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ATEX Explosion Protection
00_atex_U1_U4_200606_en.FH10 Tue Jul 11 13:27:40 2006 Seite 1
low-voltage
CONTROLS AND DISTRIBUTION
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July 2006
Explosion Protection (ATEX)
Fundamentals 2
Introduction
The full range of Ex applications under control:
ATEX (atmosphère explosive)
Introduction
In many industries, the manufacture, processing, trans-
port, or storage of combustible materials results in the
creation, or release into the surrounding environment, of
gases, vapors, or mist. Other processes create combustible
dust. An explosive atmosphere can form in conjunction
with the oxygen in the air, resulting in an explosion if igni-
ted.
Particularly in areas such as the chemical and petrochemical
industries, the transport of crude oil and natural gas, the
mining industry, milling (e.g. grain and granular solids) and
many other branches of industry, this can result in serious
injury to personnel and damage to equipment.
To guarantee the highest possible level of safety in these
areas, the legislatures of most countries have developed
appropriate obligations in the form of laws, regulations and
standards. In the course of globalization, it has been possible
to make significant progress towards harmonizing guidelines
for explosion protection.
With the Directive 94/9/EU the European Union creates the
prerequisites for complete standardization because all new
devices must be approved in accordance with this directive
since July 1, 2003.
The brochure "Explosion Protection Fundamentals" is designed
to provide users and interested readers with an overview of
explosion protection in conjunction with electrical equipment
and systems. It also assists in decoding device labels.
However, it does not replace intensive study of the relevant
fundamentals and guidelines when planning and installing
electrical systems. Introduction
3
Index
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Physical principles and parameters . . . . . . . . . . . . . 4
Classification of
explosion-protected equipment . . . . . . . . . . . . . . . 8
Low-voltage product range for hazardous areas . 13
Further information . . . . . . . . . . . . . . . . . . . . . . . . . 20 4
Physical fundamentals and parameters
3
1
2
Integrated explosion protection
Prevent the formation
of potentially
explosive atmospheres
Prevent the ignition
of potentially explosive
atmospheres
Restrict the effects
of an explosion
to a negligible level
EXPLOSION
Combustible
substances
Ignition source
Oxygen
Physical fundamentals and parameters
Explosion
An explosion is the sudden chemical reaction of a combustible
substance with oxygen, involving the release of high energy.
Combustible substances can be present in the form of gases,
mist, vapor, or dust. An explosion can only take place if the
following three factors coincide:
1. Combustible substance
(in the relevant distribution and concentration)
2. Oxygen (in the air)
3. Source of ignition (e.g. electrical spark)
Primary and secondary explosion protection
The principle of integrated explosion protection requires all
explosion protection measures to be carried out in a defined
order.
A distinction is made here between primary and secondary
protective measures.
Primary explosion protection covers all measures that prevent
the formation of a potentially explosive atmosphere.
What protective measures can be taken to ensure that the risk
of an explosion will be minimized? Avoidance of combustible substances Inerting (addition of nitrogen, carbon dioxide, etc.) Limiting of the concentration Improved ventilation
Secondary explosion protection is required if the explosion
hazard cannot be removed or can only be partially removed
using primary explosion protection measures. Physical fundamentals and parameters
5
Air concentration
100 Vol.%
100 Vol.%
0 Vol.%
0 Vol.%
Concentration of combustible substance
Mixture too lean:
No combustion
Mixture too rich:
Partial deflagration
No explosion
Explosion limit
G
Area subject
to explosion
The consideration of technical safety parameters is necessary
for the characterization of potential dangers:
Flash point
The flash point for flammable liquids specifies the lowest
temperature at which a vapor-air mixture forms over the
surface of the liquid that can be ignited by a separate source.
If the flash point of such a flammable liquid is significantly
above the maximum occurring temperatures, a potentially
explosive atmosphere cannot form there. However, the flash
point of a mixture of different liquids can also be lower than
the flash point of the individual components.
In technical regulations, flammable liquids are divided into
four hazard classes:
Explosion limits
Combustible substances form a potentially explosive
atmosphere when they are present within a certain range of
concentration.
