17.3 Class II & Class III Locations and the Zone System (Articles 502, 503, 505 & 506)

Class II locations: combustible dust (Article 502)

Class II locations contain combustible dust: Group E (metal dusts such as aluminum and magnesium), Group F (carbonaceous dusts such as coal and coke), and Group G (other dusts such as flour, grain, starch, wood, and plastics). Minnesota grain elevators, feed mills, flour mills, and woodworking plants are common examples.

Wiring methods (502.10)

Boxes and fittings: in Division 1, boxes and fittings must have threaded bosses and be dusttight; those containing taps, joints, or terminal connections, or used in Group E locations, must be identified for Class II locations. In Division 2, boxes and fittings must be dusttight.

Flexible connections: dusttight flexible connectors, LFMC or LFNC with listed fittings, interlocked-armor Type MC cable with an impervious jacket, or extra-hard-usage flexible cord, each with bonding as required.

Sealing (502.15)

Where a raceway connects an enclosure that must be dust-ignitionproof with one that need not be, dust must be kept out of the second enclosure by one of these:

1. a permanent and effective seal;
2. a horizontal raceway at least 10 ft long; or
3. a vertical raceway at least 5 ft long extending downward from the dust-ignitionproof enclosure.

Class II seals need not be explosionproof, but they must be accessible. A seal is not required where the raceway runs between an enclosure and an unclassified location.

Equipment and luminaires

• Division 1: motors, switches, controllers, and transformers must be identified for Class II, Division 1 (dust-ignitionproof); luminaires must be identified for the location, marked with the maximum lamp wattage, and protected from physical damage.
• Division 2: equipment enclosures must be dusttight or designed to minimize dust entry and accumulation; luminaires must be marked for maximum wattage and designed to minimize dust deposits.
• Pendant luminaires must be suspended by threaded RMC or IMC stems, chains with approved fittings, or other approved means, with flexible connections where needed.

Multiwire branch circuits

In Class II, Division 1 (502.40), and likewise in Class I, Division 1 (501.40), multiwire branch circuits are not permitted unless the disconnecting device opens all ungrounded conductors simultaneously. A shared neutral that stays connected when one circuit is opened could arc in the classified location.

Class III locations: ignitible fibers and flyings (Article 503)

Class III locations handle or store easily ignitible fibers or flyings, such as cotton, sawdust, wood shavings, or excelsior, that are not normally suspended in the air in ignitible amounts.

The Zone system (Articles 505 and 506)

The Zone system classifies gas and dust locations by how often an ignitible atmosphere is present, following the international IEC approach. It is an alternative to the Class and Division system, used under engineering supervision.

Protection techniques and markings

A typical marking reads Class I, Zone 1, AEx d IIC T4: suitable for Zone 1, flameproof, gas group IIC (covering acetylene and hydrogen), with a maximum surface temperature of 135°C. Zone gas groups are IIC (most severe), IIB, and IIA; equipment marked IIC may be used in IIB and IIA atmospheres. Dust groups are IIIA (combustible flyings), IIIB (nonconductive dust), and IIIC (conductive dust).

Wiring methods

Zone 1 generally permits the Class I, Division 1 wiring methods, and Zone 2 the Division 2 methods. Zone 0 is limited to intrinsically safe wiring and other methods specifically permitted for Zone 0, with seals where raceways leave Zone 0.

Worked example: a grain elevator

A Minnesota country elevator has a Class II, Division 1, Group G area at the head house and a Division 2 area in the adjacent work floor.

1. Head house wiring: threaded RMC or IMC, or MI cable, with dust-ignitionproof motors and switches identified for Group G.
2. Raceway to the work floor control panel: to keep dust out of the general-purpose panel, install a seal, or run at least 10 ft of horizontal raceway, or at least 5 ft of vertical raceway downward from the dust-ignitionproof enclosure.
3. Branch circuits: no multiwire branch circuits in the Division 1 area unless a multipole breaker opens all ungrounded conductors together.