S a fe t y
with wERMA -pRoductssafe is safe
For over 50 years WERMA has been manufacturing electrical In accordance with this philosophy, we have made it our
signal devices at the highest level. Thanks to our superior quality goal to improve the range of products designed for
standards, consistent innovation, continuous product development deployment in hazardous areas. It is here in particular,
and excellent service, we have grown steadily over the last few years. where danger is at its greatest, that responsibility of
those warning of such dangers is at its greatest.
As leaders in this field of technology, we owe it to our customers
to continue to build on this success. We therefore invest 10 % of
our expenditure in new developments. In this way, we can offer our
customers innovative products which are at the leading edge of
technological development in every detail.the explosion
An explosion is an extremely rapid oxidation
or decomposition reaction accompanied by
a sudden rise in temperature and pressure.
IgnITIon souRCe
oxygen e.g. hot surfaces,
As a rule, in order for explosions to occur
e.g. in the air electrical arcs
in atmospheric air, three elements need to
be present in the right proportions:
• Inflammable substance
• Oxygen (air)
• Ignition source
Inflammable subsTanCe
e.g. gas, vapour, dust
inflammable substances
Inflammable refers to the capacity of substances to react with oxygen releasing Inflammable solIds
light and/or thermal energy. Flammability is a precondition for combustion If inflammable solids occur in the form
and thus for an explosion. of dust, fibres or lint, these can react with
the atmospheric oxygen resulting in an
Inflammable gases
explosion.
A gas is a substance that exists in a gaseous state at a temperature of 20°C.
In addition to the chemical properties of the
Many inflammable gases, especially those employed for heating or in technical
substance, the size of the solid particles,
applications, are compounds containing carbon and/or hydrogen. In combination
i.e. the increase in the total surface area the
with (air) oxygen an explosive atmosphere can form.
finer the particles, plays an important role.
Inflammable lIquIds The required ignition energy is generally
greater than in the case of gases and
Liquid refers to substances existing in a fluid state. Dependent on the
vapours.
characteristics of the substance, liquids display different evaporation rates.
Evaporation refers to the transition to the gaseous state, a process that occurs
Examples of inflammable dust include wood
without the necessity of the liquid being brought to boiling point. This can occur,
dust, cereal dust, flour dust, inflammable
depending on the liquid, at relatively low temperatures.
plastic dust, sugar dust and inflammable
metal dust.
This means that a potentially explosive atmosphere can build at the surface
of inflammable liquids at room temperature and below.oxygen
A potentially explosive atmosphere can only occur when the concentration of the inflammable substance
in the air lies within a specific range. This „explosive range“ is defined by the upper and lower explosive
limits. If the concentration of inflammable substance is too low, no explosion will take place, i.e. the
concentration is too „lean“. If the concentration is too high there is not sufficient oxygen for an
explosion to occur, i.e. the mixture is too „strong“.
exPlosIVe Range and exPlosIVe lImITs
100 Vol. % concentration of air 0 Vol. %
mixture too lean mixture too strong
explosive range
no explosion no explosion
lower explosive limit upper
0 Vol. % concentration of inflammable substance 100 Vol. %ignition sources
The ignition source provides the explosive mixture with CHemICal ReaCTIons
the energy required to ignite. In the following the relevant As a result of exothermic chemical reactions substances can be heated and
types of ignition sources encountered in practice are thus form ignition sources. A raised ambient temperature or a defective heat
discussed in more detail: dissipation can accelerate this process.
HoT suRfaCes eleCTRIC sPaRKs
Hot surfaces can ignite an explosive atmosphere when Electric sparks can be generated by the opening and closing of electric
their temperature reaches the ignition temperature of the circuits, even at low voltages. These sparks must generally be considered
explosive mixture. Hot surfaces are sometimes intended ignition sources.
(e.g. heating), however they also occur as a result of
dIsCHaRge of sTaTIC eleCTRICITy
faults (overheating of bearings and brakes, defective
electronic components etc.). Due to charge separation, sparks can be generated by static electricity,
without the necessity of an electric voltage source. This also needs to be taken
flames and HoT gases
into consideration with non-electrical devices and components.
Flames are one of the most effective ignition sources.
Charge separation occurs for example when devices are not earthed or when
During normal operation flames and hot gases occur
they are included in a potential equalization. Electrostatic discharges can
within combustion engines and analysis equipment.
occur when wearing synthetic textiles, rolling paper and plastic foil and during
However within hazardous areas flames must be
pneumatic conveyance procedures (e.g. the extraction of bulk material, filling
avoided or at the least, securely enclosed.
a silo).
meCHanICally geneRaTed sPaRKs
HIgH eneRgy WaVes
Sparks can be generated during grinding, cutting and
Wave energy entering explosive atmospheres is also principally to be
beating processes, depending on the combination of
considered as an ignition source. Ultrasound, electromagnetic radiation in the
materials. By selecting a suitable combination of
form of radio waves, IR radiation and visible light, as well as ionising radiation

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