Hazardous Area Zones | Electrical Equipment For Explosive Atmospheres

Hazardous Area Zones

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Hazardous Area Zones

Information from ASCO.

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Some History

The classification of hazardous areas into zones established the level of protection required for electrical equipment installed in explosive atmospheres (IEC 60079-10). Since this approach proved successful, it was applied to explosive dust atmospheres (IEC-EN 61241-10).

The new edition of IEC 60079-0 (2007) [EN 60079-0 (2009)] progressively replaces IEC-EN 60079-10 and IEC-EN 61241-10 by:
• IEC-EN 60079-10-1: Classification of hazardous areas, explosive gas atmospheres
• IEC-EN 60079-10-2: Classification of hazardous areas, combustible dust atmospheres (dust groups/EPL)

The selection and erection of electrical installations is defined by standard EN 60079-14.

Definition of a place where a potentially explosive atmosphere may occur 

The classification of an installation into distinct zones has two objectives (according to ATEX 1999/92/EC):

• To define the categories of equipment used in the zones indicated, provided they are suitable for gases, vapours or mists and/or dusts.

• To classify hazardous places into zones to prevent ignition sources and be able to select the correct electrical and non-electrical equipment accordingly. The zones are defined on the basis of the occurrence of explosive gaseous or dusty atmospheres.

Gas groups

Gas Group I: Equipment intended for use in mines susceptible to firedamp.
Gas Group II: Equipment intended for use in places with an explosive gas atmosphere other than mines susceptible to firedamp.

Hazardous Area ZoneCategory (2014/34/EU)Presence of Explosive Atmospheres
Gas Group IIZone 01 G¹Continuous, frequent or for long periods
Zone 12 GIntermittent in normal operation (likely)
Zone 23 GOccasional or for short periods (never in normal operation)
Group I (mines)M1¹Presence (methane, dust)
M2Risk of presence (methane, dust)

Dust groups

(According to the fifth edition, IEC 60079-0, 2007 (EN 60079-0, 2009) (2).

Dust Group III : Equipment intended for use in places with an explosive dust atmosphere other than mines susceptible to firedamp.

Hazardous Area ZoneCategory (2014/34/EU)Presence of Explosive Atmospheres
Dust Group IIIZone 201 D¹Continuous, frequent or for long periods (air/cloud of combustible dust)
Zone 212 DIntermittent in normal operation
Zone 223 DOccasional or for short periods

 

The classification of the installation is the responsibility of the user. He must individually evaluate each installation to determine the differences between them. Separate assessments must be made for places with potentially explosive atmospheres caused by gases or vapours and for those caused by dusts.

Equipment Protection Levels – EPLs

In normal circumstances the effect of the EPLs will be to retain the normal zone/equipment protection relationship. If, however, the risk is considered especially severe, then the required EPL for the hazardous area zone may be increased. Similarly, if the risk of explosion is deemed to be especially small or negligible, the EPL may be reduced from the norm. The following table shows the normal relationship between EPL and zone/category (without supplementary risk assessment).

Equipment Protection Level (EPL)Normal Applicable Zone(s)Category (2014/34/EU)
Ga0 (and 1 and 2)1G
Gb1 (and 2)2G
Gc23G
Da20 (and 21 and 22)1D
Db21 (and 22)2D
Dc223D
Ma/MbMinesM1/M2

(¹) G = gas ; D = dust; M = mines
(²) Including IEC 61241-0 (dusts)

Examples of a Classification Into Hazardous Zones

Above drawings A and B are an example only and must not be used as a model for an actual plant whose design is, in every case, the responsibility of the constructor and operator.

How can ATEX, EN 50014, EN 50281-1-1 or EN 13463-1 approved apparatus for use in explosive atmospheres be identified?

How can ATEX, EN 50014, EN 50281-1-1 or EN 13463-1 approved apparatus for use in explosive atmospheres be identified?

How can ATEX, EN-IEC 60079-0, EN 61241-0 or EN 13463-1 approved apparatus for use in explosive atmospheres be identified?

Types of Explosion Protection

What is a type of protection for electrical apparatus for use in gas atmospheres?
It is the comprehensive range of protective measures applied to an electrical apparatus to prevent possible ignition of the surrounding atmosphere.

