Explosion-Proof Equipment: Reviewing a protection technique

By Michael Johnston | Aug 15, 2022
Illustration of a triangle surrounding by a flame. One side depicts gas and wood, one side depicts a rising thermometer and one side reads O2. Image by Shutterstock / VectorMine.
A review of protection techniques for hazardous (classified) locations reveals descriptive information about each under the “division” and “zone” classification systems. 

A review of protection techniques for hazardous (classified) locations reveals descriptive information about each under the “division” and “zone” classification systems. The type of protection technique permitted in a particular place depends on the class of location and the particular division or zone. Each technique offers methods for removing one or more components (ignition, oxygen, fuel) of the fire triangle from the hazardous (classified) location. In addition, each technique has suitable applications and limitations and must be carefully followed.

While the common wiring method for electrical installations in hazardous locations continues to be conduit methods, cables under specific conditions are often used for installations under the International Electrotechnical Commission rules. Both classification systems depend on proper application of protection techniques for safety. The various techniques under these systems offer diversity in the wiring methods allowed for electrical and electronic equipment in hazardous locations.

While it is always better to locate as much electrical equipment and wiring systems out of the hazardous (classified) location, it is not always possible. The protection technique chosen in the design and how it is installed and used is directly related to electrical safety in any hazardous (classified) location. In Section 500.7(A) of the National Electrical Code for Class I locations, these are the basics of explosion-proof equipment as a protection technique.

Where is it permitted?

Explosion-proof equipment is permitted as a protection technique in Class I, Division 1, Division 2, Zone 1 or Zone 2 locations within the gas group and temperature limitations of the specific piece of equipment. A complete review of Article 501 and 505.9(C)(1) will reveal that most explosion-proof equipment is required to be identified for Class I, Division 1 or 2 locations.

Product standards for explosion-proof equipment include prescriptive requirements for the enclosure construction, such as material, enclosure thickness, enclosure joints, shaft openings, holes in the enclosures, drain and breather fittings, supply connections (threaded hubs) to the enclosures for fixed wiring and cord-connected portable equipment, corrosion protection, materials applied to joint surfaces, devices having coated threaded joint surfaces and porosity in enclosure materials.

The mating surfaces must meet the prescriptive dimension and construction requirements and pass an explosion test during evaluation. This provides reasonable assurance that the exhaust from an internal explosion in the enclosure will not ignite the hazardous explosive atmosphere around it. If there is an explosion inside this equipment, it will contain the exploding gases and flames and then allow them to be released to the surrounding hazardous atmosphere, but only after the gases are cooled while passing across metallic joints.

Three types of construction

The three basic types of joint construction for explosion-proof equipment are flat joint, threaded joint and labyrinth path joint constructions. Flat joint construction involves two mating surfaces of metal. When an explosion happens inside the enclosure, the hot gases are forced to pass through tightly joined flat metal surfaces and the hot gases (in flame form) are cooled as they reach the outside of the enclosure.

Explosion-proof equipment using threaded joint construction depends on a minimum of metallic threads for cooling exploding gases inside the enclosure. As the exploding gases and flames pass over the metallic threads, they are moving through them while the hot gas is being cooled. For threaded-joint explosion-proof equipment, the threads must be made up wrench tight with five full threads fully engaged.

This is a long-standing NEC requirement in Section 500.8(E) to prevent sparking during fault current conditions, which ensures the integrity of the explosion-proof enclosure and the installed conduit system. Threaded conduit entries into explosion-proof enclosures must be made up with five full threads fully engaged. An exception allows only 4½ full threads fully engaged for listed explosion-proof equipment for factory NPT threads, as provided in Section 500.8(E)(1). This exception only applies to the explosion-proof enclosure, not the wiring methods.

Explosion-proof equipment with labyrinth path joint construction depends on metallic surfaces arranged in a tongue-and-groove type for cooling exploding gases inside the enclosure.

Another important requirement is sealing fittings being installed at conduit entries in explosion-proof equipment. Section 501.15(A)(1) indicates that, in general, a listed conduit seal must be installed within 18 inches of the enclosure’s entry, or as otherwise required by markings on it. Only explosion-proof unions, couplings, reducers, elbows and capped elbows not larger than the trade size of the conduit shall be permitted between the sealing fitting and the explosion-proof enclosure. See Section 501.15(A)(1) for complete information about sealing requirements.

The process and requirements for installing listed explosion-proof sealing fittings will be in September’s column.

Header image by Shutterstock / VectorMine.

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].






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