In electrical and safety engineering, hazardous locations (HAZLOC, HazLoc, or hazloc ) are places where fire or explosion hazards may exist. Sources of such hazards include gases, vapors, dust, fibers, and flyings, which are combustible or flammable. Electrical equipment installed in such locations can provide an ignition source, due to electrical arcing, or high temperatures. Standards and regulations exist to identify such locations, classify the hazards, and design equipment for safe use in such locations.

Overview

A light switch may cause a small, harmless spark when switched on or off. In an ordinary household this is of no concern, but if a flammable atmosphere is present, the arc might start an explosion. In many industrial, commercial, and scientific settings, the presence of such an atmosphere is a common, or at least commonly possible, occurrence. Protecting against fire and explosion is of interest for both personnel safety as well as reliability reasons.

Several protection strategies exist. The simplest is to minimize the amount of electrical equipment installed in a hazardous location, either by keeping the equipment out of the area altogether, or by making the area less hazardous (for example, by process changes, or ventilation with clean air).

When equipment must be placed in a hazardous location, it can be designed to reduce the risk of fire or explosion. Intrinsic safety designs equipment to operate using minimal energy, insufficient to cause ignition. Explosion-proofing designs equipment to contain ignition hazards, prevent entry of hazardous substances, and, contain any fire or explosion that could occur.

Different countries have approached the standardization and testing of equipment for hazardous areas in different ways. Terminology for both hazards and protective measures can vary. Documentation requirements likewise vary. As world trade becomes more globalized, international standards are slowly converging, so that a wider range of acceptable techniques can be approved by national regulatory agencies.

The process of determining the type and size of hazardous locations is called classification. Classification of locations, testing and listing of equipment, and inspection of installation, is typically overseen by governmental bodies. For example, in the US by the Occupational Safety and Health Administration.

Standards

North America

In the US, the independent National Fire Protection Association (NFPA) publishes several relevant standards, and they are often adopted by government agencies. Guidance on assessment of hazards is given in NFPA497 (explosive gas) and NFPA499 (dust). The American Petroleum Institute publishes analogous standards in RP500 andRP505.

NFPA 70, the National Electrical Code (NEC), defines area classification and installation principles. NEC article 500 describes the NEC Division classification system, while articles 505 and 506 describe the NEC Zone classification system. The NEC Zone system was created to harmonize with IEC classification system, and therefore reduce the complexity of management.

Canada has a similar system with CSA Group standard C22.1, the Canadian Electrical Code, which defines area classification and installation principles. Two possible classifications are described, in Section 18 (Zones), and Appendix J (Divisions).

International Electrotechnical Commission

The International Electrotechnical Commission publishes the 60079 series of standards which defines a system for classification of locations, as well as categorizing and testing of equipment designed for use in hazardous locations, known as "Ex equipment". IEC 60079-10-1 covers classification of explosive gas atmospheres, and IEC 60079-10-2 explosive dust. Equipment is placed into protection level categories according to manufacture method and suitability for different situations. Unlike ATEX which uses numbers to define the safety "Category" of equipment (namely 1, 2, and 3), the IEC continued to utilise the method used for defining the safe levels of intrinsic safety namely "a" for zone 0, "b" for zone 1 and "c" for zone 2 and apply this Equipment Level of Protection to all equipment for use in hazardous areas since 2009. IEC 60079-14: Explosive atmospheres – Part 14: Electrical installations design, selection and erection

The IEC 60079 standard set has been adapted for use in Australia and New Zealand and is published as the AS/NZS 60079 standard set.

Hazards

In an industrial plant, such as a refinery or chemical plant, handling of large quantities of flammable liquids and gases creates a risk of exposure. Coal mines, grain mills, elevators, and similar facilities likewise present the risk of a clouds of dust. In some cases, the hazardous atmosphere is present all the time, or for long periods. In other cases, the atmosphere is normally non-hazardous, but a dangerous concentration can be reasonably foreseen—such as operator error or equipment failure. Locations are thus classified by type and risk of release of gas, vapor, or dust. Various regulations use terms such as class, division, zone, and group to differentiate the various hazards.

