What exactly is a Class I location? Can you define it, or better yet, easily identify it in the field? You have a Class I location any time enough flammable gases or vapors are present in the air in quantities that may produce an explosive or ignitable mixture. When you have a Class I installation, you must follow the requirements outlined in Article 501. When doing so, you will need to refer back to Article 500 — which defines two types of Class I locations: Div. 1 and Div. 2.

You have Div. 1 where the Class I ignition sources may be present or are present [500.5(B)(1)]. But in a Div. 2 location, these sources are normally confined within closed containers (or systems) or vented (via positive mechanical ventilation). You also have a Div. 2 location when the area is adjacent to a Div. 1 space, and ignitable concentrations may occasionally waft over due to lack of positive pressure ventilation [500.5(B)(2)]. Article 501 requirements are, consequently, less stringent for Div. 2 areas.

Wiring methods. For Div. 1 spaces, you can use these wiring methods [501.10(A)]:

General wiring, which consists of:

  • Fixed wiring. Threaded rigid metal conduit or intermediate metal conduit with explosionproof fittings.

  • Type MI cable with termination fittings listed for the location.

In industrial establishments, if only qualified persons service the installation, you can also use:

  • Type MC-HL cable with fittings listed for a Class I, Div. 1 location (Fig. 1).

  • Type ITC-HL cable with fittings listed for a Class I, Div. 1 location.

Flexible wiring. You can use explosionproof flexible metal couplings when necessary for vibration, movement, or difficult bends.

Boxes and fittings. All boxes and fittings must be approved by the authority having jurisdiction (AHJ) for Class I, Div. 1 locations.

For Div. 2 spaces, you can use these wiring methods [501.10(B)]:

General wiring, which consists of:

  • Wiring methods permitted in Class I, Div. 1 areas [501.10(A)].

  • Threaded rigid metal or intermediate metal conduit.

  • Enclosed gasketed busways and enclosed gasketed wireways.

  • Type PLTC cable per Article 725 (avoid tensile stress at the termination fittings).

  • Type ITC cable [727.4(3)].

  • Cables — MI, MC, MV, or TC — with termination fittings.

Flexible wiring. Where limited flexibility is necessary, you can use one or more of:

  • Flexible metal fittings

  • Flexible metal conduit with listed fittings

  • Liquidtight flexible metal conduit (or nonmetallic conduit) with listed fittings

  • Flexible cords listed for extra-hard usage, containing an equipment grounding (bonding) conductor, and terminated with a listed bushed fitting

Nonincendive field wiring. You can use any of the wiring methods allowed for unclassified locations.

Boxes and fittings. You can use general-purpose enclosures and fittings — unless the enclosure contains make-and-break contacts for meters, instruments, and relays [501.105(B)(1)], switches, circuit breakers, or motor controllers [501.115(B)(1)], signaling, alarm, remote-control, or communications systems [501.150(B)].

Conduit and cable seals. By keeping flames inside enclosures and conduit, seals reduce the likelihood of ignition of the surrounding environment. The seal requirements in 501.15 can seem intimidating, due to their length, but don't get overwhelmed. Here are some highlights:

  • No splices or taps within a conduit seal fitting.

  • The conductor area must not exceed 25% of the cross-sectional area of rigid metal conduit, unless the seal fitting is identified for a higher percentage fill (Fig. 2).

  • Provide seals at enclosures (with some exceptions).

  • Provide seals at the boundary (with some exceptions).

  • Provide seals in each unpressurized conduit where the conduit enters a pressurized enclosure.

  • Use only explosionproof unions, couplings, reducers, elbows, etc., between the conduit seal and the explosionproof enclosure (Fig. 3).

  • Use sealing compounds listed for the specific location.

  • Use seal fittings listed for the specific sealing compounds and location.

  • Mix and install the conduit seal compound per manufacturer's instructions. The sealing compound must be from the same manufacturer as the conduit seal.

  • The minimum thickness of the conduit seal compound must not be less than the trade size of the seal fitting (with some exceptions).

  • Bond wiring and equipment to an effective ground-fault current path per 250.100.

Surge protection. For Div. 1 locations, install surge arresters, transient voltage surge suppressors (TVSSs), and capacitors in enclosures identified for Class I, Div. 1 locations. For Div. 2 spaces, you must use surge arresters and (TVSSs) designed for the specific duty, but you can install them in general-purpose enclosures [501.35].

Equipment. Part III of Article 501 addresses equipment. As before, the requirements for Div. I equipment are more stringent than those for Div. II equipment.