Explosion Protection SymbolHazardous Area ZonesDescriptionDrawing
012
“d”Type of protection in which the parts which can ignite an explosive atmosphere are placed in an enclosure which can withstand the pressure developed during an internal explosion of an explosive mixture and which prevents the transmission of the explosion to the explosive atmospheres surrounding the enclosure.
“e”Type of protection in which measures are applied so as to prevent with a higher degree of safety the possibility of excessive temperatures and of the occurrence of arcs or sparks in the interior and on the external parts of electrical apparatus, which does not produce them in normal service.
“i”“ia”Type of protection when no spark or any thermal effect in the circuit,
produced in the test conditions prescribed in the standard (which include normal operation and specific fault conditions), is capable of causing ignition.
“ib”
“m”Type of protection in which the parts which can ignite an explosive atmosphere are enclosed in a resin sufficiently resistant to the environmental influences in such a way that this explosive atmosphere cannot be ignited by either sparking or heating which may occur within the encapsulation.
“n”Method of protection for electrical equipment designed so that it will not ignite the surrounding explosive atmosphere in normal operation and under certain fault conditions specified in the standard. There are 5 categories of equipment: nA (non-sparking), nC
(hermetically sealed), nR (restricted breathing), nL (limited energy) and nP (simplified pressurisation).
“o”Type of protection in which the electrical apparatus is immersed in oil.
“p”Type of protection in which the protective inert gas inside the enclosure is maintained at a higher pressure than that of the surrounding atmosphere.
“q”Type of protection in which the enclosure is filled with a material in a finely granulated state.

Types of protection offered:
– a wide range of certified solenoid valves with “d”, “m”, “em”, “n” or “i” type of protection;
– certified air operated valves, pressure-operated valves, cylinders and air service equipment with “c” type of protection.

Types of protection for electrical apparatus for use in the presence of combustible dust (EN 60241-0)

Applicable to electrical apparatus for use in areas where combustible dust may be present in quantities which could lead to a fire or explosion hazard. EN 61241-1 = tD ; EN 61241-18 = mD ; EN 61241-11 = iD

Explosion Protection SymbolHazardous Area ZonesDescriptionDrawing
202122
“tD”Electrical apparatus protected by enclosure and surface
temperature limitation for use in areas where combustible
dust may be present in quantities which could lead to a fire or explosion hazard. The ignition protection is based on the limitation of the maximum surface temperature of the enclosure and other surfaces which may come into contact with dust and on the restriction of dust ingress into the enclosure by the use of “dust-tight” or “dust-protected” enclosures. This standard is not applicable to electrical apparatus intended for use in underground parts of mines as well as those parts of surface installations of such mines endangered by firedamp and/or combustible dust; nor does it take account of any risk due to an emission of flammable or toxic gas from the dust.
“mD”maDElectrical apparatus protected by encapsulation type of protection ‘mD’ and surface temperature limitation for use in areas where combustible dust may be present in quantities which could lead to a fire or explosion hazard.
Type of protection in which the parts which can ignite an
explosive atmosphere are enclosed in a resin sufficiently
resistant to environmental influences in such a way that a
dust cloud or layer cannot be ignited during installation or
operation.
mbD
“iD”Intrinsically safe apparatus intended for use in potentially
explosive dust cloud or dust layer environments and for
associated apparatus that is intended for connection to
intrinsically safe circuits which enter such environments.
Applicable to electrical apparatus in which the electrical circuits themselves are incapable of causing an explosion in the surrounding combustible dust environment.

TYPE OF PROTECTION FOR NON-ELECTRICAL APPARATUS (EN 13463-5 = C)

“c”This standard establishes manufacturing requirements
which have been proven safe, in order to avoid any inflammation sources such as friction or heating sparks.
It applies to apparatus where movement and friction can
occur (clutches, brakes, bearings, springs…).

 

Equipment Groups & Temperature Classes

Classification of gases into explosion groups

  • Gas Group I: Electrical equipment intended for use in the underground parts of mines, and to those parts of surface
    installations of such mines, likely to become endangered by firedamp and/or combustible dust.
  • Gas Group II: Electrical equipment intended for use in other places likely to become endangered by explosive
    atmospheres (surface industries).

