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For an explosion to occur in a hazardous area, a source of ignition must be present. It is therefore a sensible precaution to ensure that wherever possible and practicable sources of ignition are removed from the hazardous area. Where removal of a source or potential source of ignition is not possible, adequate precautions must be taken to ensure that the source of ignition does not come into contact with the explosive atmosphere. Care must be taken to ensure that all potential sources of ignition receive proper consideration.

A list of possible sources of ignition, which can commonly occur in the water industry is detailed below. For a more comprehensive list of ignition sources requiring consideration, refer to BS EN 1127-1 (22).

10.1.1

10.1.2

10.1.3

Unprotected Flames

Unprotected flame is possibly the most obvious source of ignition and could arise from a range of objects and activities. These include matches, smoking, welding, oxy- acetylene cutting etc.

Mechanically Generated Sparks

Mechanical generated sparks may result from friction, impact or abrasion. The particles which become separated from solid materials during these processes become hot owing to the energy used in the separation process. If these particles consist of oxidizable substances, for example iron or steel, they can undergo an oxidation process, thus reaching even higher temperatures. Sources include such things as sparks from tools, grinding operations, metal studs on boots, etc.

Impacts involving rust and light metals such as aluminium, magnesium or their alloys can initiate a thermite reaction which can cause ignition of explosive atmospheres. These metals could exist as housings for equipment, conduit, walkways, covers etc, Under BS EN 13463 1 2009 (22) there is no restriction in the use of Aluminium (and other light metals) in hazardous areas provided it can be demonstrated that there is no risk of ignition from incendive friction, impact or abrasion sparks through contact with rusty metal can be avoided or if the metal is sufficiently protected (e.g. with a suitable coating) so as not to present a source of ignition.

The light metals titanium and zirconium can also form incendive sparks under impact or friction against any sufficiently hard material, even in the absence of rust, so there use in hazardous areas should be avoided.

NOTE: Aluminium tools, ladders, etc. should not be taken into hazardous areas.

Static Electricity

Static electricity primarily occurs when two dissimilar materials are brought into contact and then separated, resulting in each carrying an equal but opposite charge. For conducting materials this charge is quickly recombined and no residual charge remains, however if one or more of the materials is a non-conductor then the insulating properties of this material may prevent the charge from being immediately dissipated. Solids and liquids are capable of retaining an electrostatic charge, which if the charge potential is great enough, can create an electrostatic discharge or spark to earth. Possible sources of static electricity include non-conducting solids such as plastics, man made fibres (e.g. nylon), but also liquids such as vapour droplets, jets of liquids/gases, nozzles, drive belts, fan blades etc. Whether or not an electrostatic discharge will ignite a flammable mixture will depend on the minimum ignition energy to ignite the mixture and the incendivity of the spark discharge. BS 5958 (17) provides

more detailed information and should be consulted for general considerations on control of static electricity and for recommendations applicable to particular industrial situations. BS EN 60079-0 (24) provides requirements for preventing electrostatic charges on external non-metallic materials of electrical apparatus and provides guidance on the maximum size of chargeable surfaces depending on the ignitability of the gases and vapours and the classification of the hazardous area.

GRP materials used in as covers on odour control plant, tanks and digesters will not readily accumulate electrostatic charge in normal use and will readily dissipate any charge due to the high humidity inside the covered vessel. Note: there are available GRP materials which have antistatic properties. Some types of plastic (e.g. polyethylene) have high resistivity values, (resulting in a resistance to earth path in excess of 1 GΩ), that present a greater risk of becoming charged and creating an ignition hazard. Where a charging mechanism exists for building up static on these materials, such as friction, or flow through a hose and valve and by ‘splash filling’ of containers, then the following precautions must be considered. In Zone 0 areas (Zone 20) the use of sheeting made from high resistivity materials should be avoided. In Zone 1 and Zone 2 (Zone 21 and 22) such sheeting is acceptable provided that the charge generated is small and gives rise to a low ignition risk, equivalent to that from Zone 2 electrical equipment. Continuity and earth bonding of all conductive and dissipative parts connected to non-conductors will also reduce charging potential. Note: the resistance to earth only has to be sufficiently low to provide a dissipative path and could be as high as 1MΩ, with 10Ω - 100Ω specified for convenience in monitoring. These values are considerably higher than that used for personnel protection.

10.1.4 Lightning

10.1.5

10.1.6

10.1.7

This form of ignition is dealt with in BS EN 62305 (34) and information on lightning protection is given in Section 10.13.

Electrical Equipment

Only suitably protected electrical equipment should be used within hazardous areas. NOTE: Personal portable electrical equipment (mobile telephones, pagers, other communication and test equipment, etc) shall not be taken into a hazardous area, unless such equipment is specifically designed and approved for use in such an area and is of proper condition and maintained in accordance with the manufacturers instructions. Such excluded equipment may only be used under the control of a permit to work. Simple electronic based watches are acceptable providing the case and glass is in good condition and not removed within the area. Multifunction watches (i.e. watches with calculators etc. and stop watches), torches (unless suitably certified for hazardous area use), car alarm key fobs, etc. are not to be taken into such areas.

Hot Surfaces (non-electrical equipment)

Hot surfaces may result from equipment malfunction or process operations. Magnification of sunlight through glass can also result in the production of hot surfaces.

Other Ignition Sources

BS EN 1127-1 (22) describes several other possible ignition sources including;

• Radio frequency electromagnetic waves, further information is available in BS 6656 (20).

• High frequency electromagnetic waves • Ionizing radiation

• Ultrasonics

Generally these igntion sources require significantly higher levels of power to be a risk than are normally encountered in the water industry