Hazardous locations

Wherever flammable gases, vapors, combustible dusts or fibers can be present in the air in dangerous amounts, an ordinary switch, motor or light fixture can become an ignition source. Refineries, gas plants, paint booths, fuel stations, grain elevators, sawmills, distilleries, pharmaceutical plants, wastewater facilities and battery rooms all contain such areas. Electrical work in hazardous (classified) locations follows special code rules: the area is first classified by the likelihood and type of hazard, and then equipment and wiring methods are chosen that cannot ignite the atmosphere. This lesson covers the fire triangle and explosive limits, area classification in the zone and division systems, gas and dust groups, temperature codes, protection techniques and markings, wiring methods and seals, and common applications.

Hazardous locations are covered by Articles 500 to 516 of the NEC: Articles 500 to 503 use the class and division system, Article 505 the zone system for gases and vapors (Zones 0, 1 and 2), and Article 506 the zone system for dusts and fibers (Zones 20, 21 and 22). Articles 511 to 516 cover specific occupancies such as garages, aircraft hangars, motor fuel dispensing, bulk storage and spray finishing.

How explosions happen

An explosion needs three things: fuel (a flammable gas, vapor, mist or dust) mixed with an oxidizer (usually air) in the right proportions, and an ignition source with enough energy. Removing any one prevents it. Electrical equipment can ignite atmospheres through arcs and sparks (switches, relays, motor brushes, loose connections, faults) and through hot surfaces (lamps, motors, resistors, heaters).

A flammable gas or vapor burns only between its lower explosive limit (LEL) and upper explosive limit (UEL): below the LEL the mixture is too lean, above the UEL too rich.

Area classification

Classifying an area means determining what hazardous material may be present and how likely it is to be present in ignitable concentrations. Classification is an engineering task, usually done by a qualified person with process knowledge, and documented on area classification drawings that show the boundaries of each zone or division. Guidance comes from standards and recommended practices (for example IEC 60079-10-1, API RP 505 and RP 500, NFPA 497 and 499).

The size of each zone depends on the release rate, the substance's density (heavier-than-air vapors such as gasoline and propane spread along the ground and into pits), ventilation and the surroundings. Good ventilation can reduce the extent of a zone or downgrade it; pits and trenches below a classified area often become Zone 1 because vapors collect there.

Gas groups and dust groups

Gases differ in how easily they ignite and how easily a flame passes through narrow gaps, so equipment is tested and marked for the gas groups it is suitable for:

• Zone system: Group IIA (propane, gasoline, methane in surface industry), IIB (ethylene and similar), IIC (hydrogen, acetylene). Equipment marked IIC is suitable for IIB and IIA; IIB for IIA.
• Division system: Class I Groups A (acetylene), B (hydrogen), C (ethylene), D (propane, gasoline). Group A is the most severe.
• Dusts: Groups IIIA (combustible flyings), IIIB (non-conductive dust) and IIIC (conductive dust) in the zone system; Class II Groups E (metal dusts), F (carbonaceous dusts such as coal) and G (grain, flour, plastics) in the division system, and Class III for fibers and flyings.

Temperature codes

Hot surfaces can ignite a gas if they reach its autoignition temperature. Equipment is marked with a temperature code (T-code) giving its maximum surface temperature in the specified ambient (usually 40 °C), which must be below the autoignition temperature of the gases present.

Protection techniques and markings

Equipment for hazardous locations prevents ignition in one of several ways, identified in the zone system by Ex protection codes:

Equipment protection levels (EPLs) summarize suitability: Ga, Gb and Gc for gas Zones 0, 1 and 2, and Da, Db and Dc for dust Zones 20, 21 and 22. A typical zone marking reads like "Ex db IIB T4 Gb": flameproof, Group IIB, temperature class T4, suitable for Zone 1. Equipment must be certified (or listed) by an accredited body, and installed and maintained so that its protection is not compromised: a flameproof enclosure with a missing bolt, scratched flame path or wrong cable gland is no longer flameproof.

Intrinsic safety

Intrinsically safe (IS) circuits are the preferred method for instrumentation: sensors, transmitters and switches in the hazardous area are powered through barriers or isolators in the safe area that limit voltage and current so that no spark or hot surface in the field circuit can ignite the gas, even with faults. Because the energy is limited, ordinary wiring methods may be used in the hazardous area, but IS circuits must be separated from other circuits, identified (often with light blue cable or labels), and the combined capacitance and inductance of cables and devices must stay within the barrier's permitted values (the entity concept).

Wiring methods and seals

In Zone 1 and Division 1 areas, wiring is typically threaded rigid metal conduit with explosion-proof fittings, or cables certified for the location (such as armored cables with suitable glands). Conduits connected to flameproof enclosures must be sealed to stop gases and flames traveling along the conduit and to prevent pressure piling (an explosion in one enclosure precompressing gas in the next). Seals are fittings filled with a special compound after the conductors are installed.

Articles 501 and 505 require seals in conduit entering explosion-proof enclosures that contain arcing or high-temperature devices, within 18 in of the enclosure, and at the boundary where conduit leaves a Division 1 or Division 2 (or zone) location (501.15). Threaded conduit joints must be made up wrenchtight with at least five full threads engaged (500.8(E)). Cable fittings and glands must be listed for the location.

Common applications

Electricians encounter hazardous locations in many settings:

• Motor fuel dispensing: Article 514 and Table 514.3(B)(1) classify areas around dispensers, vents and fill points; the space within a dispenser enclosure up to 4 ft is Division 1, and the area within 20 ft horizontally of the dispenser up to 18 in above grade is Division 2, with emergency shutoff switches required (514.11).
• Commercial garages and repair shops: areas near the floor and in pits where gasoline vapors collect.
• Spray booths and finishing rooms, with flammable paints and solvents.
• Grain, flour, sugar, plastics, wood and metal processing, where combustible dust accumulates; dust explosions can be devastating because an initial blast raises more dust.
• Oil and gas production, refineries, chemical plants and pipelines.
• Data centers: generator fuel day tanks and fuel rooms, and battery rooms, are reviewed for classification; with proper design and ventilation they are usually unclassified, but the analysis must be documented.

Lesson 6 covers the grounding and bonding principles on which hazardous location bonding requirements build.