Recently, an electrical contractor and his foreman visited my office to talk about electrical equipment installed in hazardous locations, specifically about explosion-proof, intrinsically safe and nonincendive equipment as well as purged and pressurized systems. They wanted an explanation of their types and use. It took most of the day to outline the codes and standards that contain the necessary requirements for installing this equipment. This month’s article is devoted to portions of that discussion and the description of such equipment.
The most widely used and recognizable equipment in Class 1, Division 1 locations is the explosion-proof type. As defined in Article 100, 500.2 and 500.7 of the National Electrical Code (NEC), such apparatuses must be capable of withstanding an explosion within. If gases or vapors enter the enclosure and an explosion occurs, not only does it have to withstand the explosion without rupturing, it also must cool down the hot products of combustion before they reach the surrounding atmosphere. The NEC assumes the entry of these vapors into the wiring system is inevitable. Therefore, requirements are designed to ensure any resulting explosions will not ignite the surrounding atmosphere.
What about joints? The joints that are widely used in the manufacturing of explosion-proof equipment are the threaded and flat types. Both joints work on the principle of a long flame path. For threaded joints to be effective, they must be made wrench tight with five fully engaged threads. The flat joint involves carefully machined mating surfaces securely bolted together. The flame path or escape path is the path the hot gases or vapors must take after ignition through the threads or across the flat joint. Note that the joints are performing properly if the gases or vapors are cooled, so they do not ignite an explosive mixture in the surrounding atmosphere.
Intrinsically safe equipment, on the other hand, is manufactured with circuitry in which any spark or thermal heat produced by such circuits will not ignite a flammable or combustible atmospheric mixture under prescribed test conditions. Very little energy is needed to ignite a flammable atmosphere. Therefore, this protection system is built to maintain the available energy to a value below what is necessary to cause ignition, even under fault conditions. An important installation requirement is to, when necessary, appropriately ground the system as well as rigorously separate circuits from normal power and other intrinsically safe wiring. Since harmful energy is not considered a problem, electricians, without the use of special equipment and tools can remove conductors from their terminals and perform troubleshooting techniques.
Keep in mind that contractors using intrinsically safe systems must verify that a qualified testing laboratory evaluates circuits and components as a complete system. The testing lab must determine the amount of energy based on the voltage and amperage and evaluate the stored energy in terms of capacitance and reactance to ensure safe levels under conditions of use.
The third type we discussed is nonincendive systems. These will not release enough energy to ignite a specific ignitable atmosphere under normal conditions but may do so under fault conditions. These systems are permitted in general-purpose enclosures. The NEC assumes the odds of a fault in a nonincendive system occurring at the same time as a vapor release are infinitesimal. Note that these circuits are not permitted in Division 1 locations, except with regular Division 1 wiring methods and enclosures. Although their circuits are not incendiary, their internal contacts qualify as outlined in 501.105(B)(1), Ex. s (3) and (4) and 501.150(B)(1), Ex. s (3) and (4), because the mass of the contact material absorbs enough circuit energy to prevent ignition.
Finally, we discussed purged and pressurized equipment. Purged and pressurized equipment in hazardous locations can be difficult to arrange and install. However, this equipment is an excellent solution where large or complicated control systems, instrumentation devices or motors are located in such locations (review 500.2 and 500.7(F) through (H)).
For purging to work properly, supply the electrical equipment of the totally enclosed type with positive pressure ventilation from a clean source of clean air (or gas). A wired interlock-timer prevents energizing the machine until the ventilation system has purged the enclosure with at least four volumes of air and 10 volumes for motors. The power supplying the equipment must be wired and arranged to automatically de-energize the equipment should the air supply fail.
To ensure equipment is selected properly, it is important that electricians closely watch and adhere to the label ratings for group, class and division. Don’t forget, for maximum reliability, observe the installation requirements of the applicable testing laboratory and when installing equipment, a high standard of workmanship must be maintained.
In the next issue, I will discuss the selection and installation requirements using this equipment. EC
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.