Different classifications of gas detection systems
Answered by John Villalovos, senior applications engineer, RKI Instruments Inc., Union City, CA.
Electrical equipment sometimes must be installed in areas where combustible vapors and gases are used or may be present. These are commonly referred to as hazardous locations, and are defined by the National Electrical Code in the United States and the Canadian Electrical Code in Canada. When equipment must be installed in hazardous locations, there are strict requirements for the construction of the installation, including materials and design requirements. To prevent inadvertent ignition of flammable gases and vapors by electrical equipment, the two most common methods of protection are "explosion proof" and "intrinsically safe."
Generally, explosion proof is the more commonly used method for detector/sensor assemblies for fixed gas detection systems, where higher voltages and power requirements may be encountered and the installation is permanent.
An explosion-proof classification for a sensor or transmitter means the housing has been engineered and constructed to contain a flash or explosion. Such housings are usually made of cast aluminum or stainless steel. They are constructed of sufficient mass and strength to safely contain an explosion should flammable gases or vapors penetrate the housing and internal electronics or wiring. The design must prevent any surface temperatures that could exceed the ignition temperature of the gases or vapors covered by its group rating. If the sensing element is a high-temperature device, it may be protected by a flame arrestor to prevent the propagation of high-temperature gases to the ambient atmosphere.
An intrinsically safe classification and design means an electronic circuit and its wiring will not cause any sparking or arcing. It cannot store sufficient energy to ignite a flammable gas or vapor, and cannot produce a surface temperature high enough to cause ignition. Such a design is not explosion proof, nor does it need to be. For permanent installations, such an installation may include intrinsically safe barriers that are located outside the hazardous location and limit the amount of energy available to the device located in the hazardous area. The intrinsically safe method can also be used for permanent installations where the detector or sensors are relatively low-power devices. Almost all portable instruments use the intrinsically safe method.