ATEX... explained

 


 

ATEX Coding

 

II 2 GD

 

Equipment Group

Equipment Category

 

 

I - Mining

M1 - energised

M2 - de-energised

 

Gas (G)

Dust (D)

II - Non-mining

1 - Very high protection

0

20

2 - High protection

1

21

3 - Normal protection

2

22

 

 

Zones (vapours & gases)

 

If you look at the size of a refinery or chemical factory and the amount of liquids and gases that circulate the various processes in that plant there must be a certain amount of risk of leaks and other hazards. In some cases the gas, vapour or dust is present all the time or for long periods. Refineries and chemical complexes should thus be divided into areas of risk of release of gas, vapour or dust known as zones. The type and size of these hazardous areas is determined using area classification.

 

Typical gas hazards are from hydrocarbon compounds.

 

Safe area

A domestic domain such as a house would be classed as safe area where the only risk of a release of explosive or flammable gas would be 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 (although on rare occasions gas explosions in domestic property are known to occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 0.25% of its lower explosive concentration limit.

Zone 2 area

This is a step up from the safe area. In this case it has been decided that in this zone the gas, vapour or dust would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 00.1% of the time. Explosion safety compliant equipment, should be used.

Zone 1 area

These areas are where special or classified electrical equipment must be used. It is expected that the gas, vapour or dust will be present or expected for long periods of time under normal running. As a guide this can be defined as 101000 hours/year or 0.110% of the time. Explosion safety equipment that has a higher safety level than Zone 2 equipment must be used.

Zone 0 area

This is the worst scenario as gas or vapour is present all of the time (over 1000 hours/year or >10% of the time). Although this is the worst case it is very rare that a zone 0 area will be in the open. Usually this would be the vapour space above the liquid in the top of a tank or drum.

 

 

 

Zones (dusts)

 

In the case of dusts there is still a chance of explosion. 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 1999/92/EU 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 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.

Zone 21

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22

A place in which an explosive atmosphere in the form of a cloud of combustible dust is not likely to occur in normal operation, but if it does occur will persist for a short period only

 

Guidance on assessing the extent of zones is given in EN61241-10.

 

 

 

Electrical Protection for Gases & Dust

 

Gas

Dust

 

Gas

Dust

Ex d

 

Flameproof

1

 

 

Ex ta

 

Enclosure

 

 

20

 

Ex tb

 

21

 

Ex tc

 

22

Ex pxb

 

Ex p

 

 

Pressurised

 

1

 

21/22

 

Ex pyb

1

Ex pzc

2

Ex q

 

Powder Filled

1

 

Ex o

 

Oil Filled

1

 

Ex e

 

Increased Safety

1

 

Ex ia

 

Intrinsic Safety

 

0

20

Ex ib

1

21

Ex ic

2

22

Ex nA

 

Non-sparking

2

 

Ex nL

 

Energy limited

 

Ex nR

 

Restricted breathing

 

Ex nC

 

Enclosed break

 

Ex ma

 

Encapsulation

 

0

20

Ex mb

1

21

Ex mc

2

22

 

 

Gas groups

 

Each chemical gas or vapour used on the refinery or chemical works comes under a certain gas group. For this industry there are three:

 

Methane (Mining only)

Representative gas for group I

Propane

Representative gas for group IIA

Ethylene

Representative gas for group IIB

Hydrogen/Acetylene

Representative gas for group IIC

 

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 vapour 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 vapour 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.

 

 

Dust groups

 

Combustible Flyings

Representative dust for group IIIA

Non-conductive Dust

Representative dust for group IIIB

Conductive Dust

Representative dust for group IIIC

 

 

Temperature classification

 

Another important consideration is the temperature classification of the electrical equipment. One thing that must never be allowed to happen is the surface temperature of the electrical equipment rise beyond the auto-ignition temperature of the gas or vapour that it is put into.

