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What is Explosion Proof?
 
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What is Explosion Proof?
 
1. FORWARD

The electrical safety equipment and their accessories that are used in areas in which an explosive atmosphere may form in such quantities as to require special safety measures for safeguarding the health and safety of the workers concerned. The flammable and/or combustible substances must be considered to be substances that may form an explosive atmosphere unless an examination of their properties establishes that they are unable to cause an explosion independently, even if they form a mixture with air.

Areas at risk from explosions are divided into zones on the basis of thefrequency and duration of the presence of explosive atmospheres.Choosing the best mode of protection and determining the hazardous areas inside plants is neither easy or immediate. Thorough familiarity with current standards is in fact required.

 

We have therefore decided to use these introductory pages and the Appendices at the end of the catalogue to provide a short guide to dealing with the problem and finding other sources of information.

 

EXPLOSIVE ATMOSPHERES
 
From standard IEC 60050-426 : 2008-02: ‘mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapour, dust, fibres, or flyings which, after ignition, permits self-sustaining
propagation’. Using this definition we can divide explosive atmospheres into two large groups: those that have gases as fuel and those that have dusts (granulometry up to 500 μm).
Some figures:
The classified combustible gases set out in document IEC 60079-20-1 :
2010-01are approximately 300.
The division into subgroups is determined by the ‘maximum experimental
safety gap’ (MESG).
 
 
Some of the gases and their relative characteristics are set out below:
-- the explodibility field, i.e. the range in which gas mixed with air (21%
oxygen is the oxidising agent) may give rise to an explosion if it
reaches the appropriate flash point;
-- ‘MIT’ (Minimum Ignition Temperature) and “MIE” (Minimum Ignition
Energy) are two faces of the same coin, namely the source of
ignition.
-- the classified combustible powders listed in BIA Report 13/97 are
approximately 4,300.
Some of the dusts and their relative characteristics are set out below:
unlike the gas classification it should be noted that:
-- the ‘MIT’ is divided into two columns: one column indicates when
the dusts are dissolved in the atmosphere and are defined as being
‘in cloud form’, whilst the other column refers to a layer of 5mm of
dusts; obviously this does not mean that there is no problem withlayers that are less or greater than 5mm but that there is a problem
with different data.
-- The unit of measurement of the “MIE” changes from μJ to mJ
-- There is a new reference: KSt indicates how powerful the explosion
will be and how fast it will spread
 
COMBUSTION AND EXPLOSION PRINCIPLES
Combustion is a chemical reaction that entails oxidation of a fuel
by an oxidising agent (which is in general the oxygen in the air), with
development of heat and electromagnetic radiation, often also including
luminous radiation.
More strictly speaking, combustion is a type of exothermic oxidoreduction
inasmuch as one compound oxidizes whilst another one is reduced (in
the case of hydrocarbons the carbon oxidizes and the oxygen is reduced)
with a release of energy and formation of new compounds, mainly carbon
dioxide and water.

The combustion or fire triangle 

The ‘combustion or fire triangle’ consists of the three elements that are
necessary for the combustion reaction to take place. These three elements
are:
fuel
oxidising agent
source of ignition
The fuel may be of various types, e.g.: hydrocarbons, lumber or coal.
the oxidising agent par excellence is the oxygen present in the air.
The fuel and the oxidising agent must be of appropriate proportions to
ensure that combustion takes place within the so-called ‘flammability
range’.
The flash point can be, for example, a source of heat or a spark.
The flash point is the activation energy that is required by the molecules of
reagents to start the reaction and must be supplied from the exterior. The energy released by the reaction that enables the reaction to sustain itself
without the addition of external energy.
In order to be able to accelerate combustion, turbulence can be used to
increase the mixing between fuel and the oxidising agent, thus accelerating
combustion.
Explosion is very rapid combustion that occurs at atmospheric pressure
and the pressure must be confined in a volume in order for the explosion
to occur.
 
