Page 56 - Electrical Installation in Hazardous Area

34 Electrical installations in hazardous areas

The decomposition products ofdust; it is also necessary to know if flammable
gases or vapours are produced by the decomposition of dust at tempera-
tures below their ignition temperatures as this can have a significant adverse
effect upon the situation.
Figure 2.3 is a typical blank form which may be used to collect this infor-
mation.

2.6.3 Information on process conditions

Having collected the necessary information on the parameters of the fuel
(gas, vapour, mist or dust) itself it is necessary, in order to carry out an area
classification exercise, to identify the way in which the fuel (gas, vapour,
liquid or dust) is contained and the likelihood of its release, and thus the
formation of an explosive atmosphere. Typical examples of such information
follow.
The contuinmenf type; is the way in which the flammable material is
contained where possible releases may occur. A typical example may be
pipe flanges.
The locution; the location of the particular containment on the plant or
process site is necessary. A typical location may be, for example, in the
tank farm above tank 'X'.
The process material; material handled within the containment and thus
forming the possibly leaking fuel. This gives a direct reference to the previ-
ously gathered information on the fuel.
The process conditions; conditions, for example, of temperature and pres-
sure at the point of possible release are necessary as these will have an effect
upon the quantity released and the possibility of its ignition. Although a
material may have a flashpoint which is above ambient temperature it may
ignite if released at high temperature.
The description of containment; the detail of containment is necessary to
determine the leakage possible. If the containment is a pipe flange then the
type of gasket becomes important.
From the above information the grade of the possible source of hazard can
be established, together with the extent of any hazardous area produced.
A typical method for carrying out this exercise is a List of Equipment for
Area Classification (LEAC) Table of which an example is given in Fig. 2.4.
This also includes the appropriate Group (IIA, IIB or IIC) for any electrical
equipment and similarly the temperature class (T1 -T6) appropriate and
becomes the basic record of the area classification exercise.

2.7 Procedures

Area classification is not simply intended to identify hazardous areas
on process plants and in similar areas where design has already been
completed using only operational and economic factors as influence, but to