For the types of explosion protection “d” and “i”, group II is subdivided into IIA, IIB, IIC.

Electrical apparatus certified for IIB may be used in applications requiring apparatus to be certified for group IIA. Electrical apparatus certified for IIC may be used in applications requiring apparatus to be certified for groups IIA and IIB. For example the “d” and “i” types of protection are respectively subdivided according to the Maximum Experimental Safe Gap (MESG) and to the Minimum Igniting Current (MIC). Electrical apparatus certified for IIB may be certified for use with a gas belonging to group IIC. In this case, the identification is supplemented with the chemical symbol or the name of the gas (example: Ex d IIB + H2 according to EN 60079-0 and EN 60079.1).

The table below indicates the groups to which some gas mixtures belong:

Group

Gas

Ignition Temperature¹ (°C)Temperature Class
T1T2T3T4T5T6
Imethane (firedamp)
IIAacetone540
acetic Acide485
ammonia630
ethane515
methylene chloride556
methane (CH4)595
carbon monoxide605
propane470
n-butane365
n-butyl370
n-hexane240
acetaldehyde140
ethyl ether170
ethyl nitrite90
Bethylene425
ethyl oxyde429-440
hydrogen sulfide270
Cacetylene (C2H2)305
carbon disulphide (CS2)102
hydrogen (H2)560

(¹) Temperature of a hot surface able to ignite a gas mixture.
The ignition temperature of the gas mixture must be higher than the maximum surface temperature. In practice, a 10 to 20% safety margin is observed between the ignition  temperature and the rated nameplate temperature. The ignition temperature of a cloud of dust is generally between 300 and 700°C. At 150 to 350°C, the ignition temperature of a layer of dust is far below that of a dust cloud. A burning dust layer can initiate a dust explosion if brought in contact with a combustible dust cloud, so these values must be taken into account to limit the risk.

Temperature Class

The temperature classification is based on the maximum surface temperature of equipment. That is the highest temperature any part of or the entire surface of an electrical device can reach under the most unfavourable operating conditions capable of igniting a surrounding explosive atmosphere.

Read more T Class Temperature Classes for Hazardous Areas

Group I: Temperature ≤ 150°C or ≤ 450°C according to coal dust accumulation on equipment.
Group II: Equipment must be classified and marked:
• preferably with the temperature class (T classification) defined by the surface temperature or,
• limited to the specified flammable gases or dusts for which it is approved, if necessary (and marked accordingly).

Temperature ClassMaximum Surface Temperature (°C)Ignition Temperature¹ (°C)
T1450> 450
T2300> 300
T3200> 200
T4135> 135
T5100> 100
T685> 85

 

Classification of dusts into explosion groups (according to the fifth edition, IEC 60079-0)

Group III: Electrical equipment intended for use in places with an explosive dust atmosphere other than mines susceptible to firedamp.

Group III is subdivided into IIIA (combustible flyings), IIIB (non-conductive dust) and IIIC (conductive dust)

Combustible dust: Finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, may burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures.

Non-conductive dust: Combustible dust with electrical resistivity greater than 103 Ωm

Conductive dust: Combustible dust with electrical resistivity equal to or less than 103 Ωm

Combustible DustIgnition Temperature¹ (°C) Self-ignition Temperature of Dust Layers¹ (°C)
Aluminium530280
Cotton560350
Cereals420290
Magnesium610410
Soybean500245
Sulphur280280
Starch440290
Tobacco450300

(¹) The maximum surface temperature must be identified and suitable for the specified type of dust present (equipment marked for hazardous area zone 21). In order to prevent the ignition of dusty atmospheres, the maximum surface temperature needs to be limited. It must not exceed:
– 2/3 of the auto-ignition temperature of the specified cloud of dust,
– the auto-ignition temperature of a 5 mm layer of dust minus 75°C.

ELECTRICAL & PROCESS INSTRUMENTATION EQUIPMENT

FOR EXPLOSIVE ATMOSPHERES

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