Does a field instrument need to be rated IS when it’s used with an intrinsic safety barrier?
If we make the obvious assumption that the area classification in which the field instrument is mounted is rated Class I, Division 1, the answer is YES. The instrument must be intrinsically safe if it’s connected to an intrinsic safety barrier. An
intrinsic safety barrier does not automatically make a standard instrument safe for use in a hazardous 'classified' location. It’s the combination of an intrinsic safety barrier and an intrinsically safe instrument that’s important.
As a further clarifying point, the field instrument must be certified by an independent, third-party, certification agency also known as a Nationally Recognized Testing Laboratory (NRTL). The certification must state the acceptable barriers that may be connected to the device, or the device must have entity parameters that compare favorably to the barrier. The only exception is when a device is considered a simple apparatus.
A simple apparatus, as defined by the National Electrical Code (NFPA 70), is “an electrical component or combination of components of simple construction with well-defined electrical parameters that does not generate more than 1.5 volts, 100 milliamps, and 25 milliwatts, or a passive component that does not dissipate more than 1.3 watts and is compatible with the intrinsic safety of the circuit in which it is used.” The NEC gives some examples of simple apparatus including switches, RTDs, and LEDs. As a final wrap-up to the original question of whether a field instrument must be IS (intrinsically safe) when it’s used with a barrier, the answer is YES.
Additional clarification regarding the proper matching of a field instrument with a barrier in terms of system certification or entity parameters will be covered in future posts.