Intrinsically safe field devices in hazardous areas

Intrinsic safety prevents instruments and other low-voltage circuits in hazardous areas from releasing sufficient energy to ignite volatile gases. 

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.
An intrinsically safe apparatus (field device) is classified either as a simple or nonsimple device. Simple apparatus is defined in paragraph 3.12 of the ANSI/ISA-RP 12.6-1987
as any device which will neither generate nor store more than 1.2 volts, 0.1 amps, 25 mW or 20 µJ. Examples are simple contacts, thermocouples, RTDs, LEDs, noninductive potentiometers, and resistors. These simple devices do not need to be approved as
intrinsically safe. If they are connected to an approved intrinsically safe associated apparatus (barrier), the circuit is considered intrinsically safe.
A nonsimple device can create or store levels of energy that exceed those listed above. Typical examples are transmitters, transducers, solenoid valves, and relays. When
these devices are approved as intrinsically safe, under the entity concept, they have the following entity parameters: Vmax (maximum voltage allowed); Imax (maximum current allowed); Ci (internal capacitance); and Li (internal inductance). 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. The Ci and Li values describe the device‘s ability to store energy in the form of internal capacitance and internal inductance.