Explosion proof equipment in hazardous areas
A Guide to the Selection of Electrical Equipment for Installation in Hazardous Areas
The information below is given as a guide to enable
the users of this catalogue to select the correct hazardous area equipment
for their application. It should not be used to define the nature of hazardous
areas, this is a task for experienced professionals in the field of hazardous
area definition. Nor should it be used as replacement for the very detailed
standards which cover the installation of electrical equipment in hazardous
areas.
Hazardous Atmospheres
What conditions are required for an explosion?
There are three elements required in order to create
a hazardous area.
1. Air.
2. A flammable medium for example gases, vapours, mists and dusts.
3. A source of ignition of sufficient energy and /or a rise in temperature.
What is an explosive atmosphere?
An explosive atmosphere is created when air and
an flammable medium mix such that a rise in temperature or an electrical
spark produces an explosion.
What is a potentially explosive atmosphere?
An atmosphere is defined as potentially explosive
when in its normal state it is not explosive but there exists the possibility
that in abnormal circumstances it will become explosive. For example in
the case of a gas leak or the vapourisation of a fluid leak.
Ignition Energy
An explosive atmosphere needs a minimum energy for ignition. The energy
produced by arcs and sparks are many times greater than this minimum.
Self Ignition Temperature
An explosive atmosphere can self ignite if the local
temperature rises above the so-called self ignition temperature. This
temperature is different for different flammable mediums. Self ignition
temperatures for a range of commonly found flammable gasses and dust are
given in Table 1 and 2.
| Table 1
Typical self ignition temperatures for Gases |
|
| Gas | Temp + °C |
| Butane | 287 |
| Ethyl acetate | 425 |
| Acetylene | 300 |
| Ethylene | 450 |
| Carbon Disulphide | 90 |
| Hydrogen | 500 |
| Acetone | 465 |
| Ammonia | 650 |
| Ethyle nitrate | 90 |
| Benzne | 498 |
| Industrial Methane | 535 |
| Carbon Monoxide | 605 |
| Table 2 Typical self ignition temperatures for Dust |
||
| Dust | Temp + °C | |
| Cloud | Layer | |
| Cellulose | 490 | 430 |
| Cocoa | 500 | 200 |
| Flour | 490 | 430 |
| Lead | 460 | 240 |
| Lignite | 380 | 225 |
| polyacrylonitrate | 540 | 400 |
| Soya Meal | 540 | 340 |
| Zinc | 570 | 440 |
The classification of hazardous areas
Gases, Vapours, Mists and Dusts.
Explosive atmospheres are caused by a mixture of gases, vapours, mists dusts and air. If the explosive atmosphere is caused by gases, vapours and mists the hazardous area is coded G. If the explosive atmosphere is caused by dusts the hazardous area is coded D.
Explosion Groups
Explosion protected electrical equipment is classified
in terms of the ignition temperature, the ignition capability and flame
transmission capacity of the explosive atmosphere in which it is designed
to operate.
In the first instance equipment is divided into two groups dependingon
its location above or below ground.
Group I Electrical
equipment for use below ground
in areas susceptible to firedamp.
Group II Electrical
equipment for all other areas.
Group II equipment
is further divided into three sub groups depending on the ignition and
flame transmission characteristics of the explosive hazard.
These sub-groups are designated IIA, IIB and IIC.Zones
| Table 3
Typical self ignition temperatures for Dusts |
|||
| I | IIA | IIB | IIC |
| Methane | Acetone | Town gas | Hydrogen |
| Benzol | Ethylene | Acetylene | |
| Acetic acid | Carbon disulphide | ||
| Methanol | |||
| Petrol | |||
| Diesel | |||
| Aviation fuel | |||
| Carbon monoxide | |||
| Heating oils | |||
| AcetoneAcetaldehyde | |||
The probability of an explosive atmosphere occurring.
Zone 0 defines
an area in which an explosive atmosphere caused by a mixture of air and
gases, vapours and mists is present continuously for long periods or very
frequently.
Zone 1 defines
areas in which an explosive atmosphere caused by a mixture air and gases,
vapours and mists is expected to occur occasionally.
Zone 2 defines
areas in which an explosive atmosphere caused by a mixture of air and
gases, vapours and mists is unlikely and if it should occur it will be
for a short time and then only rarely.
Zone 20 defines
an area in which an explosive atmosphere caused by a cloud of combustible
dust and air is present continuously for long periods or very frequently.
Zone 21 defines
areas in which an explosive atmosphere caused by a cloud of combustible
dust and air is expected to occur occasionally.
