The 2014 edition of the National Electrical Code (NEC) [Sec. 500.7(J)] describes “hermetically sealed” as one of the protection techniques allowed in classified areas. In Sec. 500.2, the term “hermetically sealed” is defined as: “Equipment sealed against the entrance of external atmosphere where the seal is made by fusion, for example, soldering, brazing, welding, or the fusion of glass to metal.” This definition is important because it specifically states how the containment of the equipment, such as contacts, is achieved.
A good example of a hermetically sealed piece of equipment is a hermetically sealed compressor-motor like the ones used in a typical, small air-conditioning unit for residential or commercial properties. In these cases, the motor is sealed completely within a welded container with the compressor.
In several places throughout the NEC, we also see the term “factory sealed” used. For example, in Sec. 501.15(A)(1)(1) Exception, subpart c., it states: “Seals shall not be required for conduit entering an enclosure under any one of the following conditions: … The switch, circuit breaker, fuse, relay, or resistor is enclosed within a factory-sealed explosionproof chamber located within the enclosure, identified for the location, and marked ‘factory sealed’ or equivalent.”
Factory- sealed equipment is not the same as hermetically sealed equipment. Though the concept is similar, factory-sealed contacts are not sealed from the environment by soldering, brazing, welding, etc. Factory sealed contacts are molded into an explosionproof chamber. This method is more economical than sealing the contacts by hermetic sealing, and it is just as effective. Factory sealed contact blocks are widely used in control stations located in Class I, Div. 2 locations, and, where used, an additional exterior conduit seal probably won’t be required.
Always follow the manufacturer’s instructions though. Some factory sealed contacts are rated for Groups C and D, but not for Group B. Others may be rated for Class I, Div. 2 locations, but not Class I, Div. 1 locations.
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