21-01-2013, 01:15 PM
Basic concepts for explosion protection
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INTRODUCTION
Internationally harmonized design regulations for electrical engineering have been drawn up in the
form of IEC standards and reports have been formulated, largely in conformance to the CENELEC
standards. The numerical sequences used in IEC, CENELEC and DIN are currently being standardised.
This reorganisation involves a lot of ongoing changes at present but it will make work easier in the
future.
In the Directive 94/9/EC the European Community has provided itself with obligatory, uniform design
requirements for the explosion protection of systems, equipment and components and this directive
is supported by the EN standards referred to in the above and the CENELEC and CEN standardisation
organisation.
With the help of these standards, the manufacturer can assume during the design and assessment of
the explosion protection that he is developing safe, explosion proof systems, equipment and
components conforming to Directive 94/9/EC which will be tested in conformance to uniform and
obligatory criteria by a notified body authorized by the EC. If the test criteria have been met successfully,
the EC-appointed notified bodie issue the type examination certificates, which ensure standard
fullfillment in Europe with respect to the required safety of the explosion proof equipment. These EC
type examination certificates, or assessments provided by the manufacturer, are the prerequisite for
the production and distribution of systems, equipment and components at a very high or high level of
protection.
Explosion
An explosion is defined as a sudden reaction involving a rapid
physical or chemical oxidation reaction or decay generating an
increase in temperature or pressure or both simultaneously. The
most familiar reactions are those of flammable gases, vapours or
dusts with the oxygen contained in the air.
Explosive range
In the internal combustion engine the three factors work together effectively: petrol, air/oxygen and
the ignition spark produce an explosion inside the enclosed cylinder. For this to take place, the ratio
of petrol to air must be correct. If the petrol tank is empty, the air filter blocked or if the ignition does
not work, one of the components for triggering this controlled, useful explosion is missing and the
motor will not start.
Combustible materials mixed with air have a lower and an upper explosive limit and the explosive
range lies between these limits. When considering the safety of work places, the lower explosive limit
is the more important value, a possible concentration of at least 20 % less than that value is often
regarded as safe.
Basis for an explosion
As a rule, for explosions to happen in atmospheric air, three factors
have to be present at the same time:
■ flammable substance
■ oxygen (air)
■ source of ignition
Explosion
oxygen
ignition source flammable gases
Prevention of explosions
Explosion proof equipment is able to exclude one of the preconditions for an explosion - the ignition
source - and is in that way an important contribution to explosion protection.
In domestic areas, constructional measures ensure that normally an explosive atmosphere cannot
form. The conscious restriction of these measures, e. g. the intended, unimpeded flow of flammable
gases or a reduction in ventilation can lead to explosions if an ignition source is also present.
The easiest and simplest way to understand small and safe explosions is by looking at a gas lighter.
When the nozzle of the lighter is opened, it releases a small amount of flammable gas. This gas mixes
with the surrounding air, the spark from the flint ignites the mixture, and a weak sound is heard - the
burning.
Some distance away from the nozzle the proportion of the flammable gas is already so low that the
explosion and the flame are restricted to the immediate vicinity of the nozzle. In other words, the
design of the gas lighter has ensured that it is safe to use.