If the concentration is too low (lean mixture) and if the
concentration is too high (rich mixture) an explosion does not
take place. Instead slow burning takes place, or no burning at
all.
Only in the area between the upper and the lower explosion
limit does the mixture react explosively if ignited.
The explosion limits depend on the surrounding pressure and
the proportion of oxygen in the air (see the table below).
We refer to a deflagration, explosion, or detonation, depen-
ding on the speed of combustion.
A potentially explosive atmosphere is present if ignition repre-
sents a hazard for personnel or materials.
A potentially explosive atmosphere, even one of low volume,
can result in hazardous explosions in an enclosed space.
Explosion limits of combustible substances
Hazard class
Flash point
AI
< 21 °C
AII
21 °C to 55 °C
AIII
> 55 °C to 100 °C
B
< 21 °C, soluble in water at 15 °C
Substance
designation
Lower explosion
limit [Vol. %]
Upper explosion
limit [Vol. %]
Acetylene
2.3
78.0
(self-decomposing)
Ethylene
2.3
32.4
Petrol
~ 0.6
~ 8
Benzene
1.2
8
Natural gas
4.0 (7.0)
13.0 (17.0)
Heating oil/diesel
~ 0.6
~ 6.5
Methane
4.4
16.5
Propane
1.7
10.9
Carbon bisulfide
0.6
60.0
City gas
4.0 (6.0)
30.0 (40.0)
Hydrogen
4.0
77.0 6
Physical fundamentals and parameters
Physical fundamentals and parameters
Dust
In industrial environments, e.g. in chemical factories or corn
mills, solids are frequently encountered in fine form e.g. as
dust.
Dust is defined in EN 50281-1-2
1)
as "small solid particles in
the atmosphere which are deposited because of their own
weight, but which still remain in the atmosphere for a period
of time as a dust/air mixture". Deposits of dust are comparable
with a porous body, and have a hollow space of up to 90 %. If
the temperature of dust deposits is increased, the result may
be spontaneous ignition of the combustible dust.
If dust deposits with a small grain size are whirled up, there is
a danger of explosion. This increases as the size is reduced,
since the surface area of the hollow space becomes larger.
Dust explosions are frequently the result of whirled up glo-
wing layers of dust which possess the basis for ignition. Explo-
sions of gas or vapor mixtures with air can whirl up dust where
the gas explosion then merges into a dust explosion. In collie-
ries, explosions of methane gas frequently lead to explosions
of coal dust whose effect was often greater than that of the
gas explosion.
1)
Parallel to DIN 50281 there is already EN 61241-1.
The danger of an explosion is prevented by using explosion-
proof devices according to their suitability. The identification
of the device category reflects the efficiency of explosion pro-
tection, and thus the application in corresponding hazardous
areas. The danger of explosive dust atmospheres and the
selection of appropriate protective measures are assessed
using safety parameters for the involved substances. Dusts are
considered according to two substance-specific properties: Conductivity
Dusts are referred to as conductive if they have a specific
electric resistance up to 10
3
Ohmmeter. Combustibility
Combustible dusts can burn or glow in air, and form explo-
sive mixtures with air at atmospheric pressure and at
temperatures from 20 °C to + 60 °C.
Safety parameters for whirled-up dusts are, for example, the
minimum ignition energy and the ignition temperature,
whereas for deposited dusts, the glow temperature is a
characteristic property. Physical fundamentals and parameters
7
Minimum ignition energy
The application of a certain amount of energy is required to
ignite a potentially explosive atmosphere.
The minimum energy is taken to be the lowest possible
converted energy, for example, the discharge of a capacitor,
that will ignite the relevant flammable mixture.
The minimum energy lies between approximately 10
-5
J for
hydrogen, and several Joules for certain dusts.
What can cause ignition? Hot surfaces Adiabatic compression Ultrasound Ionized radiation Open flames Chemical reaction Optical radiation Electromagnetic radiation Electrostatic discharge Sparks caused mechanically by