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, or non-incendive equipment
and wiring methods, is a set of practices for apparatus designed with low power levels
and low stored energy. Insufficient energy is available to produce an arc that can ignite
the surrounding 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 by the U.S. Occupational
Safety and Health Administration 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.
History Soon after the introduction of electric power
into coal mines, it was discovered that lethal explosions could be initiated by electrical
equipment such as lighting, signals, or motors. The hazard of fire damp or methane accumulation
in mines was well known by the time electricity was introduced, along with the danger of suspended
coal dust. At least two British mine explosions were attributed to an electric bell signal
system. In this system, two bare wires were run along the length of a drift, and any miner
desiring to signal the surface would momentarily touch the wires to each other or bridge the
wires with a metal tool. The inductance of the signal bell coils, combined with breaking
of contacts by exposed metal surfaces, resulted in sparks which could ignite methane, causing
an explosion. Gas divisions or zones
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 60079-10-1. For hazardous dusts, the guiding standard
is IEC 60079-10-2. Typical gas hazards are from hydrocarbon compounds,
but hydrogen and ammonia are common industrial gases that are flammable.
Non-Hazardous Area An area such as a residence or office would
be classed as Non Hazardous, 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. Non hazardous areas on chemical
and other plant are present where it is absolutely certain that the hazardous gas is diluted
to a concentration below 25% of its lower flammability limit).
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. As a general
guide for Zone 2, unwanted substances should only be present under 10 hours/year or 0–0.1%
of the time. 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 operating conditions. As a guide for Zone 1, this can be defined as 10–1000 hours/year
or 0.1–10% of the time. Zone 0 area
Gas or vapor is present all of the time. 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 Division 1 area. As a guide for Zone 0, this can be defined as over 1000
hours/year or >10% of the time. Dust zones
Flammable dusts when suspended in air can explode. 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 1999EU 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 22
A place in which an explosive atmosphere in the form of a cloud of combustible dust in
air is not likely to occur in normal operation but, if it does occur, will persist for a
short period only Zone 21
A place in which an explosive atmosphere in the form of a cloud of combustible dust in
air is likely to occur, occasionally, in normal operation.
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. Gas groups
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. 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. Each chemical gas or vapour used in industry
is classified into a gas group. Apparatus marked IIB can also be used for
IIA gases. IIC marked equipment can be used for both IIA and IIB. If a piece of equipment
has just II and no A, B, or C after then it is suitable for any gas group.
A list must be drawn up of every chemical gas or vapor that is on the refinery/chemical
complex and included in the site plan of the classified areas. The above groups are formed
in order of how volatile the gas or vapor would be if it was ignited, IIC being the
most volatile and IIA being the least. The groups also indicate how much energy is required
to ignite the gas by spark ignition, Group IIA requiring the most energy and IIC the
least. Equipment protection level
In recent years also the Equipment Protection Level is specified for several kinds of protection.
The required Protection level is linked to the intended use in the zones described below:
Temperature classification Another important consideration is the temperature
classification of the electrical equipment. The surface temperature or any parts of the
electrical equipment that may be exposed to the hazardous atmosphere should be tested
that it does not exceed 80% of the auto-ignition temperature of the specific gas or vapor in
the area where the equipment is intended to be used.
The temperature classification on the electrical equipment label will be one of the following:
The above table tells us that the surface temperature of a piece of electrical equipment
with a temperature classification of T3 will not rise above 200 °C.
Auto-ignition temperatures The auto-ignition temperature of a liquid,
gas or vapor is the temperature at which the substance will ignite without any external
heat source. The exact temperature value determined depends on the laboratory test conditions
and apparatus. Such temperatures for common substances are:
The surface of a high pressure steam pipe may be above the autoignition temperature
of some fuel/air mixtures. Auto-ignition temperatures
The auto-ignition temperature of a dust is usually higher than that of vapours & gases.
Examples for common materials are: Type of protection
To ensure safety in a given situation, equipment is placed into protection level categories
according to manufacture method and suitability for different situations. Category 1 is the
highest safety level and Category 3 the lowest. Although there are many types of protection,
a few are detailed The types of protection are subdivided into
several sub classes, linked to EPL: ma and mb, px, py and pz, ia, ib and ic. The a subdivisions
have the most stringent safety requirements, taking into account more the one independent
component faults simultaneously. Multiple protection
Many items of EEx rated equipment will employ more than one method of protection in different
components of the apparatus. These would be then labeled with each of the individual methods.
