Intrinsic safety barriers are devices that limit the current, voltage, and total energy delivered to a sensor in a hazardous area or flammable environment in order to prevent an explosion. There are several types of intrinsic safety (IS) devices. Galvanic isolators provide intrinsically safe connections and are used to isolate circuits, either optically or with a transformer, in both hazardous and non-hazardous areas. Most galvanic isolators require an external power source and are used as signal conditioners. Zener barriers are passive devices that contain Zener diodes, resistors and fuses to limit excess voltage and current. They divert potentially dangerous energy to ground and are the basic building blocks for all other types of intrinsically safe barriers. Because of resistance, there is always a voltage drop across Zener barriers. Some intrinsic safety barriers contain light emitting diode (LED) indicators or replaceable fuses for overvoltage and reverse polarity protection. Other devices provide protection against short circuits.
Selecting intrinsic safety barriers requires an analysis of specifications and approvals. Performance specifications include operating temperature and maximum number of channels (I/O). Maximum open circuit voltage (VOC) and maximum short circuit current (ISC) are also important considerations. Devices that limit alternating current (AC), direct current (DC), or both AC and DC are commonly available. In terms of agency approvals, many national and international organizations certify intrinsic safety barriers. Products that bear the UL mark meet published safety standards from Underwriters’ Laboratories. Similarly, devices that bear the CSA mark are certified by the Canadian Standards Association (CSA). Internationally, products that conform to European Union standards feature the CE mark and/or meet ATEX standards defined in Directive 94/9/EC. The European Committee for Electrotechnical Standardization (CENELEC) and Factory Mutual Research (FM), a non-profit testing organization, also provide approvals.
Intrinsic safety barriers are used in a variety of applications. Simple devices such as switches, solenoids, LED indicators, load cells, potentiometers, resistance temperature detectors (RTDs), strain gauges, and thermocouples do not require intrinsic safety barriers. With more complex devices, however, the use of intrinsic safety barriers is either required or represents an important design consideration. Examples include 4-20 mA 2-wire transmitters, current-to-pressure (I/P) converters or displays, flow meters, infrared (IR) temperature sensors, and proximity switches.
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