While I was teaching a seminar for the National Fire Protection Agency (NFPA), a student asked a series of interesting questions. Basically, the student wanted to know where in the National Electrical Code (NEC) were the requirements that permitted a general-purpose enclosure to be installed containing hermetically sealed contacts. The intended use of the enclosure was to house totally enclosed current-interrupting contacts of instrumentation components to sense and operate pertinent devices in a Class 1, Division 2 location. Also, the student asked if MC/HL cable could be installed and what other wiring techniques would be necessary to complete such an installation. I was told that the entrustment monitors flammable gas flow and pressure in a small pipeline with proper ventilation being provided. Ultimately, the student wanted to know if an installer should use the NEC to determine the classified location or if other codes and standards would be needed.
Section 500.2 defines the types of equipment permitted in classified locations. For example, hermetically sealed equipment seals against the entrance of an external atmosphere where the seal is made by fusion, such as soldering, brazing, welding or fusion of glass to metal. For further information, review ANSI/ISA-12.12.O1-2000 as suggested in the FPN.
Section 500.2 defines hermetically sealed equipment and how it must be constructed to qualify as equipment permitted for classified areas. Section 500.7(J) recognizes hermetically sealed equipment as a protection technique when used in Class 1, Division 2 locations. Next step is selecting a section that regulates the location and use of hermetically sealed contacts in general-purpose equipment.
Section 501.105(A) and (B) requires enclosures to be identified for use in a Class 1, Division 1 location unless the Ex. to 501.105(B)(1) is applied. The concept in Class 1, Division 2 areas is to totally enclose contacts that can arc and spark when they operate.
The general rule of the exception prohibits general-purpose enclosures from being equipped with current-interrupting contacts that are not totally enclosed. There are four protection techniques that the NEC recognizes for enclosing such contacts, and they are 1.) Immersed in oil, 2.) in nonincendive circuits, 3.) listed for Division 2, and 4.) hermetically sealed type. In Class 1, Division 2, it is clear that current-interrupting contacts that are totally enclosed can be installed in general-purpose enclosures.
Of course, wiring methods will vary. By reference from 501.10(B)(1)(1), a MC/HL cable can be installed in a Class 1, Division 2 location as permitted in 501.10(A)(1)(c) that allows the use of the cable in both Class 1, Divisions 1 and 2 locations. Note that these sections contain a laundry list of wiring methods permitted in these locations and MC/HL cable is definitely permitted.
When selecting a sealing fitting for a MC/HL cable, the installer must adhere to 501.15(D)(1) and apply the requirements in this section. Plainly stated, a multi-conductor Type MC/HL cable with a gas/vapor-tight continuous corrugated metallic sheath with an overall jacket of suitable polymeric material must be sealed with a listed fitting that, after removing the jacket and any other covers, the sealing compound surrounds each individual insulated conductor and has the capability of minimizing the passage of gases and vapors through the cable. The second paragraph of 300.15 requires fittings to be listed for the wiring methods in which they are used to connect raceways and cables to enclosures.
However, one must accommodate for location. According to the student, the gas is normally confined in a closed system from which it can escape only in case of accidental rupture or breakdown of such system or in case of abnormal operation of the instrument under operating conditions.
The flammable gas falls under Group D. The flash point and density as well as the ignition temperature and explosive limits are found in NFPA 497 as mentioned in the FPN 2 to 500.4(B) of the NEC.
Section 500.8(A) and (B) contains guidelines for accepting electrical equipment in classified locations. One method is to purchase listed and labeled equipment. A second method is evidence of equipment evaluation from a qualified testing laboratory or inspection agency concerned with product evaluation. An on-site field evaluation of equipment by a lab or inspection department trained and qualified for such a task is acceptable. A third method, if acceptable to the authority having jurisdiction, is evidence of a manufacturer’s self-evaluation or an owner’s engineering judgment.
Therefore, my response for the student was, applying the NEC, as the articles and sections in this issue instruct, will provide guidelines necessary to aid contractors and installers, who have the responsibility of selecting general-purpose enclosures with totally enclosed contacts (for devices) used in instrumentation equipment operating in a Class 1, Division 2 environment.
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.