Often an area classification plan view is provided to identify equipment ratings and installation techniques to be used for each classified area. The plan may contain the list of chemicals with their group and temperature rating. The classification process requires the participation of operations, maintenance, safety, electrical and instrumentation professionals; and the use of process diagrams, material flows, safety data sheets, and other pertinent documents. Area classification documentations are reviewed and updated to reflect process changes.

Explosive gas

Typical gas hazards are from hydrocarbon compounds, but hydrogen and ammonia are also common industrial gases that are flammable.

Class I, Division 1 classified locations An area where ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operating conditions. A Class I, Division 1 area encompasses the combination of Zone 0 and Zone 1 areas.

Zone 0 classified locations An area where ignitable concentrations of flammable gases, vapors or liquids are present continuously or for long periods of time under normal operating conditions. An example of 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 Class I, Division 1 area. As a guide for Zone 0, this can be defined as over 1000 hours per year or more than 10% of the time.

Zone 1 classified location An area where ignitable concentrations of flammable gases, vapors or liquids are likely to exist under normal operating conditions. As a guide for Zone 1, this can be defined as 10–1000 hours per year or 0.1–10% of the time.

Class I, Division 2 or Zone 2 classified locations An area where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions. In this area the gas, vapor or liquids would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide for Zone 2, unwanted substances should only be present under 10 hours per year or 0–0.1% of the time.

Unclassified locations Also known as non-hazardous or ordinary locations, these locations are determined to be neither Class I, Division 1 or Division 2; Zone 0, Zone 1 or Zone 2; or any combination thereof. Such areas include a residence or office 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 liquids are paint and brush cleaner. These are designated as very low risk of causing an explosion and are more of a fire risk (although gas explosions in residential buildings do occur). Unclassified locations in chemical and other plants are present where it is absolutely certain that the hazardous gas is diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).

Combustible dusts

Dust or other small particles suspended in air can explode.

NEC

Class Division Description Class II Division 1 ignitable concentrations of combustible dust can exist, under normal conditions Division 2 ignitable concentrations of combustible dust are unlikely to exist normally Class III Division 1 ignitable fibers, or materials producing combustible flyings, are handled, manufactured or used Division 2 easily ignitable fibers are stored or handled UnclassifiedNon-hazardous or ordinary locations. Determined to be none of the above.

United Kingdom

An old British standard used letters to designate 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.

Zone Description Zone 20 ignitable concentrations of dust, fibers, or flyings are present for long periods of time Zone 21 ignitable concentrations of dust, fibers, or flyings are likely to exist under normal conditions Zone 22 ignitable concentrations of dust, fibers, or flyings unlikely to exist under normal conditions

Gas and dust groups

Different explosive atmospheres have 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 (MESG), minimum igniting current (MIC) ratio, 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.

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. The flash point determines if an area needs to be classified. A material may have a relatively low autoignition temperature yet if its flash-point is above the ambient temperature, then the area may not need to be classified. Conversely if the same material is heated and handled above its flash-point, the area must be classified for proper electrical system design, as it will then form an ignitable mixture.

Each chemical gas or vapour used in industry is classified into a gas group. NEC Division System gas & dust groupsAreaGroupRepresentative materialsClass I, Divisions 1 & 2AAcetyleneBHydrogenCEthyleneDPropane, methaneClass II, Divisions 1 & 2E (Division 1 only)Metal dusts, such as magnesium (Division 1 only)FCarbonaceous dusts, such as carbon & charcoalGNon-conductive dusts, such as flour, grain, wood & plasticClass III, Divisions 1 & 2NoneIgnitible fibers/flyings, such as cotton lint, flax & rayon +NEC & IEC Zone System gas & dust groupsAreaGroupRepresentative materialsZones 0, 1 & 2IICAcetylene & hydrogen(equivalent to NEC Class I, Groups A and B)IIB+H2Hydrogen(equivalent to NEC Class I, Group B)IIBEthylene(equivalent to NEC Class I, Group C)IIAPropane(equivalent to NEC Class I, Group D)Zones 20, 21 & 22IIICConductive dusts, such as magnesium(equivalent to NEC Class II, Group E)IIIBNon-conductive dusts, such as flour, grain, wood & plastic(equivalent to NEC Class II, Groups F and G)IIIAIgnitible fibers or flyings, such as cotton lint, flax & rayon(equivalent to NEC Class III)Mines susceptible to firedampI (IEC only)Methane