Zone 22 defines
areas in which an explosive atmosphere caused by a cloud of combustible
dust and air is unlikely and if it should occur it will be for a short
time and then only rarely.
| Table 4 Defenition of zones |
||||||
| Hazard Probability | Continuous | Infrequent | Occasional | |||
| Zones | 0 | 20 | 1 | 21 | 2 | 22 |
| Type of hazard | G | D | G | D | G | D |
Ignition Temperature
The ignition temperature is the lowest temperature
on the surface at which an explosive atmosphere will ignite. Gases and
dusts have been divided into temperature classes according to their ignition
temperatures so that electrical equipment can be further sub-divided into
six temperature classes T1 to T6. The temperature classes are shown in
Table 5.
Note. The maximum operating surface temperature of a piece of electrical
equipment must always be below the ignition temperature of the explosive
mixture, see Table 6 below.
| Table 5 Temperature Classes |
|
| Temperature Class | Maximum Surface Temperature °C |
| T1 | 450 |
| T2 | 300 |
| T3 | 200 |
| T4 | 135 |
| T5 | 100 |
| T6 | 85 |
| Table 6 Self Ignition Temperature and Temparature Classes |
||||||
| Self Ignition Temperature °C | Temperature Class of the Equipment | |||||
| T6 | T5 | T4 | T3 | T2 | T1 | |
| (85°) | (100°) | (135°) | (200°) | (300°) | (450°) | |
| 85° ≤ T ≤ 100 °c | ||||||
| 100° ≤ T ≤ 135 °c | ||||||
| 135° ≤ T ≤ 200 °c | ||||||
| 200° ≤ T ≤ 3 00 °c | ||||||
| 300° ≤ T≤ 450 °c | ||||||
| 450° ≤ | ||||||
| = EXPLOSION | ||||||
Hazardous Area Equipment Categories.
Electrical equipment for hazardous areas is designed to comply with different fault levels depending on the Zone in which the equipment is to be installed. These fault levels are designated as Categories 1, 2 and 3. Their relationship with the zones, the potential for an explosive atmosphere and the type of explosive atmosphere is shown in Table 7.
| Table 7 Relationship between zones and equipment categories |
|||||||
| Zones | 0 | 20 | 1 | 21 | 2 | 22 | |
| Type of hazard | G | D | G | D | G | D | |
| Categories | 1 | 2 | 3 | ||||
Ingress Protection
Because hazardous area electrical equipment is very often used in hostile atmospheric environments the standards call for minimum levels of protection against water and dust. For example the minimum level set is IP54 however in the case of dust hazards the minimum level ingress protection is set at IP64. The range of IP levels are shown in Table 8.
| Table
8 IP Rating IPXY |
|||
| X Protected From | Y Protected From | ||
| 0 | None | 0 | None |
| 1 | Solids < 50mm diam | 1 | Water dripping vertically |
| 2 | Solids < 12.5mm diam | 2 | Water dripping at 15 deg |
| 3 | Solids < 2.5mm diam | 3 | Spray water at 60 deg |
| 4 | Solids < 1mm diam | 4 | Splash water from any direction |
| 5 | Dust protected | 5 | Jet water from any direction |
| 6 | Dust tight | 6 | Powerful water jets from any direction |
| 7 | Water when submerged for limited time | ||
| 8 | Water when submerged for an indeterminate time | ||
Reading the Label
The ATEX standards call for
the manufacturers of hazardous area electrical equipment to label their
products with the following information. A typical label for a flood light
is shown for reference.
1. Name of manufacturer
2. Product type code
3. Address of manufacturer
4. Year of manufacture
5. Marking in accordance with directive 94/9/EC II 3 GD – group II location,
category 3 equipment, for gas and dust hazards
6. Marking in accordance with European standards EEx nR II T – built and
tested to European standards, type of protection, explosion group, temperature
class
7. CE marking together with notified body registration number
8. Notified testing body and reference number
9. Serial number
10. Technical data
11. Ingress protection
Hazardous area equipment from Walsall Ltd - leading supplier of explosion proof products
We specialise in the manufacture and supply of the following products for hazardous areas:
- Hazardous area protected equipment
- Hazardous area electrical equipment
- Hazardous area manufacturer
- Hazardous area junction box
- Hazardous area enclosures
- Hazardous area lighting and lamps
- Hazardous area control panels
- Hazardous area control stations
- Hazardous area sounders
- Hazardous area power switches and switching
- Hazardous area beacon
- Hazardous area starter, motor, dol
- Hazardous area stopping plug
- Hazardous area breather drains
- Hazardous area cable glands