Transformers and capacitors

  • Div. 1: If they contain liquid that will burn, install them in vaults complying with 450.41 through 450.48 plus 501.100(A)(1) through (4). Otherwise, install them in vaults complying with 501.100 (A)(1).

  • Div. 2: Install them in vaults complying with 450.21 through 450.27 [501.100(B)].

Meters, instruments, and relays

  • Div. 1: Install these items in enclosures identified for a Class I, Div. 1 location [501.105(A)].

  • Div. 2: You can install some items in general-purpose enclosures — provided you meet the various conditions listed in 501.105(B)(1) through (6). Otherwise, use Class I, Div. 1 enclosures.

Switches, circuit breakers, motor controllers, and fuses

  • Div. 1: Install these in enclosures identified for a Class I, Div. 1 location [501.115(A)].

  • Div. 2: If the installation meets certain (extensive) requirements, you can use general enclosures [501.115(B)]. Otherwise, use Class I, Div. 1 enclosures.

Control transformers and resistors Div. 1: Install them in explosionproof enclosures identified for a Class I, Div. 1 location [501.120(A)].

Div. 2: You can use general-purpose enclosures. But you must install make-and-break contacts within enclosures identified for a Class I, Div. 1 location [501.120].

Motors and generators

Div. 1 motors and generators must be [501.125(A)]:

  1. Identified for a Class I, Div. 1 location;

  2. Of the totally enclosed type supplied with positive-pressure ventilation and arranged to automatically de-energize if the air supply fails;

  3. Of the totally enclosed inert gas-filled type and arranged to automatically de-energize if the gas supply fails; or

  4. Submerged in a liquid that is flammable only when vaporized and mixed with air, and arranged to automatically de-energize if the liquid is reduced to atmospheric pressure (vaporized).

Div. 2 motors and generators may be of the open type (squirrel-cage induction motors without arcing devices) if the motor or generator doesn't contain any brushes, switching mechanisms, or similar arc-producing devices. However, motors and generators containing switching mechanisms must be identified for a Class I, Div. 1 location [501.125(A)].

Luminaires.

Div. 1 luminaires must be: identified for use in a Class I, Div. 1 location [501.130(A)]; completely enclosed, capable of both withstanding an explosion, capable of preventing the ignition of the gas or vapor surrounding the luminaire; and marked to indicate maximum lamp wattage.

  • If fixed, then protected against physical damage by a suitable guard or by location.

  • If pendant, then suspended by and supplied through threaded conduit stems. Threaded joints must be provided with set screws or other means to prevent loosening. Stems longer than 1 foot must have permanent and effective lateral bracing or a flexible fitting or connector identified for a Class I, Div. 1 location (Fig. 4).

Div. 2 luminaires must comply with the following [501.130(B)]:

  • Fixed luminaires must be protected from physical damage by suitable guards or by location. Where the lamp temperature exceeds 80% of the ignition temperature of the gas or vapor, luminaires must be identified for a Class I, Div. 1 location.

  • Pendant luminaires must be suspended by and supplied through threaded conduit stems, and threaded joints must be provided with set screws or other means to prevent loosening. Stems longer than 1 foot must be provided with permanent and effective lateral bracing, or an identified, flexible fitting, or connector must be provided.

  • Portable lighting equipment must be listed for use in a Class I, Div. 1 location, unless the luminaire is mounted on movable stands and connected by a flexible cord as provided in 501.140.

Flexible cords.

Regardless of Division, if you meet the six requirements of 501.140(B) you can use flexible cord for:

  • Portable lighting or utilization equipment.

  • Industrial situations where you need the flexibility, provide suitable protection, and ensure qualified persons install and service the installation.

  • Extension (within a suitable raceway) between a submersible pump (in a wet-pit) and the power source.

  • Electric mixers intended for travel into/out of open-type mixing tanks or vats.

Article 501 has some additional equipment requirements, which address receptacles and attachment plugs [501.145] and signaling, alarm, remote-control, and communications systems [501.150]. These are straightforward and self-explanatory.

The purpose of all 501 requirements is to keep flammable gases (or vapors) and ignition sources from reaching each other. In some cases, you may not have enough information to determine if a location is going to be Div. 2 or if you “have to” adhere to Div. 1 requirements. Don't think in terms of what you can get by with. Ask yourself how well you are keeping fuel and ignition apart. That's one question you never want forensics engineers to ask about something you designed or built.