One of the protection techniques recognized in 500.7(K) in Article 500, covering general protection installations in a hazardous location, is the use of a combustible-gas-detection system. In the years since the 2002 National Electrical Code first accepted these systems, and especially now with the 2020 NEC , there have been quite a few changes and additions.
Gas-detection systems are required to be listed for Class I, Division 1 or 2, and listed for the detection of the specific gas or vapor to be encountered. For example, in a location containing hydrogen gas, the system must be listed specifically for this gas. The system must be permanently installed with fixed wiring methods; it cannot use portable or transportable equipment or temporary wiring methods.
The gas-detection system can only use point-type sensors. A point-type detector is often an infrared (IR) hydrocarbon gas detector that continuously monitors for combustible gases and vapors within the lower (explosive) flammable limit (LEL or LFL) and provides alarm indication. With an IR point detector, measurement of the concentration of the gas is based on the absorption of IR radiation at particular wavelengths as the radiation passes through a certain volume of gas. IR gas-detection systems can be classified into two types: point detectors and open-path detectors. For a point detector, the absorption path length is fixed and determined by the detector design to be a few inches. For an open-path IR detector, the absorption path length can be as long as 130 m (426.5 feet).
Based on the NEC , the gas-detection system may be augmented with open-path-type (line-of-sight) sensors, but these sensors cannot be the basis for this protection technique. The type of gas-detection equipment, its listing, installation location(s), alarm and shutdown criteria and the calibration frequency must be documented where the system will be used.
Finally, the use applications for combustible-gas-detection systems as a protection technique is limited to areas with inadequate ventilation, building interiors or enclosed spaces and control panel interiors, based on the 2002 NEC but with many expanded requirements in the 2020 version.
The new text in 500.7(K)(2) dealing with inadequate ventilation has been revised to state the following: “A location, enclosed space, or building that is classified as a Class I, Division 1 location due to inadequate ventilation, that is provided with a combustible gas detection system, shall be permitted to utilize electrical equipment, installation methods, and wiring practices suitable for Class I, Division 2 installations. Sensing a gas concentration of not more than 40% of the lower flammable limit or a gas detector system malfunction shall activate an alarm that is audible or visual, or both, as is most appropriate for the area.”
For the interior of a building, 500.7(K)(3) of the 2020 NEC was significantly rewritten with many added requirements. Any building or enclosed space that does not contain a source of flammable gas or vapors that is located in, or with an opening into, a Class I, Division 2 hazardous (classified) location that is provided with a combustible-gas-detection system shall be permitted to use electrical equipment, installation methods and wiring practices suitable for unclassified installations under all of the following conditions: (1) An alarm (audible, visual or both) must be sounded at not more than 20% of the LFL; (2) Sensing a gas concentration of not more than 40% of the LFL or a gas detector system malfunction must activate an alarm (audible, visual or both, as most appropriate for the area) and initiate automatic disconnection of power from all electrical devices in the area that are not suitable for Class I, Division 2; and (3) The power-disconnecting device(s) must be suitable for Class I, Division 1 if located inside the building or enclosed space.
If the disconnecting device(s) is located outside the building or enclosed space, it must be suitable for the location where it is installed. In addition, redundant or duplicate equipment (such as sensors) are permitted to be installed to avoid disconnecting electrical power when equipment malfunctions are indicated. When automatic shutdown could introduce additional or increased hazards, this technique is not permitted.
For the interior of a control panel that contains instrumentation or other equipment using or measuring flammable liquids, gases or vapors and that is provided with combustible-gas-detection equipment, it is permitted to use electrical equipment, installation methods and wiring practices suitable for Class I, Division 2 installations. An alarm (audible, visual or both) shall be sounded at not more than 40% of the LFL.
Major changes to combustible-gas-sensing protection techniques must be analyzed before trying this protection.