Article 501: Class I Locations

Article 501: Class I Locations

By Mike Holt for EC&M Magazine

Article 501
You have a Class I location any time enough flammable gases or vapors are present that they may produce an explosive or ignitible mixture. When you have a Class I installation, you must follow Article 501. When doing so, you will need to refer back to Article 500—which defines two types of Class 1 locations: Division 1 and Division 2.

You have Division 1 where the Class 1 ignition sources may be present or are present.

But in a Division 2 location, these sources are normally confined within closed containers (or systems) or vented (via positive mechanical ventilation). You also have a Division 2 location when the area is adjacent to a Division 1 and ignitable concentrations may occasionally waft over (due to lack of positive pressure ventilation). Article 501 requirements are, consequently, less stringent for Division 2.

Wiring methods
For Division 1, you can use these wiring methods (Figure 501-1) [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, Division 1 location (Figure 501-2).
Type ITC-HL cable with fittings listed for a Class I, Division 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 for Class I, Division 1 locations.

For Division 2, you can use these wiring methods (Figure 501-1) [501.10(B)]:

General wiring, which consists of (Figure 501-3)

Wiring methods permitted in Class I, Division 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].
Cables—MI, MC, MV, or TC cable 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

Nonicendive 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 flame 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. Here are some highlights:

No splices or taps within a conduit seal fitting.
The conductor area must not exceed 25 percent of the cross-sectional area of rigid metal conduit, unless the seal fitting is identified for a higher percentage fill (Figure 501-13).
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 (Figure 501-6).
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 Division 1, install surge arresters, transient voltage surge suppressors (TVSSs), and capacitors in enclosures identified for Class I, Division 1 locations. For Division 2, 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 Division I equipment are more stringent than those for Division II equipment.

Transformers and Capacitors

Division I: 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 500.100(A).
Division 2: Install them in vaults complying with 450.21 through 450.27.

Meters, Instruments, and Relays

Division 1: Install in enclosures identified for a Class I, Division 1 location.
Division 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, Division 1 enclosures.

Switches, circuit breakers, motor controllers, and fuses

Division 1: Install these in enclosures identified for a Class I, Division 1 location.
Division 2: If the installation meets certain (extensive) requirements, you can use general enclosures [501.115(B)]. Otherwise, use Class I, Division 1 enclosures.

Control Transformers and Relays

Division 1. Install them in explosionproof enclosures identified for a Class I, Division 1 location.

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

Motors and Generators

Division 1 motors and generators must be:

Identified for a Class I, Division 1 location; or
Of the totally enclosed type supplied with positive-pressure ventilation and arranged to automatically de-energize if the air supply fails; or
Of the totally enclosed inert gas-filled type and arranged to automatically de-energize if the gas supply fails; or
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).

Division 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, Division 1 location [501.125(A)] (Figure 501-22).

Luminaires

Division 1 luminaires must be identified for use in a Class I, Division 1 location; completely enclosed, capable of both withstanding an explosion, capable of preventing the ignition of the gas or vapor surrounding the luminaire; 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 ft must have permanent and effective lateral bracing or a flexible fitting or connector identified for a Class I, Division 1 location (Figure 501-23).

Division 2 luminaires must comply with the following:

Fixed luminaires must be protected from physical damage by suitable guards or by location. Where the lamp temperature exceeds 80 percent of the ignition temperature of the gas or vapor, luminaires must be identified for a Class I, Division 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 ft 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, Division 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 6 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. These 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 Division 2 or if you “have to” adhere to Division 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.