For example a socket outlet labeled EEx'de' might have a case made to EEx 'e' and switches
that are made to EEx 'd'. ANSI/NFPA areas description
Class I, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids are
present continuously or frequently within the atmosphere under normal operation conditions.
Class I, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are
present within the atmosphere under abnormal operating conditions.
Class II, Div. 1 - Where ignitable concentrations of combustible dusts are present within the
atmosphere under normal operation conditions. Class II, Div. 2 - Where ignitable concentrations
of combustible dust are present within the atmosphere under abnormal operating conditions.
Class III, Div. 1 - Where easily ignitable fibers or materials producing combustible
flyings are present within the atmosphere under normal operation conditions.
Class III, Div. 2 - Where easily ignitable fibers or materials producing combustible
flyings are present within the atmosphere under abnormal operating conditions.
Common Materials within Associated Class & Group Ratings, such as "Class I, Division 1, Group
A": Class I Areas: Group A: Acetylene / Group
B: Hydrogen / Group C: Propane and Ethylene / Group D: Benzene, Butane, Methane & Propane
Class II Areas: Group E: Metal Dust / Group F: Carbon & Charcoal / Group G: Flour, Starch,
Wood & Plastic Class III Areas: NO GROUP: Cotton & Sawdust
For more information see Article 500 of NFPA 70 - The National Electric Code, as published
by the National Fire Protection Association. Equipment category
The equipment category indicates the level of protection offered by the equipment.
Category 1 equipment may be used in zone 0, zone 1 or zone 2 areas.
Category 2 equipment may be used in zone 1 or zone 2 areas.
Category 3 equipment may only be used in zone 2 areas.
Labeling All equipment certified for use in hazardous
areas must be labelled to show the type and level of protection applied.
Europe
In Europe the label must show the CE mark and the code number of the certifying body.
The CE marking is complemented with the Ex mark, followed by the indication of the Group,
Category and, if group II equipment, the indication relating to gases or dust. For example: Ex
II 1 G Specific type or types of protection being used will be marked.
EEx ia IIC T4.. EEx nA II T3 X.
Industrial electrical equipment for hazardous area has to conform to appropriate parts of
standard EN 60079 and in some cases, certified as meeting that standard. Independent test
houses are established in most European countries, and a certificate from any of these will be
accepted across the EU. North America
In North America the suitability of equipment for the specific hazardous area must be tested
by a Nationally Recognized Testing Laboratory. Such institutes are UL, MET, FM, CSA or Intertek,
for example. The label will always list the Class(es),
Division(s) and may list the Group(s) and temperature Code. Directly adjacent on the
label one will find the mark of the listing agency.
Some manufacturers claim "suitability" or "built-to" hazardous areas in their technical
literature, but in effect lack the testing agency's certification and thus unacceptable
for the AHJ to permit operation of the electrical installation/system.
All equipment in Division 1 areas must have an approval label, but certain materials,
such as rigid metallic conduit, does not have a specific label indicating the Cl./Div.1
suitability and their listing as approved method of installation in the NEC serves as
the permission. Some equipment in Division 2 areas do not require a specific label, such
as standard 3 phase induction motors that do not contain normally arcing components.
Also included in the marking are the manufacturers name or trademark and address, the apparatus
type, name and serial number, year of manufacture and any special conditions of use. The NEMA
enclosure rating or IP code may also be indicated, but it is usually independent of the Classified
Area suitability. See also
Electrical conduit Intrinsic safety
ATEX directive Pressure piling
Mineral-insulated copper-clad cable Grounding kit
CompEx Competency standard References
^ Bossert & Hurst 1986, p. 17. ^ a b c "Hazardous Area Classification and
Control of Ignition Sources". Health and Safety Executive. 22 September 2004. Retrieved 7
May 2011. ^ Bossert & Hurst 1986, Chapter 9.
^ BS EN 60079: "Explosive atmospheres" Bossert, John; Hurst, Randolph. Hazardous
Locations: A Guide for the Design, Construction, and Installation of Electrical Equipment.
Toronto: Canadian Standards Association. ISBN 0-9690124-5-4. Further reading
Alan McMillan, Electrical Installations in Hazardous Areas, Butterworth-Heineman 1998,
ISBN 0-7506-3768-4 Peter Schram Electrical Installations in Hazardous
Locations, Jones and Bartlett, 1997, ISBN 0-87765-423-9
Intrinsic Safety in Cellphones, Smartphones and PDAs, Airo Wireless
External links ATEX Explained
ATEX "Use" Directive ATEX Directive in Detail
Hazardous area basics IECEx System
Safety Zones Explained