11.3 Hazardous (Classified) Locations — Articles 500 through 506

The Classification System

The traditional North American system uses Class, Division, and Group. Articles 505 and 506 provide the alternative Zone system harmonized with IEC practice.

Class — what the material is

Division — how likely it is to be there

Group — the ignition characteristics of the specific material

The mnemonic for Class I groups is Acetylene, Butadiene and hydrogen, Carbon monoxide and ethylene, Diesel and propane — in order of decreasing hazard. Group A is the most hazardous; Group D is the most common.

Class III has no groups. Division 1 is where fibers or flyings are handled, manufactured, or used; Division 2 is where they are stored or handled other than in the process of manufacture.

Protection Techniques — 500.7

500.7 lists the recognized protection techniques, which include:

• Explosionproof equipment (500.7(A)) — for Class I, Division 1;
• Dust-ignitionproof (500.7(B)) — Class II, Division 1;
• Dusttight (500.7(C)) — Class II, Division 2 and Class III;
• Purged and pressurized (500.7(D));
• Intrinsic safety (500.7(E)) — Class I, II, III, Divisions 1 and 2;
• Nonincendive circuit (500.7(F)), nonincendive equipment (G), nonincendive component (H) — Division 2 only;
• Oil immersion (500.7(I)), hermetically sealed (J), combustible gas detection system (K), encapsulation (L).

Understanding explosionproof matters: it does not mean the enclosure keeps the atmosphere out. It means the enclosure is strong enough to contain an internal explosion and to cool the escaping gases through machined flame paths so they do not ignite the surrounding atmosphere. This is why the flat, machined surfaces of an explosionproof fitting must be clean and fully engaged, and why all threaded joints must have at least five threads fully engaged under 500.8(E)(1).

Intrinsic safety takes the opposite approach: it limits the energy in the circuit below the level that can ignite the atmosphere. Article 504 governs intrinsically safe systems, requiring separation of intrinsically safe conductors from nonintrinsically safe conductors by at least 50 mm (2 in.) unless separated by a grounded metal partition or an approved insulating partition (504.30(A)(2)), and requiring identification by labels reading "Intrinsic Safety Wiring" at intervals not exceeding 7.5 m (25 ft) (504.80).

Wiring Methods

501.10(A) Class I, Division 1. Permitted wiring methods include:

1. Threaded rigid metal conduit or threaded steel intermediate metal conduit, with listed fittings and threaded joints made up with at least five threads fully engaged;
2. Type MI cable with listed termination fittings;
3. In industrial establishments with restricted public access, Type MC-HL cable with a gas/vaportight continuous corrugated metallic sheath, an overall jacket of suitable polymeric material, and a separate equipment grounding conductor;
4. Type ITC-HL cable under the same conditions;
5. Optical fiber cable in one of the above raceways.

Flexible connections where necessary shall use flexible fittings listed for the location, flexible metal fittings, liquidtight flexible metal conduit with listed fittings, liquidtight flexible nonmetallic conduit with listed fittings, or flexible cord listed for extra-hard usage and terminated with listed fittings, with a separate equipment grounding conductor.

501.10(B) Class I, Division 2. The permitted list is much broader and includes all of the Division 1 methods plus rigid metal conduit, intermediate metal conduit, PVC conduit, RTRC, EMT, and enclosed gasketed busways and wireways, plus Types MC, MI, MV, TC, PLTC, ITC, and TC-ER cable under stated conditions.

502.10 and 503.10 give the parallel lists for Class II and Class III.

Sealing — 501.15

Seals prevent the passage of gases, vapors, or flames from one portion of the electrical installation to another through the raceway, and limit pressure piling during an internal explosion.

501.15(A) Conduit Seals, Class I, Division 1.