 

The temperature classification on the electrical equipment label will be one of the following (in degrees Celsius):

 

America C

UK C

Germany C
Continuous - Short Time

T1 - 450

T3A - 180

T1 - 450

G1: 360 - 400

T2 - 300

T3B - 165

T2 - 300

G2: 240 - 270

T2A - 280

T3C - 160

T3 - 200

G3: 160 - 180

T2B - 260

T4 - 135

T4 - 135

G4: 110 - 125

T2C - 230

T4A - 120

T5 - 100

G5: 80 - 90

T2D - 215

T5 - 100

T6 - 85

T3 - 200

T6 - 85

 

 

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 200C at an ambient temperature of 40C. T5 will not exceed 100C, etc.

 

 

 

Auto-ignition temperatures (vapours & gases)

 

The auto-ignition temperature of a liquid, gas or vapour is the temperature which the substance will ignite automatically by itself without any external heat source. Such temperatures for common substances are:

 

Hydrogen

560 C

Ethylene

425 C

Acetylene

305 C

Naphtha

290 C

Carbon Disulfide

102 C

 

 

 

Auto-ignition temperatures (dust)

 

The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:

 

Sugar

460 C

Wood

340 C

Flour

340 C

Grain dust

300 C

Tea

300 C

 

 

 

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 below.

 

Protection

Ex Code

Notified Body Name

Description

Location

Use

Flameproof

d

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment is robust can stand an explosion from within, without transmitting the flame to the outside
 

Equipment has flameproof gaps (max 0.006" propane/ethylene, 0.004" acetylene/hydrogen)

Zone 1 if gas group & temp. class correct

Motors, lighting, junction boxes

Increased Safety

 

e

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment is very robust and components are made to a high quality

Motors, lighting, junction boxes

 

Oil Filled

o

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment components are completely covered with a layer of oil

Zone 2 or Zone 1, depending on edition of the standard used

Heavy current equipment

Sand/Powder/

Quartz Filled

q

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment components are completely covered with a layer of Sand, powder or quartz

Zone 2

Electronics, telephones, chokes

Encapsulated

m

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment components of the equipment are usually encased in a resin type material

Zone 1 (Ex mb) or Zone 0 (Ex ma)

Electronics (no heat)

Pressurised/

purged

p

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Equipment is pressurised with a positive pressure; gas cannot get in for air coming out or equipment is purged with a diluting gas such as air. If air is used, it is ducted in from outside the hazardous area

Zone 1

Analysers, motors, control boxes, computers

Intrinsically safe

i

Will have a CENELEC Hexagon followed by apparatus group and Safety Category

Any arcs or sparks in this equipment has insufficient energy (heat) to ignite a vapour
 

Equipment can be installed in ANY housing provided to IP54
A 'Zener Barrier' or 'opto isol' or 'galvanic' unit may be used to assist with certification.

'ia': Zone 0 & 1
'ib': Zone 1

Instrumentation, measurement, control

Non Incendive

n

Now CENELEC recognised; so will have a hexagon followed by apparatus group and Safety Category

Equipment is non-incendive or non-sparking

Zone 2

Motors, lighting, junction boxes, electronic equipment

Special Protection

S

Has a BASEEFA Crown

Not CENELEC recognised; no hexagon

Categorised 's' because it does not fit into any other protection type. This is an old fashioned protection method, the modern method does not include the 's' coding and certifies the product directly against the safety requirements of the European Directive 94/9/EC

Zone 1 depending upon Manufacturers Certification

As its certification states

 

 

 

Dual Protection

 

Many items of equipment use dual protection, so a label could be 'de' and you might find that the enclosure of the equipment is made to EEx 'e' and any switches that are a part of the equipment are EEx 'd'. An example of this would be a mains socket outlet.

 

 

Disclaimer:

 

The above information is offered only as guide in choosing a suitable product.  West Controls Ltd. accept no responsibility for the validity of the information above, or any work area assessment.  We will however ensure that any products offered by us meet the requirement of the application as specified.

 

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