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Making instruments intrinsically safe need not seem like a nightmare. Here, the basics of intrinsic safety circuit design are discussed. Start With The Field Device All intrinsically safe circuits have three components: the field device, referred to as the intrinsically safe
apparatus; the energy-limiting device, also known as a barrier or intrinsically safe associated apparatus; and the field wiring. When designing an intrinsically safe circuit, begin the analysis with the field device. This will determine the type of barrier that can be used so that the circuit functions properly under normal operating conditions but still is safe under fault conditions. The Vmax and Imax values are straightforward. Under a fault condition, excess voltage or current could be transferred to the intrinsically safe apparatus (field device). If the voltage or current exceeds the apparatus’ Vmax or Imax, the device can heat up or spark and ignite the gases in the hazardous area. Limiting Energy To The Field Device To protect the intrinsically safe apparatus in a hazardous area, an energy-limiting device must be installed. This is commonly referred to as an intrinsically safe associated apparatus or barrier. Under normal conditions, the device is passive and allows the intrinsically safe apparatus to function properly. Determining Safe Energy Levels Voltage and current limitations are ascertained by ignition curves, as seen in Figure 4. A circuit with a combination of 30 V and 150 mA would fall on the ignition level of gases in Group A. This combination of voltage and current could create a spark large enough to ignite the mixture of gases and oxygen.
Tags : safety circuit, circuit functions, intrinsic safety, safe energy, fault conditions, hazardous area, vmax, energy levels, imax, gases, curves, nightmare, oxygen, mixture, circuits
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