Hazardous Area Zones, Definitions & Explosion Protection Technical Guide

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The Redapt Hazardous Area Technical Guide provides an overview for the design and use of equipment used within potentially explosive atmospheres created by gases, vapours and ignitable dusts or fibres, commonly referred to as hazardous area zones.

Typically this includes such installations as oil/gas rigs, processing refineries, chemical production facilities, flammable liquids storage facilities, fuel transportation, petrol stations, paint production, paper production etc, however this list is not exhaustive and many new areas may be re-classified as hazardous. A “hazardous area” is defined as an area in which the atmosphere contains, or may contain in sufficient quantities, flammable or explosive gases, dusts or vapours. In such an atmosphere a fire or explosion is possible when three basic conditions are met. This is often referred to as the “hazardous area” or “combustion” triangle.

Hazardous Areas

In order to protect installations from a potential explosion a method of analysing and classifying a potentially hazardous area is required. The purpose of this is to ensure the correct selection and installation of equipment to ultimately prevent an explosion and to ensure safety of life. The methods used to classify an installation can vary depending upon which part of the world it is located, but generally there are two main types of classification. In countries that have adopted the IEC (International Electrotechnical Commission) philosophy this is referred to as Zoning whilst in North American installations are classified by Classes, Divisions and Groups to ascertain the level of safety required.

Hazardous Area Classification

IEC and ATEX Standards

A Hazardous Area is defined by three main criteria, these being:

  • The type of hazard (groups)
  • The auto-ignition temperature of the hazardous material (temperature or “T” rating)
  • The likelihood of the hazard being present in flammable concentrations (zones)

The Type of Hazard

The type of hazard will be in the form of either a gas or vapour or a dust or fibre.

The classification of these hazardous zones is primarily divided into two groups depending on whether it is in a mining or above surface industry. These are defined below:

Group I -electrical equipment for use in mines and underground installations susceptible to firedamp Group II and Group III -electrical equipment for use in surface installations
Groups II & III are further sub-divided depending upon the hazard. Group II gases are grouped together based upon the amount of energy required to ignite the most explosive mixture of the gas with air. Group III dusts are subdivided according to the nature of the explosive atmosphere for which it is intended.

MiningSurface Industry
Group IGroup IIGroup III
Electrical equipment for mines susceptible to firedampElectrical equipment for places with an explosive gas atmosphereElectrical equipment for places with an explosive dust atmosphere
Sub-DivisionIgnition EnergySub-DivisionExplosive Atmosphere
IIA260 MicrojoulesIIIACombustible flyings
IIB95 MicrojoulesIIIBNon-conductive dust
IIC18 MicrojoulesIIICConductive dust

Auto Ignition Temperature or “T” Rating

The hazard level of the gases increases from gas group IIA to IIC with group IIC being the most severe. Substances in this group can be ignited very easily with Hydrogen being the most at risk to ignition. The temperature class or T-Class Rating is based on the auto-ignition temperature of the gas, details of which are given below.

Apparatus Groups and Temperature Classes for Common Flammable Gases and Vapours – Group II

Gas GroupTemperature Class
T1T2T3T4T5T6
IMethane
IIAAcetoneEthanolDiesel FuelAcetaldehyde
MethaneCyclohexaneAircraft Fuel
EthanePropanol 2Fuel Oil
BenzeneN-Butyl AlcoholN-Hexane
MethanolN-ButaneHeptane
TolueneKerosene
Propane
Acetic Acid
Ammonia
IIB



Coal GasEthylene GlycolEthyl Methyl Ether
Ethylene OxideHydrogen Sulphide
Propanol 1Tetrahydrofuran
Methyl Ethyl Ketone
IICHydrogenAcetylene

