant for the purpose of control. 

The use of computers and microprocessors (also known as programmable electronic systems [PES]) in process control continues to grow. They have brought about many improvements but have also been responsible for some failures. If we can learn from these failures, we may be able to prevent them from happening again. A number of them are therefore described below. Although PES is the most precise descnption of the equipment used, I refer to it as a computer, as this is the term usually used by the nonexpert. 

IEC (1998), IEC 61508, Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems, Parts 1-7, Geneva International Electrotechnical Commission. 

IEC 61508, 2000. Functional Safety of electronical/electronic/programmable electronic safety-related systems, Bureau Central de la Commission Electrotechnique International, Geneve. 

Draft Standard IEC 61508, "Functional Safety of Electrical/ Electronic/ Programmable Electronic Safety-Related Systems", (Contact BSI/IEC for availability)... 

Safety Related Instrument Systems for the Process Industries (Including Programmable Electronic Systems)", Publication No 160, The Engineering Equipment Manufacturers and Users Association (EEMUA), 14/15 Belgrave Square, London SW1X 8PS ... 

Eight hrs were required to hand calculate the data on 16 elements from computer-calculated net areas. The hand calculations were performed on programmable electronic calculators. 

The history of human mathematical computations goes back for several millenia. The need for numerical computations has increased since the age of enlightenment and the industrial revolution three centuries ago. For the last 50 years, the human race has become more and more dependent on numerical computations and digital computers. Computational techniques have developed from early hand computations, through table look up, mechanical adding and multiplying devices, the slide rule etc, to programmable electronic computers, mainframes, PCs, laptops, and notebooks. 

NOTE 3 Parts 2 and 3 deal with box 9 (realisation) but they also deal, where relevant, with the programmable electronic (hardware and software) aspects of boxes 13, 14 and 15. 

Since the 1970s, process controls have evolved from pneumatic analog technology to electronic analog technology to microprocessor-based controls. Electronic and pneumatic controllers have now virtually disappeared from process control systems, which are dominated by programmable electronic systems based on microprocessor technology. 

Application of Safety Instrumented Systems for the Process Industries, ISA-S84.01, Instrument Society of America, ISBN 1-55617-590-6, Research Triangle Park, NC, 1996. The objective of this document is to define the requirements for Safety Instrumented Systems (SIS) for the process industries. The SIS addressed here includes electrical, electronic, and programmable electronic technology. 

The scope of the computer system being validated must be clearly imderstood. Computer systems can be thought of as comprising a programmable electronic device that may be connected to actuators and sensors (and possibly other computer systems) via communication links. Closely associated with the computer system are data and equipment, people, and procedures. A schematic of a computer-related system based on some work by the U.S. Pharmaceutical Manufacturers Association (PMA) is shown in Figure 6.1. 

U.K. Ministry of Defence, Defence Standard 00-58 A Guideline for HAZOP Studies in Systems which include a Programmable Electronic System, MOD Directorate of Standardisation, Glasgow, U.K. 

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