Mining and Drilling

In non-hazardous locations, there are several different NEMA ratings for specific enclosure "types", their applications, and the environmental conditions they are designed to protect against.  For complete definitions,descriptions, and test criteria,see the National Electrical Manufacturers Association (NEMA) Standards Publication No. 250.

NEMA 1
Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt.

NEMA 2
Same as NEMA 1 including protection against dripping and light splashing of liquids.

NEMA 3
Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 3R
Same as NEMA 3 excluding protection against windblown dust.

NEMA 3S
Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and in which the external mechanism (s) remain operable when ice laden.

NEMA 4
Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 4X
Same as NEMA 4 including protection against corrosion.

NEMA 5
Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against settling airborne dust, lint, fibers, and to provide a degree of protection against dripping and light splashing of liquids.

NEMA 6
Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against hose-directed water and the entry of water during occasional temporary submersion at a limited depth; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 6P
Same as NEMA 6 including protection against the entry of water during prolonged submersion at a limited depth.

NEMA 7
Enclosures are for indoor use in locations classified as Class I, Groups A, B, C, or D and shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficiently that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion, hydro-static, and temperature design tests.

NEMA 9
Enclosures are intended for indoor use in locations classified as Class II, Groups E, F, or G, and shall be capable of preventing the entrance of dust. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting or discoloring dust on the enclosure or igniting dust-air mixtures in the surrounding atmosphere. Enclosures shall meet dust penetration and temperature design tests, and aging of gaskets (if used).

NEMA 12
Enclosures constructed (without knockouts) for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and against dripping and light splashing of liquids.

NEMA 12K
Same as NEMA 12 including enclosures constructed with knockouts.

NEMA 13
Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and against the spraying, splashing, and seepage of water, oil, and non-corrosive coolants.

Explosion-proof products are capable of containing an explosion. The term "explosion-proof" does not indicate that the product is capable of withstanding an external explosion, but only of withstanding an internal explosion without allowing flames or hot gases to escape from the transducer housing to trigger an explosion in the surrounding atmosphere.

The "explosion-proof" term is assigned to those products which are certified by the national rating agencies such as Underwriters Laboratories and Factory Mutual Research after meeting their specifications and passing their tests. Unless certified by one of these agencies, the product does not meet the "explosion-proof" requirements of the National Electrical Code.

Article 100, the Code offers this general definition of "explosion-proof apparatus": "Apparatus enclosed in a case that is capable of withstanding an explosion of a specific gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby."

Note that the Code definition does not require the apparatus to be designed so that an internal explosion cannot occur. The only requirement is that any such explosion be confined to the apparatus interior.

Second, Article 500 of the Code defines specific environments as follows:

* Class I: environments containing flammable gases or vapors.

* Class II: environments containing combustible dusts.

* Class III: environments containing "easily ignitible fibers or flyings."

Within Classes I and II are several "groups" of materials having similar characteristics. For example, under Class I are Groups A (acetylene); B (predominantly hydrogen); C (ethyl ether or ethylene); and D (gasoline, acetone, ammonia, butane, methane, natural gas, etc.). Under Class II we find Groups E (combustible metal dusts); F (carbonaceous materials such as carbon black or coal); and G (other dusts such as flour, wood, and plastic).

Non-incendive Devices, Circuits, and Components: incapable of generating thermal or electrical energy sufficient to ignite a volatile atmosphere under normal operating conditions-although sufficient energy for ignition could be generated under fault conditions.  A Non-Incendive device is designed for use in environments where the specified hazard may be present, but is not likely to exist under normal operating conditions.

Class I is part of the National Electric Code definitions of hazardous location classifications and protection techniques. The Class I classification is a segment of the basic designation which is listed by “class” and “division”.

Class I locations are areas where flammable gases may be present in sufficient quantities to produce explosive or flammable mixtures. Class II locations can be described as hazardous because of the presence of combustible dust. Class III locations contain easily ignitable fibers and flyings. Division 1 designates an environment where flammable gases, vapors, liquids, combustible dusts or ignitable fibers and flyings are likely to exist under normal operating conditions. On the other hand, Division 2 is an environment where flammable gases, vapors, liquids, combustible dusts or ignitable fibers and flyings are not likely to exist under normal operating conditions. Hazardous atmospheres are further defined by “groups.”

Intrinsically Safe (I-Safe) Devices, Circuits, and Components: are incapable of generating thermal or electrical energy sufficient to ignite a volatile atmosphere under either normal or abnormal operating conditions. Consequently, intrinsically safe systems have much wider application than their non-incendive counterparts. Non-incendive systems are generally less costly and easier to maintain than either explosion-proof or intrinsically safe systems.

Class 1, Division 2 Safe Device: Device which is safe to operate in locations (1) in which volatile flammable liquids or gases are handled, processed or used but which are normally confined in enclosed containers or systems, (2) in which ignitable concentrations of gases or vapors are normally prevented by ventilation, (3) which are adjacent to Class I Division I locations and not separated by a vapor tight barrier.  An intrinsically safe device is approved for use in the specified class and division and will not produce any spark or thermal effects that will ignite a specified gas mixture.

ATEX: Derived from the French “ATmosphere EXplosible” (explosive atmosphere). Refers to Atex Directive 04/9/EC, the European regulation governing equipment and protective systems intended for use in potentially explosive atmospheres.

Intrinsic safety (IS) is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms. As a discipline, it is an application of inherent safety in instrumentation.

