The field of explosion protection can prove difficult to navigate due to standards, rules and regulations for hazardous areas that leave room for interpretation and misunderstandings. Some common beliefs relating to key concepts like ignition protection, classes, and zones ought to be revisited, according to R Stahl.
Proposition 1: “Anywhere an explosive atmosphere occurs at any time, a hazardous area and zone designation must be established.“
Clarification: An explosive atmosphere does not necessarily require the designation of a zone-type hazardous area. Areas are classified as zones 0, 1, 2 (gas, mist or vapour) or 20, 21 and 22 (combustible dust) whenever there is a certain probability that an explosive atmosphere develops in these locations, which in combination with a source of ignition could result in an explosion. The factor that determines the required level of protection is the time frame during which a flammable mixture is actually present. If an explosive atmosphere only occurs for certain foreseeable periods, e.g. only when and while a container is opened, a protection zone need not be declared. Instead, as part of a hazard assessment, certain protective measures can be implemented solely for the duration of the presence of the explosive atmosphere.
Proposition 2: “Temperature classes and groups are irrelevant for zone 2 hazardous areas.“
Clarification: Temperature classes and groups are independent of zone classifications. They are a reference framework for possible ignition sources like hot surfaces or flammable energies. The maximum surface temperature of electrical equipment must always be lower than the ignition temperature of the air-gas or air-vapour mixture it is exposed to. For instance, some devices are approved for temperature class T6, hence they can be operated within gas and vapour atmospheres with ignition temperatures between +85 °C and +100 °C. Surface temperatures for these devices may not exceed +85 °C. The lowest temperature class T1 allows for operating equipment in areas with gas and vapours that can only be ignited at temperatures in excess of +450 °C. In addition to the temperature classes, equipment is classified into groups IIA (e.g. propane), IIB (e.g. ethylene) and IIC (e.g. hydrogen). Temperature classes and groups are to be observed irrespective of zone classifications.
Proposition 3: “Maximum surface temperatures of equipment used within an atmosphere containing combustible dust must not exceed two thirds of the minimum ignition temperature of that atmosphere.“
Clarification: This is only partially accurate. For combustible dusts, temperature classes do not apply. The crucial factor for explosion hazards is the dust’s minimum ignition temperature in relation to the equipment’s surface temperature. However, a minimum ignition temperature must be separately determined for clouds containing airborne particles on the one hand and layers of dust on the other. It is true that the equipment’s surface temperature may not exceed two thirds of a dust cloud’s minimum ignition temperature. For residual layers of dust on equipment, the minimum ignition temperature is a different matter altogether. It is defined as the lowest temperature of a hot surface required to ignite a layer of 5 mm of dust. This value must be observed for maximum device surface temperatures with a safety margin of 75 °C. Since thicker dust layers will ignite at lower temperatures due to increased heat insulation, the safety factor can be determined by means of a table in the standard for installing electrical systems in hazardous areas. Wherever devices are completely covered in dust, or if dust layers of more than 50 mm may accumulate, or if the ignition temperatures of dust layers up to 50 mm layers are below +250°C, the ignition temperature must be determined via lab tests.
Proposition 4: „Equipment marked as explosion-protected in gas and dust atmospheres may be used in environments containing both.“
Clarification: If an atmosphere contains both combustible dusts and explosive gases or vapours, a so-called hybrid mixture forms. A hybrid mixture may possess different critical characteristics than the individual substances it is composed of. Appendix M of the IEC 60079-14 standard, last revised in 2013, provides guidelines that should be observed in such cases. Actual conditions may vary considerably – a hazard assessment must therefore be conducted to determine appropriate safety measures. Equipment suitably protected for both gas- and dust-related explosive atmospheres cannot automatically be safely operated in environments with hybrid mixtures of gas and dust. For example, temperature classes for equipment marked for gas atmospheres do not take layers of dust into account. However, such layers increase the surface temperature of enclosures or devices installed within them, which corresponds to a greater risk of explosions. Flameproof enclosures exposed to hybrid mixtures are at risk of gathering dust in the flameproof joint. An internal explosion may cause accumulated hot particles to be ejected and turn into an ignition source in their own right. For these reasons, separate tests are required for explosion-protected devices that are to be exposed to hybrid mixtures.
