Hazardous areas device installation

LIMITATIONS ON INSTALLATION OF ELECTRICAL DEVICES IN HAZARDOUS AREAS... 

Emergency alarm system design and installation should be in accordance with NFPA 72. Electrical aspects of the fire alarm systems should be designed and installed in accordance with NEPA 70. When devices are located in hazardous areas, they should meet the electrical requirements suitable for that hazardous area. 

Figure 6.231 A fieldbus is designed according to explosion Group IIB. To extend it with field devices into a hazardous area endangered by Group IIC gases, an additional current limitation is installed in the terminal box.

Normally a guard is installed on a moving part of the machine, which acts as a barrier to prevent the entry of any part of the human body in the hazardous area. It is also possible that a safety device is installed that prevents or interrupts the operation if part of the operator s body is in a hazardous area or requires its withdrawal prior to machine operation. The guard or safety device itself must not constitute a hazard, must be safe, low maintenance, easy-to-use, automatically controlled, or fixed on the machines. There are different types of guards and safety devices available according to the design and demands of machines. 

Conventional systems are mainly based on two aspects each device has a separate barrier and they shall be wired separately from other field circuits. Probably the first definition of the required parameter for intrinsic application in a fieldbus system was done in the FOUNDATION fieldbus. Fieldbus devices are low-power devices, so IS is very much suitable for fieldbus installations in hazardous areas. The following are distinct features found in fieldbus intrinsic applications ... 

Intrinsically safe A term applied to the installation of electrical devices in an explosion hazard area. An. intrinsically safe system is one designed, in-st ed, and maintained so that imder the most adverse conditions it cannot produce an arc of sufficient energy to be considered a source of ignition. 

Fire-detection devices are usually installed in systems which combine manually activated fire-alarm stations and audible and visual warning devices. They may also be connected to fire-suppression systems in some hazardous areas. 

Fusible Link a mechanical release device actiwited by the heat effects of a fire. It usually consist of two pieces of metal joined by a low melting solder. Fusible links are manufactured as various incremental temperature ratings and are subjected to varying normal maximum tension. When installed and the fixed temperature is reached, the solder melts and the two metal parts separate, initiating the desired actions Hazard Analysis the systematic identification of chemical or physical characteristics and/ or processing conditions and/or operating conditions that could lead to undesired events Hazardous Area, Electrical a US classification for an area in which explosive gas/air mixtures are, or may be expected to be, present in quantities such as to require special precautions for the construction and use of electrical apparatus HAZOP an acronym for Hazard and Operability study, which is a qualitative process risk analysis tool used to identify hazards and evaluate if suitable protective arrangements are in place if the process were not to perform as intended and unexpected consequences were to result... 

Electiical classification. Article 500 of the National Electric-Code provides for the classification of the hazardous nature of the process area in which the measurement device will be installed. If the measurement device is not inherently compatible with this classification, suitable enclosures must be purchased and included in the installation costs. 

The mere fact that voltage, current, or even both, are at low levels does not guarantee a circuit to be intrinsically safe, even though intrinsically safe circuits do utilize relatively low voltage and current levels. Intrinsically safe systems employ electrical barriers to assure that the system remains intrinsically safe. The barriers limit the voltage and current combinations so as not to present an ignition hazard should a malfunction develop. Typically, devices upstream of barriers are not intrinsically safe and are installed in control rooms or other unclassified locations. All devices and wiring on the downstream side of the barriers are intrinsically safe and can be installed in classified areas. 

Integrity of installed electrical equipment maintained 1 Class I Division 2 installations provided with sealed devices. Explosion proof equipment provided or purged reliably and good electrical isolation between hazardous and nonhazardous areas. 

The real world test parameters discussed above have been incorporated into the fire hazard assessment model used by UL (Webber 1983) to determine the compliance of transformer fluids with section 450.23 of the National Electrical Code. In order to be compliant with the code, transformers insulated with less-flammable liquids are permitted to be installed without a vault in Type I and Type II buildings of approved noncombustible materials in areas in which no combustible materials are stored, provided there is a liquid confinement area, the liquid has a fire point of not less than 300 C and the installation complies with all restrictions provided for in the listing of the liquid. UL has identified the need for pressure relief devices and current limiting fusing to limit the effect of possible high current arcing faults. 

Workstation Environments (Fabrication Area). Electrical equipment and devices within five feet of workstations in which flammable liquids or gases are used shall be in accordance with the Electrical Code for Class I, Division 2 hazardous locations, except when the air removal from the workstation or dilution will provide nonflammable atmospheres on a continuous basis (UFC 5104.5). The requirements for hazardous locations need not be applied when the average air change is at least four times that set forth in UBC 307.11.2.4 (UBC 307.11.2.6). The volumes of air moving through and exhausted from a cleanroom t5 ically meet the requirements of these exceptions. If the air volume is sufficient, continuous, and on emergency power, the electrical installation within five feet of the workstation will not have to meet the requirements of Class I, Division 2 locations. 

When using certain gases, it is desirable to maintain them at a set temperature for reasons related to the process and for safety. To do this, there are a number of different types of chillers and heaters. Circulating temperature controllers, use a temperature controlled liquid that is circulated through clam shell t)q)e jackets or coils placed around the cylinder. This type of system can have the actual control unit in a remote location and can easily be used in areas classified as hazardous. Another t5q)e controls the temperature of the air that is circulated through the cabinet. The air either transfers to or absorbs heat from the cylinder. Also available to heat cylinders are electrical resistance t5q)e devices. Some of these electrical devices cannot be installed into areas classified as hazardous. 

All manual valves and valve operators shall be located in areas readily accessible to operating personnel. If valves are installed in areas of high radiation or other hazards they should be provided with extended stems or equivalent devices allowing actuation from a lower radiation area. Handwheels and chain or extension handle operators should be located at a reasonable height above either the floor or a permanently installed platform to allow easy operation. Piping ducts and valves should be located and marked such that the operators can easily identify the flow paths. Pipes or ducts that cross the normal and safety pathways should be at least 2 m above the floor. 

Manual activation devices are normally located in the main egress routes from the facility or process area. Usually placement in the immediate high hazard location egress route and the periphery evacuation routes or muster location of the installation is appropriate. 

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