Temperature Class Rating for Group II Electrical Apparatus

Group II Temperature Class
T CodeMaximum Surface TemperatureIgnition Temperature
T1450ºC > 450ºC
T2300ºC> 300ºC ≤ 450ºC
T3200ºC> 200ºC ≤ 300ºC
T4130ºC> 130ºC ≤ 200ºC
T5100ºC> 100ºC ≤ 135ºC
T685ºC > 85ºC ≤ 100ºC

 

If a hazard is present the equipment used within the installation must be given an appropriate “T” classification in order to maintain the integrity. If that hazardous gas is, say, hydrogen, then all equipment used must meet the “T6” rating. This means that all hazardous area equipment used must not have a surface temperature of greater than 85°C. Any hazardous area equipment used that can generate a hotter surface temperature of greater than 85°C must not be used as this will then increase the likelihood of an explosion by igniting the hydrogen in the atmosphere.

Apparatus Groups and Temperature Classes for Common Flammable Dusts and Fibres – Group III

When considering installations that are at risk of a potential explosion due to dust ignition, the equipment used is classified in much the same way as with gases. No equipment should be installed where the surface temperature of the equipment is greater than the ignition temperature of the given hazard. Below are some common dust hazardous and their minimum ignition temperature.

Ignition Temperatures for Common Flammable Dusts and Fibers
MaterialIgnition Temperature
CloudLayer
Coal Dust380°C225°C
Polythene420°C(melts)
Methyl Cellulose420°C320°C
Starch460°C435°C
Flour490°C340°C
Sugar490°C460°C
Grain Dust510°C300°C
Phenolic Resin530°C> 450°C
Aluminium590°C> 450°C
PVC700°C> 450°C
Soot810°C570°C

 

The Likelihood of the Hazard

The likelihood of the hazard being present in a concentration high enough to cause an ignition will vary from location to location. For most installations this risk is dependant upon how long the hazard (gas, vapour, dust or fibre) is present. In order to classify this danger an installation is divided into areas of risk depending upon the amount of time the hazardous is present. These areas are referred to as Zones. For gases and vapours and dusts and fibres there are three zones.

Zone 0 | Zone 1 | Zone 2

GasDustHazardous Area Zone Characteristics
Zone 0Zone 20A hazardous atmosphere is highly likely to be present and may be present for long periods of time (>1000 hours per year) or even continuously
Zone 1Zone 21A hazardous atmosphere is possible but unlikely to be present for long periods of time (>10 <1000 hours per year)
Zone 2Zone 22A hazardous atmosphere is not likely to be present in normal operation or infrequently and for short periods of time (<10 hours per year)

Explosion Protection Concepts

There are varying types of equipment that can be used within these hazardous area zones to ensure that the potential for an explosion is removed or greatly reduced. This equipment must be designed and manufactured in accordance with particular construction parameters known as protection concepts. Essentially these concepts fall under four main methods. These methods are detailed below along with a brief description of some of the concepts:

Type of Explosion Protection MethodEquipment CodeDescriptionInternational StandardSuitable for Zones
Intended to prevent a potential ignition arisingEx eIncreased safetyIEC 60079-71, 2
Ex nAType -n protectionIEC 60079-152
Intended to limit the ignition energy of the equipmentEx iaIntrinsic safety ‘ia’IEC 60079-110, 1, 2
Ex ibIntrinsic safety ‘ib’IEC 60079-111, 2
Ex icIntrinsic Safety ‘ic’IEC 60079-112
Ex nLType -n protectionIEC 60079-152
Intended to prevent the explosive atmosphere contacting the ignition sourceEx pPurge/pressurized protectionIEC 60079-21, 2
Ex pxPurge/pressurized protection ‘px’IEC 60079-21, 2
Ex pyPurge/pressurized protection ‘py’IEC 60079-21, 2
Ex pzPurge/pressurized protection ‘pz’IEC 60079-22
Ex mEncapsulationIEC 60079-181, 2
Ex maEncapsulationIEC 60079-180, 1, 2
Ex mbEncapsulationIEC 60079-181, 2
Ex oOil immersionIEC 60079-181, 2
Ex nRType -n protectionIEC 60079-152
Intended to prevent an ignition from escaping outside the equipmentEx dFlameproof protectionIEC 60079-11, 2
Ex qSand / powder (quartz) fi llingIEC 60079-51, 2
Ex nCType -n protectionIEC 60079-152
SpecialEx sSpecial protectionSee IEC 60079-00, 1, 2