• (A)(1) In each conduit entering an enclosure containing arcing, sparking, or high-temperature devices — switches, circuit breakers, fuses, relays, resistors — the seal shall be installed within 18 inches (450 mm) of the enclosure. Only explosionproof unions, couplings, reducers, elbows, capped elbows, and conduit bodies of a type comparable to the conduit are permitted between the sealing fitting and the enclosure.
• (A)(2) Conduit seals shall be installed in each conduit run entering an enclosure that houses terminals, splices, or taps, where the conduit is trade size 2 or larger. The seal shall be installed within 18 inches of the enclosure.
• (A)(4) Where two enclosures requiring seals are connected by a nipple or conduit run not more than 36 inches long, a single seal in the run between the enclosures shall be permitted if located not more than 18 inches from either enclosure.
• (A)(5) Class I, Division 1 Boundary. Conduit seals shall be installed in each conduit run leaving a Class I, Division 1 location. The sealing fitting shall be permitted on either side of the boundary within 3.05 m (10 ft) of the boundary, and shall be designed and installed so as to minimize the amount of gas or vapor within the Division 1 portion of the conduit from being communicated to the conduit beyond the seal. There shall be no union, coupling, box, or fitting, other than listed explosionproof reducers at the conduit seal, in the conduit between the sealing fitting and the point at which the conduit leaves the Division 1 location.

501.15(B) Conduit Seals, Class I, Division 2.

• (B)(1) In each conduit run entering an enclosure or fitting that is required to be explosionproof.
• (B)(2) Class I, Division 2 Boundary. Conduit seals shall be installed in each conduit run passing from a Class I, Division 2 location into an unclassified location. The sealing fitting shall be permitted on either side of the boundary within 10 feet (3.05 m). The seal shall be designed and installed so as to minimize the amount of gas or vapor within the Division 2 portion of the conduit from being communicated to the conduit beyond the seal. Rigid metal conduit or threaded steel intermediate metal conduit shall be used between the sealing fitting and the point at which the conduit leaves the Division 2 location, and a threaded connection shall be used at the sealing fitting. There shall be no union, coupling, box, or fitting in the conduit between the sealing fitting and the point at which the conduit leaves the Division 2 location.

Exception: A conduit seal shall not be required where a conduit passes completely through a Class I, Division 2 location with no fittings within 12 inches of each boundary, and where the termination points of the unbroken conduit are in unclassified locations.

501.15(C) Class I, Divisions 1 and 2 — Requirements. Seals shall comply with the following: (1) Fittings shall be listed for the location; (2) Compound shall provide a seal against passage of gas or vapor through the seal fitting, shall not be affected by the surrounding atmosphere or liquids, and shall have a melting point of not less than 93 degrees C (200 degrees F); (3) Thickness of compounds — in completed seals, the minimum thickness of the sealing compound shall not be less than the trade size of the sealing fitting and, in no case, less than 16 mm (5/8 in.); (4) Splices and taps shall not be made in fittings intended only for sealing with compound, nor shall other fittings in which splices or taps are made be filled with compound; (5) Assemblies — where an enclosure is factory sealed, an additional seal is not required; (6) Conductor fill — the cross-sectional area of the conductors permitted in a seal shall not exceed 25 percent of the cross-sectional area of a rigid metal conduit of the same trade size unless it is specifically listed for a higher percentage of fill.

The two numbers to hold: 5/8 inch minimum compound thickness and 25 percent maximum conductor fill in a seal.

502.15 gives the Class II sealing rules, which are different in intent: they prevent dust from passing between a dust-ignitionproof enclosure and one that is not, and they do not require the compound-thickness dimensions of 501.15.

Grounding and Bonding — 501.30, 502.30, 503.30

501.30(A) Bonding. The locknut-bushing and double-locknut types of contacts shall not be depended on for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used. Such means of bonding shall apply to all intervening raceways, fittings, boxes, enclosures, and so forth, between Class I locations and the point of grounding for service equipment or point of grounding of a separately derived system.

250.100 states the same requirement more generally: regardless of the voltage of the electrical system, electrical continuity of non-current-carrying metal parts of equipment, raceways, and other enclosures in any hazardous (classified) location shall be ensured by the bonding methods specified in 250.92(B)(2) through (B)(4).

501.30(B) Types of Equipment Grounding Conductors. Where flexible metal conduit or liquidtight flexible metal conduit is used as permitted in 501.10(B) and is to be used as a grounding means, it shall be installed with internal or external bonding jumpers in parallel with each conduit and complying with 250.102.

Exception No. 1: In Class I, Division 2 locations, the bonding jumper shall be permitted to be deleted where all of the following conditions are met: the liquidtight flexible metal conduit is not more than 1.8 m (6 ft) in length, with fittings listed for grounding; overcurrent protection in the circuit is limited to 10 amperes or less; and the load is not a power utilization load.