Explosion group

Some experimentalists chose to maximize precautions by eliminating energetic fuels and phlegmatizing their mixes. A classic explosive group that took this approach was the Minelites. 

Groups (I) and (II) comprise non-permitted explosives, groups (III)—(V)—permitted explosives. 

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. 

Classification and marking of intrinsically safe apparatus concerning explosion groups (I, IIA, IIB, IIC) and temperature classes (T1-T6) are organized in the same way as for, e.g., flameproof enclosure - d (see Tables 4.1 and 4.2, Chapter 4, and Chapter 5 for marking). Some special features, however, which are of importance for type of protection intrinsic safety - i only, will be explained in the following. 

The selection of apparatus according to the data of a gas/the gases (explosion Groups I, IIA/B/C, temperature classes T1-T6) does not differ from rules valid for e.g. flameproof enclosure - d For normal zone 1 applications, category ib is sufficient. For circuits in zone 0 only, category ia shall be selected. 

An apparatus showing data as given in Fig. 6.219 complies with explosion Group IIC. For IIB applications, however, the increased maximum values C0 and L0 (as indicated for IIB) may be used, resulting from the reference curves for explosion Group IIB. The maximum values for voltage and current, U0 and Ior are laid down by design, so that the increased values permissible for IIB in principle cannot be applied here. 

Only in the case of two apparatus identical with respect to category and explosion group will their interconnection show the same properties. If this does not apply, it is ... 

Caused by addition of currents and increasing values (U0, IQ) produced thereby, the explosion group of the interconnected apparatus may change. Both associated apparatus are assumed to be specified for IIC. Their interconnection, however, may exceed the values permissible for IIC, so that the total assembly complies with IIB only. 

Figure 6.231 A fieldbus is designed according to explosion Group IIB. To extend it with field devices into a hazardous area endangered by Group IIC gases, an additional current limitation is installed in the terminal box.

In particular, for explosion Group IIC the inductance values permissible for higher currents are considerably lower than 0.1 mH. So, leakage inductances arising everywhere shall be considered during planning and installation. 

Table 7.3 Classification of materials in explosion groups based on their maximum experimental safe gap (MESG) according to [1, 14, 16]...

Explosion group Maximum experimental safe gap in mm Test gas... 

In order to correctly choose the flame arrester, the maximum gap width (cf. Sect. 2.1.1.6) must be known. This parameter serves to categorize materials in explosion groups, as shown in Table 7.3. 

Explosion group and subgroup Temperature class Ambient specification... 

EXPLOSION GROUP GAS IIC HERE DUST me HERE(Cond) 15 AMBIENT TEMPERATURE TEMPERATURE RANGE... 

In the result of the determination of the safety characteristic Flame-proof gap width and Ignition temperature an explosive atmosphere can be assigned to an Explosion Group (Ex-Group) and a Temperature Class (T-Class). 

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