Equipment for Class I Hazardous Locations
The key to installing electrical equipment in any hazardous location is to prevent the ignition of its surrounding flammable atmosphere.
Electrical equipment installed in hazardous locations must be of a type designed to prevent the ignition of a flammable atmosphere, propagation of fire, or creation of an explosion. There are four main types of equipment and systems used for these purposes.
- Explosionproof equipment,
- Intrinsically safe equipment,
- Purged and pressurized equipment, and
- Nonincendive equipment.
Explosionproof equipment. The most widely used and recognizable equipment in Class I, Division 1 locations is explosionproof equipment, which is defined in Art. 100 of the National Electrical Code (NEC). The electrical apparatus must be capable of withstanding an explosion within it. If gases or vapors enter the enclosure and an explosion occurs, it must withstand the explosion without rupturing and cool down the hot products of combustion before they reach the surrounding atmosphere.
In Class I, Division 1 locations, electrical systems are not designed to keep explosive vapors out of the wiring system. On the contrary, the NEC assumes that the entry of those vapors into the wiring system is inevitable. The rules are designed to assure any resulting explosions will not ignite the surrounding atmosphere.
Threaded and flat joints are commonly used in explosionproof equipment. Both joints work on the principle of a long flame path. The flame path or escape path is the path the hot gases or vapors must take after ignition. For threaded joints to be effective, they must be made up wrench tight with five fully engaged threads. The flat joint involves carefully machined mating surfaces, bolted securely together. The joints perform properly, if the hot gases are cooled enough so they do not ignite an explosive mixture in the surrounding atmosphere. The flame must not propagate outside the enclosure.
Intrinsically safe equipment. Intrinsically safe equipment is designed with special circuitry that maintains the available energy to a value below that required to cause ignition - even under fault conditions. A qualified testing laboratory must carefully evaluate these systems. But keep in mind, the amount of energy flowing, as a function of voltage and amperage is only one factor considered; stored energy is also considered in terms of system capacitance and reactance.
Properly grounding the system is an important installation requirement. It's also important to separate these circuits from all nonintrinsically safe circuits. This helps to ensure the system's reliability.
Purged equipment. Purged equipment in hazardous locations can be difficult to arrange. However, they're an excellent solution where large or complicated control systems, instrumentation, or motors must be located in the hazardous location.
For purging to work properly, you must supply electrical equipment (of the totally enclosed type) with positive pressure ventilation from a clean source of air. A wired interlock-timer prevents energizing the machine until the ventilation system has purged the enclosure with at least four volumes of gas (ten volumes for motors). The power supplied to the equipment has to be wired and arranged in such a manner as to automatically deenergize the equipment - should the air supply fail.
NFPA 496 recognizes three types of purging. Type X purging converts the interior of an enclosure from Division 1 to nonhazardous, Type Y purging coverts the interior of an enclosure from Division 1 to Division 2, and Type Z purging from Division 2 to nonhazardous.
Current interrupting equipment. All equipment used in Class I, Division 2 locations with arcing or sparking devices require the ignition source to be enclosed and isolated.
One example involves ignition-capable contacts that are immersed in oil. This method protects against the danger of the contacts making, breaking, or igniting in, and thereby igniting, a flammable mixture in Class I, Division 2 locations. Immersion of ignition-capable contacts in oil is known in the electrical industry as oil-immersed contacts.
Another method of protecting contacts in Class I, Division 2 locations is enclosing the contacts in hermetically-sealed chambers. These chambers consist of fusion joints such as welding, silver soldering, brazing, or equal rather than gasket joints. You can use general-purpose enclosures with motors that don't have arcing contacts, such as starter switches, thermal protectors, etc. Conventional 3-phase, squirrel cage induction motors, with Type TEFC enclosures, work well in these applications.
Nonincendive equipment. Nonincendive systems don't release enough energy to ignite a specific ignitable atmosphere under normal conditions, but may do so under some fault conditions. These systems are permitted in general purpose enclosures, in Division 2 locations. The NEC assumes that the odds of a fault in a nonincendive system occurring at the same time as vapor release are infinitesimal. These circuits are not permitted in Division 1 locations, except with regular Division 1 wiring methods and enclosures.
Rotary dial telephone circuits often fall into this category in Group D atmospheres. Although their circuits are not incendiary, their internal contacts qualify under Sec. 501-3(b)(1) Exs. (c) and (d) because the mass of the contact material absorbs enough circuit energy to prevent ignition.
The key to proper application. The key to proper equipment application is to closely watch and adhere to the label ratings for Group, Class, and Division. You must also observe the requirements of the applicable testing laboratory, and maintain a high standard of workmanship for maximum reliability.