Safety and compatibility requirements for types of protection to explosion applied to electrical equipment intended for use in potentially explosive atmospheres

. The principles of protection for electrical equipment against the ignition of a potentially explosive atmosphere differ from one type of equipment to another and they can be combined, but only if the principles clearly defined in the literature are respected. There are situations where two or more principles of protection applied to the same equipment are not compatible with each other in order to ensure a level of protection appropriate to the classification of the area endangered by the presence of a potentially explosive atmosphere. In this case, an appropriate selection of technical solutions is required to ensure the explosion protection requirements for designated equipment. This paper is intended to be a synthesis of the requirements imposed by the technical standards in the specific field of potentially explosive atmospheres - ATEX, as well as highlighting some situations of incorrect application of the types of explosion protection chosen, during commissioning, but also during the exploitation of equipment in installations operating in areas endangered by the presence of potentially explosive atmospheres.


Introduction
An area where explosive atmospheres may occur in such high concentrations as to require special precautions to protect the health and safety of the workers involved is considered hazardous [1].
As explosions can cause human and material losses, assessing the risk of explosions and establishing appropriate measures to reduce it to acceptable levels according to the norms [2] and standards in force is of particular importance for the safety and health of people and property.Assessing the risk of explosion is a complex activity that must be carried out by qualified and experienced personnel, based on the most realistic assessment of the actual situation.
Explosive atmosphere [3] is defined as a mixture with air, under atmospheric conditions, of a flammable material mixed with air in which, after ignition, the combustion propagates throughout the whole unconsumed mixture.In situations where an explosive atmosphere may occur, the following measures should be provided [4]: -eliminate the probability of an explosive atmosphere occurring near the ignition source, or -eliminate sources of ignition.
Installations where flammable materials are handled or stored must be designed, operated and maintained in such a way that all releases of flammable materials, and consequently the extent of hazardous areas, are kept to a minimum, in terms of frequency, duration and quantity.
If these measures cannot be applied, preventive measures must be chosen and applied, so that the probability of the simultaneous occurrence of the two elements listed above is brought to a sufficiently low level to be accepted.
For an explosive atmosphere to exist, the flammable substance must be present in certain concentrations.If the concentration is too low (poor mixture) or too high (rich mixture) no explosion can occur, in fact a weak combustion reaction may occur but not a reaction in the whole mixture.Thus, the explosion can occur only in the presence of an ignition source and when the concentration is in the explosive range of the substance, respectively between the lower flammability limit (LFL) and the upper flammability limit (UFL) [5].

Hazardous Ex areas classification
Hazardous Ex areas are classified into zones, according to the frequency of occurrence and duration of the presence of explosive mixture generated as follows [5,6].Thus, we can have the following cases: • Zone 0 (gases) or 20 (dusts): area where the combustible gas or dust mixed with air is present permanently, or for long periods of time; • Zone 1 (gases) or 21 (dusts): area in which the combustible gas or dust mixed with air is likely to occur in normal operation; • Zone 2 (gases) or 22 (dusts): Area in which an combustible gas or dust mixed with air is not likely to occur during normal operation and in which, if it does occur, it is likely to occur only rarely and only for a short period of time.Combustibles gas is a generic name given to low-density, colourless, easily deformable and expandable fluid bodies in suspension, which in mixture with air results in explosive mixtures, and in this context they are found in the form of: gases, vapours or mists.
Combustible dust is defined as small solid particles with a nominal size of 500µm or less, which may be found suspended in air or deposit under their own weight, and may form explosive mixtures with air at atmospheric pressure and normal temperatures.
Any escape and/or release, whether or not intentional, of flammable gases, vapours, mists or combustible dusts which may give rise to explosion hazards must be suitably diverted or removed to a safe place.If that is not practicable, safely contained or rendered safe by some other suitable method.If an explosive atmosphere contains several types of flammable and/or combustible gases, vapours, mists or dusts, protective measures shall be appropriate to the greatest potential risk.
Because there are not exact rules for defining the presence of explosive atmosphere (duration and probability) in relation to areas classified as Ex, Zone 0, Zone 1 or Zone 2. However good engineering practice are used and the data from Table 1 can be considered as a starting reference for the classification of potentially explosive areas.Hazardous area classification is a method of analysis and classification of the environment in which gaseous explosive atmospheres may occur, so as to facilitate the correct choice and installation of usable equipment without generating a hazard in this environment.This is done taking into account the gas groups and gas temperature classes and the ignition temperature.
Starting from the explosion protection principles stated above, in Table 2 are summarized the requirements for technical equipment, depending on the intended use.
Table 2. Requirements for equipment depending on the Ex zone to be use.
In particular, the following categories of equipment must be used in the zones indicated, provided they are suitable for gases, vapours and/or dusts as appropriate: -in zone 0 or zone 20, category 1 equipment and Ga/Da EPL; -in zone 1 or zone 21, category 1 or 2 equipment and Ga/Da or Gb/Db EPL; -in zone 2 or zone 22, category 1, 2 or 3 equipment, and Ga/Da, Gb/Db or Gc/Dc EPL.The recognized types of protection for electrical equipment in accordance with European standards [5] have been given protection levels (EPL) according to Table 3. 2 Risc assessment of tehnical equipment operating in potentially explosive atmospheres In case of any product designed for use in potentially explosive atmospheres in accordance with the requirements of the ATEX Directive [3], the explosion protection characteristics against the generation of an explosion may be altered due to multiple factors such as: • an incorrect selection of the equipment according to the final destination; • an incorrect choice of the Ex protection type solution, for the equipment intended for installation, related to a specific process; • improper performance of inspection and maintenance operations of equipment intended for use into potentially explosive atmospheres; • modifications made to the Ex certified equipment; • inadequate repairs applied to the Ex certified equipment.
For the above mentioned operations that can compromise the explosion protection characteristics, only the manufacturer instructions and the requirements of the ATEX Directive are not enough.In these conditions the member states can adopted their own legislation regarding those issues.In this context, in Romania in 2007, a normative document was adopted [2] that provides some aspects regarding the requirements that must be considered when putting into operation an installation that will operate in potentially explosive atmospheres.According to this normative at commissioning of a technical installation operating in potentially explosive atmospheres a verification of the technical documentation and an on-site assessment must be performed.This is to assess the technical installation in order to check if all the measures required to ensure the protection to explosion are adopted, especially those regarding the equipment/protective systems used in Ex classified areas [8].

