An Inside Look at Hazardous (Classified) Locations

Jan 1, 2006 12:00 PM, By Mike Holt, NEC Consultant

Article 500 is the foundation for correctly applying Articles 501 through 517

When you're looking for answers on hazardous (classified) location questions, where should you turn? Think of Articles 500 through 517 as a mini codebook dedicated specifically to that subject. Article 500 is the foundation — within it you'll find the definitions and general requirements that apply to all hazardous locations.

More specifically, the properties of materials present (or likely to be present) determine the hazard classification — and which 501-503 Article(s) apply. Article 504 provides the requirements for a certain protection technique known as “Intrinsically Safe Systems.” Articles 505 and 506 provide requirements for the “Zone” method of area classification, which is an alternative method to Articles 500-503. The NEC classifies specific hazardous locations by use in Articles 511 through 517. Article 500 is also loaded with Fine Print Notes. While not Code requirements [90.5(C]), they are immensely helpful in making a hazardous location safe.

Key concepts. Grasping a few key concepts upfront will enhance your ability to comply with hazardous (classified) location requirements:

The Fire Triangle (Fig. 1) helps clarify the rationale behind hazardous (classified) location requirements. The three components are fuel, oxygen, and a source of ignition. Fire needs all three.
Locating wiring and equipment outside of the classified location provides the safest electrical installation — usually at the least cost [500.5(A) FPN].
As an installation standard [90.2(A)], the NEC doesn't cover battery-operated equipment. OSHA regulates the use of battery-operated equipment in hazardous (classified) locations.
“Explosionproof” means the device contains the explosion so external temperature won't ignite the surrounding flammable atmosphere. It does not mean the device protects against explosion [500.2].
Proper documentation is a requirement for all hazardous (classified) locations. It must be available to those who are authorized to design, install, inspect, maintain, or operate the electrical equipment [500.4].

Fig. 1. It takes three components to create a fire or explosion – fuel, oxygen, and an ignition source.

Classification. Classification is not set by the entire facility. Rather, it is determined by individual room, section, or area [500.5(A)]. In fact, the same structure might contain a mix of different classifications. Although detailed descriptions of these classifications may seem overwhelming, they can be simplified by Class:

I: Presence of flammable gases or vapors that may be present in the air and in quantities sufficient to produce explosive or ignitible mixtures [500.5(B)].
II: Presence of combustible dust that may be suspended in the air with quantities sufficient to ignite or explode [500.5(C].
III: Presence of easily ignitible fibers or flyings that aren't likely to be suspended in the air in quantities sufficient to produce ignitible mixtures [500.5(D)].

Division. Confusion may also result when we append a “Division” to the Classification. We can cut through the fog there, as well.

Class I:

Class I, Division 1. Ignitible concentrations of flammable gases or vapors may exist in the course of normal operations [500.5(B)(1)].

Class I, Division 2. Volatile flammable gases or vapors would become hazardous only in case of an accident or some unusual operating condition — or under certain conditions [500.5(B)(2)]:

Where volatile flammable liquids or gases are handled, processed, or used, but are normally confined within closed containers and the gases would escape only in the case of accidental rupture or breakdown — or by abnormal operation of equipment.
Where ignitible concentrations of flammable gases or vapors are normally prevented by positive mechanical ventilation, but might become hazardous through abnormal operation of ventilating equipment.
Areas adjacent to a Class I, Division 1 location where flammable gases or vapors might occasionally be present unless prevented by adequate positive-pressure ventilation with effective safeguards against ventilation failure.

Class II:

Class II, Division 1. Where combustible dust may exist in any of these conditions [500.5(C)(1)]:

Nonconductive combustible dust is suspended in the air under normal conditions in sufficient quantities to produce mixtures that will ignite or explode.
Faulty equipment releases ignitible mixtures of dust, and the equipment becomes an ignition source.
Group E (to be defined shortly) electrically conductive combustible dust may be present in sufficient quantities to ignite or explode.

Class II, Division 2. Where a hazard exists because combustible dust [500.5(C)(2):

May be present in the air, due to abnormal operations, in quantities sufficient to produce explosive or ignitible mixtures.
Accumulation is normally insufficient to interfere with the normal operation of electrical equipment or apparatus, but where equipment malfunction may result in combustible dust being suspended in the air.
Accumulations on, in, or near electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitible by abnormal operation of electrical equipment.

Class III:

Class III, Division 1. Ignitible fibers or flyings are manufactured, handled, or used [500.5(D)(1)].

Class III, Division 2. Ignitible fibers or flyings are stored or handled other than in the manufacturing process [500.5(D)(2)].

