Temperature Classifications

Fig. VIII-4. Nomenclature of the earth s atmosphere based on a temperature classification. The stratosphere is the region of temperature inversion, that is, the temperature increases with height and is stable against vertical mixing since dense cold air is at the bottom of the layer.

Group I equipment (for coal mines endangered by firedamp and/or coal dust) is not subjected to any temperature classification. Despite the high ignition temperature of methane-air mixtures (see Table 1.3), the maximum surface temperature is limited to 150°C (or 302°F) in all cases where deposits of coal dust with a low glow temperature (see Table 1.4) may be expected. If such deposits are not likely to occur, the surface temperature is limited to 450°C (or 842°F)... 

In this case the apparatus is not subjected to the temperature classification T1...T6 because of its safe environment. For Group II intrinsically safe circuits, the MESG or MIC grouping IIA/IIB/IIC shall be marked according to the relevant standards (IEC 60079-11, EN 50020). 

The General requirements cover apparatus grouping and temperature classification, constructional requirements for enclosures and fasteners, connection facilities and terminal compartments, cable entries and supplementary requirements for rotating electrical machines, switchgear, plugs and sockets and luminaires. A third part of the standards cover type verifications and tests as well as marking requirements. 

Apparatus grouping and temperature classification are described in Chapter 4, especially in Table 4.1 Part A and Table 4.2 Part A for EN and IEC specifications. As a general rule, the ambient temperature (in service) for explosion protected electrical apparatus is limited to the range from 20°C to +40°C. Deviating ambient temperature ranges shall be indicated, e.g. -30°C = Tamb =s +60°C, or the apparatus shall be marked with the symbol X, which indicates special conditions in general. 

Type of protection EEx d IIC T4/T5/T6 (temperature classification depending on power losses of internal components) Certificate PTB Ex-94.C.1065 Enclosure material steel. 

Identical type and power rating complies with the certificate and temperature classification... 

Hazardous area classification zone, temperature classification, and gas group... 

The stable crystalline form is corundum (a-alumlna). Kappa-alumina is one of several structurally related, metastable forms. These occur In "active alumina" and are much studied (5) due to their Importance in adsorbents and catalysts. Llppens and Steggerda (5) summarized the classifications of metastable forms, their crystal structures and conditions of formation by dehydration. Okuralya et al. (6) recently published an extensive study on the formation and structural relations of 1C-AI2O3 and its precursors le -AlgOg and tohdlte. Kappa-alumina belongs to the "nearly anhydrous", high-temperature classification (5) based on its formation temperture of 900 to 1000 C (somewhat lower in vacuo). The Irreversible transition x+o is thermally activated at 1200 C (2, 5). Thermochemical studies cited In this table presume that the samples of x-Al Og were essentially anhydrous. 

The solvent recoverers may not be able to predict the solvents handled on their plant in the future, but must specify a temperature classification for the electrical equipment based on the appropriate material code and be sure that this danger is imderstood by the staff. Table 9.1 shows that 200 °C is an adequate limit for covering the majority of solvents. Protection against the sparks produced by electrical equipment is also covered by appropriate national codes. 

Temperature classification Maximum temperature inside equipment ( C) ... 

Electrical equipment for use in flammable atmospheres should be marked both with a group number, which relates to the relative ease with which flammable gases may be ignited by electrical sparking from the equipment, and by a temperature classification which relates to the maximum surface temperature of the equipment. 

Group I equipment is used in the mining industry and Group II equipment in other industries. Letters A, B or C markings following the Croup number indicate the suitability of the equipment for various gases subject also to the marking for the appropriate temperature classification. 

Flameproof equipment is regarded as safe for use when exposed to the risk of explosive atmosphere for which certification has been given. Electrical apparatus, defined as flameproof, has an enclosure that will withstand an internal explosion of the flammable vapour or gas in question which may enter the enclosure. The joints of the enclosure which are designed with clearance gaps to prevent a build-up of internal pressure also prevent any internal explosion igniting vapour or gas surrounding the equipment. The surface temperature of the enclosure must be below the ignition temperature of the vapour or gas in question, hence the importance of the temperature classification referred to above. Flameproof enclosures are primarily intended for use in Zone 1 or Zone 2 classification but not in Zone 0. 

A temperature classification of T4 (135°C) is achievable to meet the requirements of most of the industrial gases. 

Another eonsideration for eleetrical equipment is its temperature classification, or T rating. This indicates the maximum surface temperature of those surfaces of the equipment that can be exposed to the flammable atmosphere. So the T rating of electrical equipment should be below the minimum ignition temperature of the vapours or gases likely to be present in the flammable atmospheres. The T ratings in terms of maximum apparatus surface temperature are ... 

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