Requirements for electrical equipment
Electrical equipment designed for use in potentially explosive atmospheres must comply with the requirements of the general specific standard [5] and the standards specific to the Ex types of protection that were used to produce that equipment.The Ex types of protection for electrical equipment intended for use in explosive atmospheres generated by gases, vapors or flammable mists, can be grouped into three categories, depending on the presence of the ignition source and the potentially explosive atmosphere: -types of protection that allow direct contact between the ignition source and the hazardous atmosphere (eg.flameproof enclosure "d", intrinsic safety "i"); -types of protection that do not allow direct contact between the ignition source and the hazardous atmosphere (eg.oil immersion "o"; pressurized enclosure "p", powder filling "q", encapsulation "m"); -types of protection which by construction try to eliminate the ignition source under specified conditions (eg.increased safety "e"); This classification is actually a ranking of the level of safety offered by each type of protection.In order to choose the type or types of protection of the explosion-proof electrical equipment, an analysis must be made regarding the framing of the potentially explosive atmospheres in which the electrical equipment operates, determining: -explosive properties of the potentially explosive atmosphere; -the source and frequency of its occurrence.Thus, in order to correctly select the electrical equipment for Ex hazardous areas, the following information is required [8]: -classification of Ex hazardous area; -classification of gases and vapours in relation to the group and subgroup of electrical equipment; -the temperature class or the ignition temperature of the gases or dusts which create the explosive mixture; -external influences and ambient temperature.The assessment of technical installations operating in potentially explosive atmospheres is of high importance considering the existing explosion risk.The risk must be minimised to ensure the health and safety of people and to prevent damage to property and, not least, the protection of the environment.[4] The provisions of the applicable standards and regulations are used when performing the evaluation regarding the assurance of security requirements.In this sense, obtaining information regarding about installation, maintenance [9] and repair, must be taken into account, in order to fulfil these requirements.This information consists of: -manufacturer's instructions regarding selection, installation and initial inspection; -the necessary documentation to ensure the correct choice and installation of equipment for the specific application, regarding the compatibility of the equipment for the Ex zone and the environment condition in which they will be exposed, (e.g.temperature ranges, types of protection, IP classification, corrosion resistance, etc.); -the information necessary to ensure the correct installation of the equipment provided in a form that is suitable for the personnel responsible for this activity; -plans showing the types of cable systems and details about them; -records regarding the selection criteria for cable entry systems, for compliance with the requirements of certain Ex types of protection; -drawings and schemes related to the clear identification of the circuits, in order to separate those with strong currents from those with weak currents; -records of initial and subsequent preventive inspections -records of initial and subsequent preventive inspections (eg. a list of equipment and their locations, necessary spare parts, relevant technical information); -documents regarding the declarations of conformity for the company that installs the equipments in the installation that will operate in Ex zone; -documents about the qualification of the personnel involved in the installation process and Ex equipment inspections.