Explosion pentagon

Classification of the appliances
The Directive divides into the following groups:
• Group I - Products designed for use in mines and in their surface
plants.
• Group II - Products designed for use on surface sites in the presence of
explosive atmospheres.
The products are then subdivided within the Groups in the following
manner:
Group I
-- category M1 - Equipment ensuring a very high level of protection; they
must remain powered in the presence of an explosive atmosphere.
-- category M2 - Equipment ensuring a high level of protection; it
must be possible to disconnect them in the presence of an explosive
atmosphere.
Group II
-- Category 1 - Equipment ensuring a very high level of protection;
they are intended for places in which there is always an explosive
atmosphere or in which there is an explosive atmosphere for long
periods.
-- Category 2 - Equipment ensuring a high level of protection; they are
intended for places in which an explosive atmosphere will probably
develop .
-- Category 3 - Equipment ensuring a normal level of protection; they
are intended for places in which there is a small probability that an
explosive atmosphere will develop.
RISK ANALYSIS
Risk analysis is a fundamental process for understanding if we are inside
or outside the problem.
This process consists of evaluating, depending on the required level of
protection (normal, high, very high), whether the appliance has its own
potential sources of ignition that are able to cause an explosion. Thus if
the analysis shows that our equipment, in the various types of operation
required, does not have its own potential sources of ignition we are outside
the scope of the Directive; on the other hand, we must take measures to
ensure that its own potential sources of ignition do not become effective.
Risk analysis is normally constituted by the following four logic phases:
1) Hazard identification: systematic procedure aimed at identifying
all dangers associated with the product. After identifying an hazard, it is
possible to change the design to minimise the hazard, regardless of whether
the degree of risk has been estimated. If the hazard is not identified, it will
not be possible to eliminate it during the design phase.
2) Hazard estimation: determining the probability that the identified
hazards could occur and the level of seriousness of possible damages
arising from the considered hazards.
3) Hazard evaluation: comparison of the estimated risk and the criteria
that enable us to decide whether the risk is acceptable or when the design
of the product needs to be modified to reduce the risk in question.
4) Analysis of the hazard-reduction options: the last phase of the risk
analysis is the process of identifying, selecting and modifying variations to
the project to reduce the overall risk arising from the products. Although it
is always simple to reduce risks further, they can rarely be reduced to zero
without eliminating the activities.
The following potential sources of ignition must be considered:
-- hot surfaces
-- flames and hot gases (including hot particles)
-- mechanically generated sparks
-- electric apparatus
-- stray electric currents, protection against cathode corrosion
-- static electricity
-- lightning
-- electromagnetic waves
-- ionizing radiation
-- ultrasonics
-- adiabatic compression and shock waves
-- exothermic reactions, including self-ignition of dusts
‘EX’ EQUIPMENT
1 Types of protection
The types of protection are techniques that are provided by the harmonised
standards in order to meet the Essential Health and Safety Requirements.
These techniques ‘play’ on the fact that if only one of the elements is
removed that constitute the explosion pentagon the explosion cannot occur.
Thus by limiting energy (intrinsic safety), limiting heat (increased safety,
constructional safety), by removing the fuel (pressurisation, immersion
in liquid, encapsulation), by containing the explosion (flameproof
enclosures), the objective is achieved.
The European Commission periodically publishes in the Official Journal
of the European Union the list of the harmonised technical standards that
are presumed to conform to the requirements of the directive ‘ATEX 95’; the
last publication was on 27 January 2009 (O.J.E.C., series C, n° 20).
The diagram of the main mode protection for both electrical and nonelectrical
appliances is set out below.
Caption: those with a red background are ‘enabled’ for category 1 and/or
M1; those with an orange background are ‘enabled’ for category 2 and/or
M2; those with the pink background are ‘enabled’ for category 3.
2 the degree of protection of packaging: the IP code
 
3 the temperature class
The temperature class is the maximum temperature, surface or absolute,
depending on the mode protection, that the equipment can reach during
the operations for which its category is designed.
The less the equipment heat up the less likely it is that they may cause
explosions. It should be remembered that most gases have an ignition
temperature above 200-250 °C (T3).
A comparison between European/International temperature classes and
North American temperature classes is set out below.
Group II
The following protection modes for powders have been currently coded:
t; pD; iD; mD
 
9.2 the degree of protection of packaging: the IP code
 
9.3 the temperature class
The temperature class is the maximum temperature, surface or absolute,
depending on the mode protection, that the equipment can reach during
the operations for which its category is designed.
The less the equipment heat up the less likely it is that they may cause
explosions. It should be remembered that most gases have an ignition
temperature above 200-250 °C (T3).
A comparison between European/International temperature classes and
North American temperature classes is set out below.

Group II

 

4.Marking
Some marking examples follow.

a) electrical equipment, group II, category 2G: for example a junction box

 

 

b) electrical equipment, group II, category 2D: for example a command

and control unit

 

 

c) electrical equipment, group II, category 2(1)G: for example a command and control unit


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