 

Ex i Intrinsic Safety

Ex i is an explosion protection concept in which the electrical energy within the equipment is restricted to a level which is below that what may cause an ignition or to limit the heating of the surface of the hazardous area equipment. There are two main sub types to Ex i protection, these being “ia” and “ib”. Type “ia” protection allows for the occurrence of two faults during operation Type “ib” explosion protection allows for the occurrence of one fault during operation.

Ex d Flameproof

The equipment that may cause an explosion is contained within an enclosure which can withstand the force of an explosion and prevent transmission to the outside hazardous atmosphere. The Ex d flameproof method of explosion protection also prevents the hazardous atmosphere from entering the enclosure and coming into contact with equipment.

Ex m Encapsulation

Ex m is an explosion protection concept where by equipment that could potentially cause an ignition is encapsulated within a compound or resin so as to prevent contact with the explosive atmosphere.  The concept also limits the surface temperature of the equipment under normal operating conditions.

Ex e Increased Safety

Ex e is an explosion protection concept applied to the installation to ensure increased security against the possibility of excessive temperatures and sparks from hazardous area electrical equipment. Equipment that normally causes sparks is excluded from use within this method of protection.

Ex p Pressurised

One process ensures that the pressure inside an Ex p enclosure is sufficient to prevent the entrance of a flammable gas, vapour, dust, or fibre and prevent a possible ignition. Another process maintains a constant flow  of air (or an inert gas) to dilute to take away any potentially explosive atmosphere.

Ex o Oil Immersion

All equipment that has the potential to arc and potentially cause an ignition is immersed in a protective liquid or oil. The oil provides an insulating method to prevent ignition.

Ex q Powder Filling

All equipment that has the potential to arc is contained within an enclosure filled with quartz or glass powder particles. The powder filling prevents the possibility of an ignition.

Ex n Non-Sparking

A type of explosion protection where precautions are taken so that hazardous area electrical equipment that has the potential to arc is not capable of igniting a surrounding explosive atmosphere. This can be further categorised as follows:

Ex nA -Where components used in construction are non-sparking
Ex nC -Where components used in construction are non-incendive
Ex nR – Where components used are tightly enclosed to restrict the breathing and prevent ignition
Ex nL -Where components used in construction do not contain enough energy to cause an ignition

Ex s Special
This method of explosion protection, as its name indicates, has no specific parameters or construction rules. In essence it is any method of explosion protection which can provide a pre-determined level of safety to ensure that there is no potential for an ignition. As such it does not fall under any specific protection method and may in fact be a combination of more than one.

Ingress Protection
Another consideration in the protection of equipment in hazardous areas is the safeguarding against the ingress of solid foreign objects and water. This is known as the degree of ingress protection and is commonly referred to as the IP Code. The relevant standard for the degree of ingress protection is IEC 60529. An overview of the IP code as defined in the standard is detailed below:

1st NumeralDegree of Protection2nd NumeralDegree of Protection
0No protection at all against solid objects0No protection at all against the ingress of water
1Protection against solid objects greater than 50 mm in diameter1Protected against falling water drops
2Protection against solid objects greater than 12.5 mm in diameter2Protected against falling water drops at an angle of up to 15°
3Protection against solid objects greater than 2.5 mm in diameter3Protected against sprayed water at an angle of up to 60°
4Protection against solid objects greater than 1.0 mm in diameter4Protected against the splashing of water from any direction
5Protected against the ingress of dust in such an amount that it will not interfere with the operation of the equipment5Protected against water jets from any direction
6Total protection against the ingress of any dust6Protected against powerful water jets from any direction
7Protected against the ingress of water when temporary  immersed between 0.15 m and 1 m
8Protected against the ingress of water when continuously
immersed to a specified depth

