Temperature classes

The maximum surface temperature must remain below the temperature class specified in Table 7.4 for a specific application. 

Table 7.6 Temperature class and limiting surface temperature with regard to gas ignition...

Temperature class Ignition temperature (°C) Limiting surface temperature, (°C)... 

Surface temperature should not exceed the ignition temperature noted in Table 7.6 for a particular temperature class under any conditions of operation. There is no limit to the temperature rise to the permissible limits for a particular class of insulation of windings or other parts of the machine, except the limiting surface temperature as in Table 7.6. For such an application, a normal 1P 55 enclosure may also be employed. 

To contain the temperature of the electrical circuits within safe limits for a particular temperature class of the surroundings, the maximum current rating for a minimum size of a conductor is also stipulated in ICC 60079-1 I. The constructional requirements also stipulate the minimum clearances and creepage distances in air between the conducting parts of all the intrinsically safe L lectrical eirctiits. 

The temperature class marked on equipment shall be as shown in Table 505-10(b). 

Temperature classes for transformers A, E, B, F, H and C are generally recognized. All classes are applicable to both oil-immersed and air-cooled transformers. Classes A, B, H and C are most commonly used but the use of Class F is increasing. 

The spectral features observed by astronomers have led to the classification of stars into seven broad classes outlined in Table 4.1, together with their surface temperatures. The highest-temperature class, class O, contains may ionised atoms in the spectrum whereas the older stars in class M have a much lower temperature and many more elements present in the spectrum of the star. Observation of a large number of the stars has lead to extensive stellar catalogues, recently extended by the increased sensitivity of the Hubble Space Telescope. Making sense of this vast quantity of information is difficult but in the early 19th century two astronomers... 

In the light of the above, contact abrasion methods can be classified into ordinary, used for measurements at room temperature, and high temperature. Classed among abrasion resistance test methods at room temperature are the following Bohme s, Mindt s, Harvey s, McGee s, Blondel s, dry friction with balls, dry friction without balls, and Siever s method. 

This marking system correlates the adequate zone on the explosion protected apparatus with traditional data, e.g. group, type of protection and temperature class (see Chapter 5). 

Figure 2.4 The zone 2 concept according to VDE 0165/1991-02. Requirements for electrical apparatus without arcs or sparks in normal operation and/or temperatures not exceeding a certain temperature limit, e.g. complying with a given temperature class.

These subdivisions and their temperature class shall be marked on the apparatus e.g. ... 

The maximum surface temperature may be marked as a temperature value or as a temperature class according to IEC 60079-0 (or both)... 

Lower temperature classes have not been installed with respect to the fact that carbon disulphide, CS2, shows the lowest known ignition temperature (see Table 1.3)... 

Article 503 of the NEC (National Electrical Code, USA) limits maximum temperatures for Class III equipment (suitable for ignitable fibres and flyings) to 165°C (equipment not subjected to overload) and to 120°C for equipment which may be overloaded. Thus, only temperature classes T3B, T3C, T4 and T4A, T5, T6 are applicable... 

Table 4.4 gives a survey of the classification of combustible (gas, vapour or mist) mixtures with air according to temperature class and group/subgroup. 

Selection in this chapter is strictly focused on safety related data in the field of explosion protection, neglecting all the other aspects like maintenance or financial considerations (this will be the objective of Chapter 9). Here, selection means the appropriate choice of electrical apparatus according to temperature class and MESG or MIC/MIC (CH4) grouping, taking into account the combustible raw materials and products in the relevant plant. 

IEC and EN Standards ask for marking the type of protection as an abbreviation completed by the classification scheme (temperature class, explosion group) as described in the previous chapter. Table 5.1 gives a survey of the types of explosion protection and their abbreviations. Table 5.2 summarizes the IEC and EN marking code. 

Table 5.5 gives a guideline for the selection of explosion protected electrical apparatus, i.e. the appropriate choice of temperature class and group referring to the hazardous substances in the plant to be considered. Very generally Tower temperature classes do comply with the requirements for... 

Table 5.5 contains the minimum requirements throughout all the examples. Chapter 9 will demonstrate that the design of an apparatus for lower temperature classes (T4, T5, T6) often adversely affects its economic efficiency. 

Type of protection EEx ia IIC T4/T5/T6 Certificate BVS 98.E.2068X The temperature class rating depends on ambient temperature (maximum +60°C) and rated power. Rated power (max.) 2 W Supply voltage 24 V AC/DC. 

An essential point considering e-motors is their thermal behaviour. For the simple case of permanent operation at rated power, the windings shall exceed neither the temperature limit of the corresponding temperature class (T1. .. T6) nor the limiting temperature due to the thermal class of insulating material given in Table 6.13, Part A. 

This is of general importance, with a special reference to the bare parts of a cage rotor, and establishes an additional limit for the (insulated) windings. The rotor of a squirrel cage motor shall exceed neither +300°C nor the temperature limit of the corresponding temperature class. These values shall not be exceeded even during starting. 

Table 6.21 tE values of cage induction motors (thermal class F, 50 cps, for temperature classes T1/T2 and T3) (by courtesy of Siemens AG, Standard Drives Division, Erlangen/Germany)... 

Temperature class T... Rated power kW Efficiency % COS

[Pg.206]

The fact that all internal parts of a motor in increased safety shall comply with the temperature limitations according to the temperature class, results in somewhat uneconomical motor dimensions for classes T4, T5 and T6, or, in other words, the rated power is considerably reduced (Table 6.22) compared with normal industrial design. ... 

Avery general requirement of e -standards is the prevention from exceeding the limiting temperature (according to the temperature class). This shall be ensured by one of the following means ... 

In the same manner, temperatures exceeding the ignition temperatures (see Table 1.3 in Section 1.2.2) or the maximum surface temperatures according to the temperature class of the apparatus (see Table 4.1 in Chapter 4) are permissible inside of d. This covers windings and rotors in motors, especially for low temperature classes T4, T5 and T6 as well as the discharge tubes of high pressure sodium or mercury vapour lamps in case of a broken lamp bulb, or the ovens of gas chromatographs. 

Type of protection Temperature class Rated power kW Efficiency t] % COS 4> Weight kg... 

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