Safety requirements for Ex electrical equipment
A very important aspect that must be taken into account when installing electrical equipment intended to be used in potentially explosive atmospheres is the choice of cable entry systems and sealing elements, in order not to endanger the type of protection (e.g.flameproof enclosure "d") [9].

Selection of Ex cable entries
The cable entry must be selected to match the cable diameter.The use of sealing tapes, heatshrinkable tubes or other materials is not allowed to adapt the cable diameter to the diameter of the cable entries.Cable entries and/or cables should be chosen so as to reduce the effects of the cold plastic deformation characteristic of the cable.Cold plastic deformation can lead to a reduction in the insulation resistance of the cable and cables with low resistance to smoke and/or fire usually show considerable cold plastic deformation characteristics.
Cable entries, adapters and blanking elements must be in accordance with the specific standard requirement [5] and must be chosen so as to maintain the requirements of the method of protection [9], in accordance with the type of protection of the enclosure according to Table 4.

Protection method for cable entries, adapters and blanking elements
Ex "d" Ex "e" Ex "n" Ex "t" Ex "d" X Ex "e" X X Ex "i" şi Ex "nL" -Group II a X X X Ex "i" -Group III a X Ex "m" Ex "m" would not normally apply to connections.The protection method for the connections must correspond to the cable system used Ex "n" X X X Ex "o" Ex "o" would not normally apply to connections.The protection method for the connections must correspond to the cable system used Ex "p" X X X b Ex "pD" X Ex "q" Ex "q" would not normally apply to connections.The protection method for the connections must correspond to the cable system used Ex "t" X Note: X -indicates permitted use.a If a single "i" circuit is applied then no requirements are specified for cable entries.b Permitted only for Gc installations.

Cables used for connecting to Ex equipment
Cable entries must be installed in such a way that, after installation, they can only be loosened or removed by means of a tool.If additional fastening is required to prevent cable pulling and twisting from transmitting forces to the conductor ends inside the housing, a clamp should be provided with the cable as close as possible to the cable entry.
Cables should be routed straight from the cable entry to avoid side tension which could compromise the seal around the cable.Where cable entries, blanking elements and tapered adapters are used in enclosures fitted with unthreaded cable entry plates, care must be taken to use appropriate fittings to maintain enclosure integrity.
The connection of cables to electrical equipment must be made through cable entries appropriate to the type of cable used and must maintain the integrity of the relevant explosion protection type.Where the threaded entry or hole size is different from the cable entry, a threaded adapter must be provided in accordance with Table 4.Some common issues that were found during the assessment are: 1. Non-Ex equipment (cash machine) located in the proximity of a fuel dispenser whose normal operating principle generates a potentially explosive atmosphere [8]   4. Non-compliance with electrical separation requirements [11,13] and insulation distances, between circuits with and those without intrinsic safety "i".When we talk about the risks induced due to the improper installation and maintenance of equipment intended for operation in potentially explosive environments, we must take into account the overall risk level that can be defined with the following simplified formula: where: R(1)i -the partial risk associated with risk factor i, ki -the rank of risk factor i; Nrisc -the overall risk level

Conclusion
Even if a product designed for use in potentially explosive atmospheres is placed on the market considering the conformity assessment procedures of the ATEX Directive [3], the explosion protection characteristics can be altered due to multiple factors.They can be: incorrect selection of the equipment, incorrect installation of the product, inadequate performance of inspection operations, inadequate maintenance of equipment, inadequate overhaul or repairing of equipment.
Assessing the risk of explosion due to the operation of technical equipment in hazardous areas is a complex task that must be carried out by qualified and experienced personnel.
In Romania, at commissioning of a technical installation operating in potentially explosive atmospheres a verification of the technical documentation and an on-site assessment must be performed by ATEX experts whithin INSEMEX -Petrosani in accordance with NEx 01-06 Normative which was adopted in 2007.
Some examples of non-conformities identified during the assessment were shown in this paper.The main issues highlighted in this paper were the choice and combination of two or more types of explosion protection applied to technical equipment.Any of these types of protection considered individually are effective, but by incorrect mounting/combination may invalidate the explosion protection of such equipment designed and constructed to operate in an explosive atmosphere as defined in the specialized literature [3].

Fig. 1 .
Fig. 1.Cash machine located too close from the fuel dispenser

Fig. 2 .
Fig. 2. Ex-certified junction box, hanging only in the connecting cables

Fig. 3 .
Fig. 3. Incorrect cable entries chosen, for the type of protection Ex "d"

Fig. 4 .
Fig. 4. Non-compliance with electrical isolation distances between a non-Ex and Ex "i" circuits

Table 1 .
Relationship of probability and duration of presence of the explosive atmosphere.

Table 3 .
The relationship between EPL, Ex protection types, ATEX Category and Ex Zone