 

Operational Temperatures
All equipment used within hazardous areas has an operational temperature band or limit. This is often referred to as the “Tamb” and defines the upper and lower ambient temperatures of which the equipment is approved for use in. As defi ned in IEC 60079-0 the standard limits are – 20°C to +40°C. Where the operation temperatures of the equipment fall between these parameters no additional marking is required. However, if they are outside these parameters than the specific temperatures need to be identified.

Marking of Hazardous Area Equipment
All equipment for use in hazardous areas should be marked as prescribed in 60079-0. As a general rule this includes, where appropriate, such information as:
– Company/Manufacturers name and address
– Hazardous area certificate number(s)
– Protection concept – Gas group(s)
– Temperature class
– Ambient temperature range
– Product identification
– Serial number and year of manufacture
– Electrical parameters
– CE marking and ATEX notified Body ID number
– ATEX coding – IP code

HazarDous area Classification

North American Standards

In North American installations, hazardous areas are defined by classes, divisions, and groups to classify the level of safety required for equipment installed in these locations.

Classes define the type of hazard in terms of whether it is a gas or vapour, a combustible or conductive dust or an ignitable fibre or flying. Divisions define the probability of the presence of the hazard being present during normal or abnormal conditions. Groups classify the exact type and nature of the hazardous substance.

An overview of this classification system can be defined as follows:

Classes

ClassDefinition
IA location in which a flammable gas or vapour is or may be present in sufficient quantity to cause an explosive atmosphere
IIA location in which a conductive or combustible dust is or may be present in sufficient quantity to cause a fi re or an explosive hazard
IIIA location in which easily ignitable fibres or flyings are present in sufficient quantity to present a serious risk of fire

 

Classes

ClassDefinition
1The defined hazard is present during normal operational conditions
2The defined hazard is present only during abnormal conditions such as equipment failure

 

Groups

ATEX

 

ClassRepresentative Hazard
AAcetylene
BHydrogen
Ethylene
DPropane
EMetal Dust
FCoal Dust
GGrain Dust

 

Marking of Hazardous Location Equipment

The marking of hazardous location equipment varies in accordance with the relevant Electrical Code. As a general rule equipment should be marked as follows:
– Company/Manufacturers name
– Class I, II and/or III
– Division 1 and/or 2
– Group A, B, C, D, E, F and/or G
– Approving NRTL (Nationally Recognized Testing Laboratory) logo
– Product identification
– Serial number
– Other relevant safety information
– Enclosure Type Rating

Basic UL / CSA / NEMA Enclosure Types

Type 3
An enclosure which is intended for outdoor use primarily to provide a degree of protection against windblown dust, rain, sleet and damage from external ice formation
Type 3R
An enclosure which is intended for outdoor use primarily to provide a degree of protection against falling rain and damage from external ice formation
Type 3S
An enclosure which is intended for outdoor use primarily to provide a degree of protection against rain, sleet, windblown dust, and to provide for operation of external mechanisms when ice laden
Type 4
An enclosure which is intended for indoor or outdoor use primarily to provide a degree of protection against windblown rain and dust, splashing water, hose directed water and damage from external ice formation
Type 4X
An enclosure which is intended for indoor or outdoor use primarily to provide a degree of protection against corrosion, windblown rain and dust, splashing water, hose directed water and damage from external ice formation
Type 6
An enclosure which is intended for indoor or outdoor use primarily to provide a degree of protection against falling dirt, hose directed water, the entry of water during occasional temporary submersion at a specified depth and damage from external ice formation
Type 6P
An enclosure which is intended for indoor or outdoor use to primarily to provide a degree of protection against falling dirt, hose directed water and the entry of water during prolonged submersion at a specified depth and damage from external ice formation.

 

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