The theory behind intrinsic safety is to ensure that the available electrical and thermal energy in the system is always low enough that ignition of the hazardous atmosphere cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area, and that all electric supply and signal wires are protected by zener safety barriers. Sometimes an alternative type of barrier known as a galvanic isolation barrier may be used.

In normal uses, electrical equipment often creates internal tiny sparks in switches, motor brushes, connectors, and in other places. Such sparks can ignite flammable substances present in air. A device termed intrinsically safe is designed to not contain any components that produce sparks or which can hold enough energy to produce a spark of sufficient energy to cause an ignition. For example, during marine transfer operations when flammable products are transferred between the marine terminal and tanker ships or barges, two-way radio communication needs to be constantly maintained in case the transfer needs to stop for unforeseen reasons such as a spill. The United States Coast Guard requires that the two way radio must be certified as intrinsically safe.

Another aspect of intrinsic safety is controlling abnormal small component temperatures. Under certain fault conditions (such as an internal short inside a semiconductor device), the temperature of a component case can rise to a much higher level than in normal use. Safeguards, such as current limiting by resistors and fuses, must be employed to ensure that in no case can a component reach a temperature that could cause autoignition of a combustible atmosphere.
No single field device or wiring is intrinsically safe by itself (except for battery-operated, self contained devices), but is intrinsically safe only when employed in a properly designed IS system. All systems are provided with detailed instructions with the proper instructions to ensure safe use.

"Intrinsically safe" products receive their classification because their electrical power usage is below the level of power required to set off an explosion within a given hazardous area. In addition, "intrinsically safe'" products are incapable of storing large amounts of energy which might spark an explosion when discharged.

In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts or high surface temperature of equipment.

For example a household light switch may emit a small, harmless visible spark when switching; in an ordinary atmosphere this arc is of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery is designed either to contain any explosion within the device, or is designed not to produce sparks with sufficient energy to trigger an explosion.

Many strategies exist for safety in electrical installations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety and non-incendive equipment and wiring methods are practices where apparatus is designed with low power levels and low stored energy, so that an arc produced during normal functioning of the equipment or as the result of equipment failure has insufficient energy to initiate ignition of the explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the autoignition temperature of the material involved. Both external and internal temperatures are taken into consideration.

As in most fields of electrical installation, different countries have approached the standardization and testing of equipment for hazardous areas in different ways. As world trade becomes more important in distribution of electrical products, international standards are slowly converging so that a wider range of acceptable techniques can be approved by national regulatory agencies.

Area classification is required by governmental bodies, for example OSHA and compliance is enforced. Documentation requirements are varied. Often an area classification plan-view is provided to identify equipment ratings and installation techniques to be used for each classified plant area. The plan may contain the list of chemicals with their group and temperature rating, and elevation details shaded to indicate Class, Division(Zone) and group combination. The area classification process would require the participation of operations, maintenance, safety, electrical and instrumentation professionals, the use of process diagrams and material flows, MSDS and any pertinent documents, information and knowledge to determine the hazards and their extent and the countermeasures to be employed. Area classification documentations are reviewed and updated to reflect process changes.

In an industrial plant such as a refinery or Chemical process plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones. The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA 497 Standard, or API 500 and according to their adaptation by other areas gas zones is given in the current edition of IEC 60079.10. For hazardous dusts, the guiding standard is IEC 61421.10.

Typical gas hazards are from hydrocarbon compounds.

Safe area
An area such as a residence or office would be classed as safe area, where the only risk of a release of explosive or flammable gas would be such things as the propellant in an aerosol spray. The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk of causing an explosion and are more of a fire risk (although gas explosions in residential buildings do occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).

Division 2 or Zone 2 area
This is a step up from the safe area. In this zone the gas, vapor or mist would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time.[citation needed]

Division 1 or Zone 1 area
Gas, vapor or mist will be present or expected to be present for long periods of time under normal running. As a guide this can be defined as 10–1000 hours/year or 0.1–10% of the time.[citation needed]

Zone 0 area
Gas or vapor is present all of the time (over 1000 hours/year or >10% of the time). Usually this would be the vapor space above the liquid in the top of a tank or drum. The ANSI/NEC classification method consider this environment a Division 1 area

Zones (dusts)

In the case of dusts there is still a chance of explosion. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002

The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on.

The zone definitions are:

Zone 20
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently.

Zone 21
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22
A place in which an explosive atmosphere in the form of a cloud of combustible dust is not likely to occur in normal operation, but if it does occur will persist for a short period only

<p>Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.<br /><br />Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. It is a critical data in determining whether the area needs to be classified or not. A material may have a relatively low autoignition temperature yet its flash-point is above the ambient temperature then the area may not need to be classified. Conversely if the same material is processed(heated) above ambient temperature and the handling temperature is above its flash-point, the area MUST be classified. (NFPA reference needed)<br /><br />Each chemical gas or vapor used in industry is classified into a gas group.</p>
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<td style="background-color: #ebeaea;"><strong> Group            <br /></strong></td>
<td style="background-color: #ebeaea;"><strong> Representative Gases</strong></td>
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<td></td>
<td>I</td>
<td>All Underground Coal Mining. Firedamp (methane)</td>
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<td>IIA</td>
<td>Industrial methane, propane, petrol and the majority of industrial</td>
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<td>IIB</td>
<td>Ethylene, coke oven gas and other industrial gases</td>
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<td></td>
<td>IIC</td>
<td>Hydrogen, acetylene, carbon disulphide</td>
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