Hazards intrinsic safety

RP12.1 Recommended Practice for Electrical Instruments in Hazardous (Classified) Atmospheres Intrinsic Safety... 

In the elaboration of a safe process it is necessary to look for those synthetic routes which do not incorporate hazardous raw materials, solvents, and additives. This is unfortunately rarely achievable. Therefore, the best way to insure higher intrinsic safety is ... 

Capp, B. and Widginton, D.W., "The Intrinsic Safety of Resistive Circuits", 2nd International Conference on Electrical Safety in Hazardous Environments, 9-11 Dec 1975, pages 43-47 Electrical Equipment for Hazardous Locations, Underwriters Laboratories, Inc., Northbrook, IL... 

Redding, R.J., "The Use of Solid State Circuitry Within Hazardous Areas", 3rd International Conference on Electrical Safety in Hazardous Environments, 1-3 Dec 1982, pages 219-233 Weatherhead, D., "Intrinsic Safety", Measurement and Control, Vol 10 No. 9, Sept 1977, pages 341-349... 

Intrinsic safety is based on the principal of restricting the electrical energy available in hazardous area circuits such that any sparks or hot surfaces that may occur as a result of electrical faults are too weak to cause an ignition. The useful power is about 1 watt, which is sufficient for most current instrumentation. It also provides a personnel safety factor since the voltages are low and it can allow field equipment to be maintained and calibrated "live" without the need for a gas free environment verification. Electrical components or equipment can be manufacturer as intrinsically safe and there readily usable in areas where combustible gases or vapors may be present. 

With regard to hazardous substance substitution, the guiding principle of the intrinsic safety of chemical substances, products or application systems was examined within the scope of the SubChem project. An essential approach for implementation of the guiding principle of the intrinsically safe substance or the intrinsically safe application system, especially with regard to the health and ecological consequences, would be the use of chemicals of short ranges (chemicals with low environmental persistence and transport distance) e.g. by ... 

The guiding principle of intrinsic safety also contains the idea that the chemical stmcture of a substance already predefines its potential effects and that it can therefore be possible to determine presumably more safe and presumably more problematic to hazardous substance groups on the basis of their stmcture (QSAR). Further guiding principles, which were developed within the scope of the workshop, are outlined in brief in Table 2. 

TGL 30042 has defined a four-zone concept for areas hazardous due to combustible gases, vapours and mists EG 1, comparable with zone 0 EG 2 and EG 3, covering zone 1 EG 4, comparable with zone 2. In contradiction to the majority of standards covering the zone 1 requirements with all types of protection such as flameproof enclosure, increased safety, pressurization, oil immersion, intrinsic safety, powder filling, EG 2 excludes the general application of, e.g., increased safety e, which has been the domain of EG 3. 

Associated apparatus is commonly installed in a safe area. Many applications of intrinsic safety in remote control and monitoring instrumentation are assembled in such a way that an intrinsically safe apparatus, e.g. a sensor or actuator in the hazardous area, is connected with an associated apparatus, e.g. a safety barrier or an Ex i-isolator in the safe area (see Fig. 6.196). With that, the associated apparatus takes over the function to safely limit current and voltage in the intrinsically safe circuit to permissible values. 

Another special feature of intrinsic safety - i - also results from its appropriateness for zone 0 applications. Hazardous areas classified as zone 0 are characteristic of the presence of an explosive gas atmosphere at a high probability, e.g. in the interior of a ventilated tank. Thus, special safety precautions are required. 

In the United States the discrimination between categories ia and ib is necessary only if the IEC-based classification of hazardous areas has been adopted. According to the differentiation into Divisions 1 and 2 (still predominant), one single type of intrinsic safety (IS) has been defined, requiring a two-fault assessment. 

As a rule, they are installed in the safe area as associated apparatus. They are used to limit the electrical values for sensors, actuators etc. located in the hazardous area to levels permissible for intrinsic safety. A certificate is required each time. 

Currently, remote I/O systems of several manufacturers are available for operation in zone 1 or Division 1. To a large extent, these systems are based upon the technology of intrinsic safety. The power supply only is designed according to another type of protection, e.g. flameproof enclosure - d A remote I/O suitable for hazardous areas may be an integration of input (I) and output (O) assembly units of automation systems and of Ex i-isolators of classic design. The installation may be made in a hazardous area and supersede the classic field distribution box (terminal box). Standardized fieldbus systems may be used for data transmission from/to the automation system. These remote I/O systems for hazardous areas are characterized as follows ... 