Proposition 5: “The letter ’U’ following the certificate number on an enclosure’s test certificate signifies ‘uncritical’.“
Clarification: A certification ending in “U”, e.g. “PTB 09 ATEX 1107 U”, does not mean “uncritical”, but “incomplete” (“U” for German “unvollständig”). Such certificates for enclosures without installed components do not serve as an advance certification for a device in a fully fitted enclosure. Only the
characteristics of the empty housing have been taken into account, and the operating instructions provided with it also only apply to the bare enclosure. For instance, for the “increased safety” (Ex e) type of protection, the tight sealing, rigidity, and thermal resistance of the enclosure materials will be tested. Partial test certificates of this kind merely serve to facilitate later certification of fully-fledged devices – for enclosures that come with a “U” certificate, further tests pertaining to the enclosure properties need not be conducted for the encased devices. However, a device as a whole always requires a comprehensive, complete test certificate based on examinations that include the installation conditions and the types and quantities of installed components.
Proposition 6: “In hazardous area zones, only low-sparking tools may be used.“
Clarification: Sparks originating from friction, mechanical impact, grinding or drilling constitute ignition sources as per EN 1127-1. If and to what degree sparking caused by tool use is permissible depends on the designated protection zone and hazard category of the explosive substance in the atmosphere. In zones 0 and 20, tools that can produce any sparks at all are prohibited. In zones 1 and 2, steel tools like screwdrivers or wrenches that ordinarily only produce single sparks, but neither showers of sparks nor hot surfaces, may be used. This does not apply to zone 1 atmospheres containing
explosion group IIC substances (acetylene, carbon disulphide and hydrogen), hydrogen sulphide, carbon monoxide or ethylene oxide. Special measures are required in these cases to prevent the formation of ignitable sparks. Steel tools are generally permitted in zones 21 and 22 as long as they do not produce showers of sparks or hot surfaces. If this cannot be ensured, the workplace must be free of dust layers and safely shielded from the adjacent hazardous area zone. Moreover, it must be kept moist as long as tools are used to ensure dust cannot be stirred up and no smouldering hot spots can develop. Use of low-spark tools is therefore not compulsory in hazardous zones, but may be advisable in some cases.
Proposition 7: ”A retrofit LED tube may be used in an Ex e explosion-protected luminaire.“
Clarification: Retrofit LED lamps are not fluorescent lamps. However, conventional explosion-protected linear fluorescent luminaires featuring Ex e (increased safety) and nA (non-sparking) protection are designed for fluorescent lamps equipped with electronic ballasts (EB) according to the IEC 60081 standard. The EB ensures the safe ignition of the lamp without pre-heating. It also ensures that the fluorescent tube is switched off if the power conversion inside the electrodes of the lamp breaches a specific limit, which would lead to a surface temperature in excess of the permissible maximum. Since the electronic ballasts are usually only approved for fluorescent lamps with G13 bi-pin connections, retrofit LED tubes may not be used in Ex e luminaires. Coloured fluorescent tubes may be installed if the illuminants are suited for lamps with G13 bi-pin connections as
per IEC 60081. Installing energy-saving lamps with E27 bases in explosion-protected luminaires that would accommodate them requires a test certificate. Fluorescent lamps with colour filters possess different thermal characteristics – such filters may overheat the illuminant. These types of lamps should therefore not be used with Ex e type of protection luminaires.
Proposition 8: ”Small electrical appliances like wrist watches, calculators or hearing devices are potential sources of ignition and must be kept out of hazardous areas.“
Clarification: Committee 235 of the German Electrotechnical Commission has found that timepieces worn on the wrist cannot be penetrated by explosive atmospheres due to their small size and the construction of their enclosures. This applies across the board and even in particularly unfavourable circumstances. Electric wrist watches without special features (e.g. calculator or computer functions) therefore do not constitute explosion hazards in zones 1 and 2. Wall clocks, on the other hand, must ensure explosion protection, e.g. with an II 2 G Ex e mb IIC T6 certification. In the case of hearing devices, both the battery voltage and potentially ignitable inductive circuits must be taken into consideration. However, the risk of ignition is considered so marginal for compact ear devices that wearing them inside zones 1 and 2 is generally permitted. It is worth noting, though, that remote controls for these devices may not be taken along. Last not least, a use of calculators, including solar-powered devices, in hazardous areas requires a case-by-case examination and certification. A sufficient number of photovoltaic cells may generate voltages that can potentially cause ignitions.