Safety is a primary concern when instruments are used in hazardous locations, especially in applications where the potential for explosion is involved due to a concentration of flammable gases, vapors or dust. When specifying a sensor or any instrumentation circuit for use in a hazardous location, the user needs to exercise caution in complying with local and/or national electrical codes and safety regulations.
It is important to consider the failure modes of the instrumentation and any catastrophic effects these modes could have based on the environment they are used in. The integrity and suitability of any instrumentation for use in hazardous locations is ultimately the responsibility of the end user and/or of those making the specification decisions.
Generally, when instrumentation is to be used in a hazardous location, two commonly used methods to minimize the risk of ignitions and explosions are intrinsically safe and explosion-proof systems. Intrinsically safe systems are centered around prevention whereas explosion proof systems are focused on containment.
Intrinsically safe systems operate on low power and are designed to limit the thermal and electrical energy of the instrument and associated connections to a level where ignition is not possible. Also, the devices cannot store enough energy to cause a spark when energy is released.
Explosion proof systems are based on the principle of containment. In other words, an explosion proof enclosure prevents any generated flames, sparks or hot gases from escaping. These devices are designed to contain, control, cool, and/or vent any possible ignition due to a failure mode, without igniting the surrounding atmosphere.
When it comes to RTDs and thermocouples, it is important to note that these temperature measurement sensors are defined as simple apparatus by the National Fire Protection Association (NFPA). This means that they simply operate on, store and/or generate too low level an energy, i.e. low amperage and low voltage, to cause an ignition. This status can change, however, depending on what the sensor is connected to or how hot the surface of the sensor assembly may get outside of the process. The required method of protection can vary depending on many different factors and the entire instrumentation system must be examined before use in hazardous locations.
In summary, RTD and thermocouple sensors are considered a simple apparatus as defined in the National Electric Code NFPA 70 Article 100 (Rev. 2020). By definition, these sensors generate too low an energy to be an ignition source. For use in hazardous environments, the sensor is typically connected to an intrinsically safe or explosion proof apparatus. Final application suitability and compliance with regional standards are to be determined and approved by the end user.