Redding, R. J. Intrinsic Safety The Safe Use of Electronics in Hazardous Locations, McGraw-Hill, London, 1971... 

The enhanced intrinsic safety as a benefit of scale reduction may be illustrated by the maximum hazards associated with the operation of a sub-micro reactor containing a few tenths of a gram of catalyst when working with flammable gases or liquids, the low flow rates would only sustain a flame of the size of a small candle. In the case of a toxic gas such as carbon monoxide, the emission associated with a typical flow rate would be less than the amount of CO given off by a burning cigarette. 

Avoiding the use of MIC removes a significant amount of the hazard associated with the process and as a result gives an inherently safer process. However, the alternative process still uses phosgene, which is extremely toxic. A process that avoids phosgene would provide further intrinsic safety. 

The most effective method of avoiding accidents is through intrinsic safety. For a piece of equipment to be intrinsically safesafety has been built into the design of the equipment and is not an add-on function. Intrinsic safety can be achieved by (1) eliminating the hazard entirely or (2) limiting the hazard to a level below which it can do no harm. Under either condition, no possible accident can result from the hazard in question. Hazard elimination... 

Intrinsic safety ia, ib, ic (note 2) EN 60079- 11 The equipment used in the explosion hazard zone only contains intrinsically safe circuits. A circuit is intrinsically safe if neither a spark nor a thermal effect can cause the ignition of a standard explosible atmosphere. The corresponding tests are carried out under standardized conditions, which comprise both normal operation and certain faulty states. Measurement and control devices, communication devices, sensors, actors... 

By definition, an intrinsically safe circuit is one in which no spark or thermal effect is capable of causing ignition of a given explosive atmosphere. The intrinsic safety of the equipment in a hazardous area is assured by incorporating a Zener diode safety barrier into the control circuit such as that shown in Fig. 13.6. In normal operation, the voltage across a... 

B. Svacina, B. Larson, Understanding Hazardous Area Sensing Intrinsic Sensing Handbook bttp //www.parrinst.com/wp-content/uploads/downloads/2011/06/Svacina-Larson Understanding-Hazardous-Area-Sensing Intrinsic-Safety.pdf... 

Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation. Part 1 Intrinsic Safety ANSI/ISA-RP12.06.01-2003. 

Besides the strain gauge type transducer, other pressure transducers may employ variable capacitance, variable inductance, or piezoelectric crystal sensors to detect pressure changes. The low level output pressure transducer can be used in systems controlling flammable and other hazardous gases if the system has been designed such that the electronic power levels being used to excite the transducer remain very low. They can even be used in Class I, Division II areas if the proper intrinsic safety barriers are used in conjunction. Like the pressure gauge, the transducer may have a... 

Establishing a procurement policy, which includes a pre-procurement risk assessment, will ensure that correct selection of plant and equipment for use in hazardous areas (where a flammable or explosive atmosphere has the potential to cause a fire or explosion) will minimise the risk relating to such equipment. BS EN 50020 2002, Electrical apparatus for potentially explosive atmospheres - Intrinsic safety i , specifies the construction and testing of intrinsically safe apparatus, intended for use in potentially explosive atmospheres and for associated apparatus, which is intended for connection to intrinsically safe circuits which enter such atmospheres. This document may well form the basis from which an organisation establishes its procurement policy for such equipment. 

Can intrinsic safety be achieved Mechanical hazards should be eliminated as far as possible at the design stage. 

Job safety analysis (JSA) is concerned with uncovering and rectifying potential hazards intrinsic to or inherent in the workplace. Usually, workers, supervisors, safety professionals, and management participate in JSA. JSA is composed of fhe following five steps [1,12] ... 

Hence, it becomes possible to prevent electronic instrumentation from becoming a source of ignition in the plant areas if its temperature and stored energy levels can be limited by design, to values below the thresholds for the categories of gases on the plant. We know this from of protection as INTRINSIC SAFETY. This is a form of hazard prevention. It must not be confused with a safety instrumented system but it does feature in the list of risk reduction measures. 

Safety Barriers. Figure 1 illustrates an application employing intrinsically safe electrical circuits for the demilitarization of ammunition. Three separate areas are required for this application - one area, classified as non-hazardous, to serve as the control and loading area a second area, classified as hazardous, where the actual demilitarization is accomplished and a third area, classified as non-hazardous, is required for the hydraulic pump due to the level of noise produced. 

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