Material group. The material group designates what the atmosphere in a given location contains. Groups A through D apply to Class I. Groups E, F, and G apply to Class II [500.6]. Class III hazardous locations do not have material groups. Here's a quick overview:

A: Acetylene.
B: Manufactured gas, hydrogen, butadiene, ethylene oxide, and propylene oxide.
C: Ethyl ether, ethylene, and acetaldehyde.
D: Cyclopropane, gasoline, propane, natural gas, methane, benzene, butane, and ethane.
E: Combustible metal dusts such as magnesium or aluminum powders.
F: Carbon black, charcoal, coal, or coke dusts.
G: Combustible dusts such as flour, grain, wood, or plastic.

Protection. You must protect electrical equipment and wiring within hazardous locations [500.7], but how? Use any of the following:

Explosionproof enclosures (Class I locations)
These withstand and contain the force of an internal explosion — the hot gases within the enclosure cool as they escape [500.2].
Dust-ignitionproof enclosures (Class II locations)
These exclude dusts. They will not permit arcs, sparks, or heat within the enclosure to cause ignition of exterior dust [500.2].
Dusttight enclosures (Class II, Division 2 and Class III locations)
These prevent the entrance of dust or flyings. They have no openings to allow electrical sparks or burning material to escape [500.2 and 502.115(B)].
Purged and pressurized systems
For Class I Locations (containing flammable gases or vapors), these permit general-purpose enclosures [500.2]. For Class II Locations (contains combustible dust), these supply positive pressure to general-purpose enclosures [500.2].
Intrinsically safe systems (all locations)
These are incapable of releasing sufficient electrical or thermal energy to cause ignition of flammable gases or vapors [500.2]. None of the requirements in Articles 501 through 503, or 510 through 516 apply to intrinsically safe system installations, except as required by Article 504.
Nonincendive circuits (Class I, Division 2; Class II, Division 2; or Class III, locations)
These are incapable of releasing sufficient electrical or thermal energy to cause ignition of flammable gases, vapors, or dust [500.2].
Oil-immersed make-and-break contacts (Class I, Division 2)
You can install these in a general-purpose enclosure in an area that doesn't contain explosive or ignitible mixtures under normal conditions [500.2].
Hermetically sealed contacts (Class I, Division 2; Class II, Division 2; or Class III, Division 1 and 2 locations)
You can install these in a general-purpose enclosure in an area that doesn't contain explosive or ignitible mixtures under normal conditions [500.2].

Fig. 2. A general-purpose enclosure without make-and-break contacts can be installed in a Class I, Division 2 location.

You can implement “other protection techniques used in equipment identified for use in hazardous (classified) locations” [500.7(L)]. The NEC doesn't explain what this means. But the use of the term “identified” provides insight. Article 100 defines it as, “Recognizable as suitable for the specific purpose, function, use, environment, application, and so forth, where described in a particular Code requirement.”

Equipment. Equipment installed in any hazardous location must be identified for the “Class” and explosive, combustible, or ignitible properties of the specific gas, vapor, dust, fiber, or flyings that will be present (Group) [500.6]. But you can still use general-purpose enclosures for:

Class I locations that don't contain explosive or ignitible mixtures under normal conditions (Class I, Division 2) (Fig. 2), if the enclosures don't contain make-and-break contacts [501.10(B)(4)].
Signaling, alarm, remote control, communications systems and motors, instruments, and relays in Class II locations where the quantities of combustible dust aren't sufficient to produce a fire or explosion under normal conditions. See 502.150(A)(2) Ex, 502.150(A)(3) Ex, 502.150(B)(1) Ex, and 502.150(B)(3) Ex.
Intrinsically safe systems in any hazardous (classified) location. See 500.7(E), 504.10(B), and 504.20.

Fig. 3. Pay particular attention to the temperature ratings of all heat-producing equipment you install in a hazardous (classified) location.

When installing heat-producing equipment (e.g., luminaires, motors), in a hazardous (classified) location, observe its markings for operation temperature or temperature range (T-Rating) as shown in Fig. 3. Table 500.8(B) has detailed information.

Thread conduit with a National Pipe Thread (NPT) taper of ¾ inch per foot. Assemble wrenchtight, with at least five threads fully engaged [500.8(D)]. Exception: For listed explosionproof equipment, factory-threaded entries must be made up with at least 4½ threads fully engaged.

So now you've seen the modular structure of the Articles that address hazardous locations. And you can see that breaking down the hazardous location designations by class, division, and material group — in that order — simplifies the task of correctly identifying and implementing the requirements for protecting equipment in hazardous locations. Remember the key concepts, and you'll have little problem correctly applying the other Articles in the Code.