Hazardous areas

These are defined by three main criteria:

• The type of hazard

• The likelihood of the hazard being present in flammable concentrations

• The (auto) ignition temperature of the hazardous material

 

The type of hazard (Groups)

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

 

Gases and Vapours

Gases and vapours are categorized in terms of their ignition energy or the maximum experimental safe gap (in respect of flameproof protection). This categorization leads to the Gas Groups:

 

Mining	Surface Industry
Group I	Group II
Methane	IIA	IIB	IIC
	Propane	Ethylene	Hydrogen

 

(The gases noted in the table are typical gases for each group.)

Group IIC is the most severe group. Hazards in this group can be ignited very easily indeed.

 

Dusts and Fibres (Group III)

Dusts and fibres are also defined in terms of their ignition properties including dust cloud ignition properties.

 

 

The likelihood of the hazard being present in flammable concentrations (Zones)

The likelihood of the hazard being present in flammable concentrations will vary from place to place. A location very close to an open source of hazard will have a high likelihood of a flammable atmosphere. On the other hand, outside a flanged pipe containing a flammable liquid, the likelihood of a flammable atmosphere being present is much lower since it will only occur if the flange leaks. Rather than work with an infinite range of possibilities, three zones are defined.

 

 

Gases and Vapours

There are three zones for gases and vapours:

 

Zone 0	Flammable atmosphere highly likely to be present - may be present for long periods or even continuously
Zone 1	Flammable atmosphere possible but unlikely to be present for long periods
Zone 2	Flammable atmosphere unlikely to be present except for short periods of time - typically as a result of a process fault condition.

 

Zone zero is the most severe zone (the highest probability of flammable atmosphere presence). Equipment for this zone needs to be very well protected against providing a source of ignition.

 

Dusts

There are three zones for dusts:

 

Zone 20	Dust cloud likely to be present continuously or for long periods
Zone 21	Dust cloud likely to be present occasionally in normal operation
Zone 22	Dust cloud unlikely to occur in normal operation, but if it does, will only exist for a short period

 

(The presence of dust layers does not automatically lead to the dust zone. The likelihood of the dust layer being disturbed to create a cloud needs to be considered. Dust layers also need careful consideration in terms of ignition temperature. Because the dust layer can make the equipment under it hotter then normal, a factor of safety is applied to the layer ignition temperature.)

 

 

The (auto) ignition temperature of the hazardous material (Temperature Classes)

As well as considering the protection against electrical arcs and sparks igniting a flammable atmosphere, consideration needs to be given to the surface temperature of equipment. (Most electrical apparatus dissipates some heat!) Flammable materials are categorized according to their ignition temperature. Again, rather than work with an infinite range, six temperature classes are defined as follows:

 

T-Class	Hazards which will not ignite at temperatures below:
T1	450°C
T2	300°C
T3	200°C
T4	135°C
T5	100°C
T6	85°C

 

The bigger the T-number the lower is the ignition temperature of the hazard. So equipment to be installed in hazardous areas where the hazard has temperature class T5 or T6 must be ensured not to get hot. Electrical equipment is marked with a T-Class. The T-Class on the equipment states that the equipment will not provide an ignition capable surface hotter than the temperature represented by the stated T-Class. For equipment in hazardous areas, the bigger the T-Number on the equipment the less hot it will get.

 

 

Protecting Electrical Apparatus to make it suitable for use in hazardous areas

Electrical apparatus for use in hazardous areas needs to be designed and constructed in such a way that it will not provide a source of ignition. There are ten recognized types of protection for hazardous area electrical apparatus. Each type of protection achieves its safety from ignition in different ways and not all are equally safe. In addition to the equipment being suitable for the Gas Group and the Temperature Class required, the type of protection must be suitable for the zone in which it is to be installed. The different types of protection and the zones for which they are suitable are as follows:

 

Equipment Code	Description	Suitable for zones...
Ex ia	Intrinsic safety 'ia'	0, 1, 2
Ex ib	Intrinsic safety 'ib'	1,2
Ex ic	Intrinsic Safety 'ic'	2
Ex d	Flameproof protection	1,2
Ex p	Purge/pressurized protection	1,2
Ex px	Purge/pressurized protection 'px'	1,2
Ex py	Purge/pressurized protection 'py'	1,2
Ex pz	Purge/pressurized protection 'pz'	2
Ex e	Increased safety	1,2
Ex m	Encapsulation	1,2
Ex ma	Encapsulation	0,1,2
Ex mb	Encapsulation	1.2
Ex o	Oil immersion	1,2
Ex q	Sand / powder (quartz) filling	1,2
Ex n	Type - n protection	2
Ex s	Special protection	Normally 1 and 2

 

Equipment complying with European (CENELEC) standards will bear the code EEx..

 

 

Standards

There are various standards which give details of hazardous area requirements. In the main, the IEC standards are now parallel voted by CENELEC (Europe).

 

Top level standard

EN1127-1 Explosive atmospheres: Explosion prevention and protection. Part 1: Basic concepts and methodology

 

Standards for Equipment

IEC 60079- ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES

 

Part 0 General requirements

Part 1 Construction and test of flameproof enclosures of electrical apparatus

Part 2 Electrical apparatus - type of protection ‘p’

Part 3 Spark test apparatus for intrinsically safe circuits

Part 4 Method of test for ignition temperature

Part 5 Powder filling ‘q’

Part 6 Oil-immersion ‘o’

Part 7 Increased safety ‘e'

Part 11 Construction and test of intrinsically safe and associated apparatus

Part 15 Electrical apparatus for explosive gas atmospheres, type of protection ‘n’

Part 18 Encapsulation ‘m’

Part 25 Intrinsically safe systems

Part 26 Special requirements for construction test and marking of zone 0 electrical apparatus

Part 27 Fieldbus intrinsically safe concept (FISCO)

 

 

Standards for Installation etc. (gases and vapours)

 

IEC 60079- ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES

 

Part 10 Classification of hazardous areas

Part 14 Installation (and selection) of equipment in hazardous areas

Part 17 Inspection and maintenance of equipment

 

 

Standards for Dust Hazard Equipment and Installation

IEC 61241 ELECTRICAL APPARATUS FOR USE IN THE PRESENCE OF COMBUSTIBLE DUST

Part 0 General Requirements

Part 1 Protection by enclosure ‘tD’

Part 2-1 Methods for determining the minimum ignition temperature of dusts

Part 2-2 Method for determining the resistivity of dust in layers

Part 2-3 Method for determining minimum ignition energy of dust air mixtures

Part 4 Type of protection ‘pD’

Part 10 Classification of areas where combustible dusts are or may be present

Part 11 Protection by intrinsic safety ‘iD’

Part 14 Selection and installation

Part 17 Inspection and maintenance

Part 18 Protection by encapsulation ‘mD’