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252 HANDBOOK OF ELECTRICAL ENGINEERING

occur. This is called the Upper Explosive Limit (UEL), above which no combustion or explosion is
possible. In terms of hazardous area analysis the UEL is not normally of practical significance. Various
institutions have determined the LEL for most of the regularly encountered gases and vapours. Both
the LEL and UEL are expressed in percentage of volume. Reference 1 Appendix 8 and Reference 2
Article 500 Table 2.1 give comprehensive listings of LEL, UEL and other important data.

When it is necessary to identify the hazardous gas or vapour the designer will also need to
know whether or not the gas or vapour is heavier or lighter than air. It is normally assumed that the
hazard appears and persists in ‘still air’ conditions. The effect of wind or forced air ventilation is
not considered for this purpose, because ‘still air’ conditions can always arise in practice and so this
becomes the ‘worst case’ condition. Heavier than air gases and vapours have a relative vapour density
greater than air, air has a value of unity. Likewise lighter than air gases and vapours have a density less
than unity. This aspect is important when the boundaries of the hazardous area are being determined,
especially in elevated parts of a plant, where open flooring is used and where open drains exist.

10.3.1.2 Temperature class
In order to classify a gas or vapour by its ability to be ignited by a hot surface, the definition Tem-
perature Class is used. Ignition by this classification is spontaneous or automatic. The classification is
simple to apply and consists of a two-digit code, the first digit is the letter ‘T’ and second a number
between 1 and 6. The lower the number the higher the maximum allowable surface temperature.
Hence a T6 gas or vapour is relatively more hazardous than one with a T1 code, see Table 10.3.
The above table complies with the same classification used in the USA, except that the NEC
sub-divides the classes in some cases to further A, B and C classes.

10.3.1.3 Grouping of gases and vapours

In order to classify a gas or vapour by its ability to be ignited by a spark, the definition Gas Group
is used. Again the classification is simple to apply. It consists of a three-digit code. The first and
second digits are I or II [The IEC may add Group III in future for dust hazards.] The third digit is
letter A, B or C.
For the oil industry the Group I is of little concern because it pertains only to underground
mining. Hence Group IIA, IIB and IIC are of concern in this sub-section. A Group IIA gas or vapour
is the hardest of the three sub-groups to ignite by a spark. Conversely a Group IIC gas or vapour is
the easiest to ignite.

Table 10.3. Temperature class for gases and
vapours

‘T’ classification by Maximum surface
◦
IEC60079 Part 0 temperature, C
T1 450
T2 300
T3 200
T4 135
T5 100
T6 85