Page 174 - Electrical Installation in Hazardous Area

140 Electrical installations in hazardous areas

(based upon Table 3.2 taking account of the maximum release of the source
of hazard, but with a minimum of 0.5m) will exist where the ventilation
exhausts into the outside air. In this case, the fan motor will need to be
considered as previously to allow for repair delays. This will have already
been considered as the fan motor will be in a Zone 2 if in-line.
As previously, the use of extract ventilation will mean that all leaks of air
will be into the building and so no external Zone 1 will occur. Any air inlets,
however, will be surrounded by Zone 2 based upon the maximum leakage
rate and should be sized in accordance with Table 3.2, with a minimum
of 0.5 m.
Continuous grade sources of release are not considered as acceptable in
normal circumstances within this type of building.

5.4.2 The application of additional local ventilation
Section 5.3.1 describes the situation which arises when general ventilation
only is provided for. It is possible to additionally arrange for local ventila-
tion at a single point of release or location to limit the extent of hazardous
areas produced by specific sources of release in areas which are not gener-
ally freely ventilated; be they unventilated or provided with general natural
or forced ventilation. There are two basic ways of doing this.

Provision of specific ventilation for individual sources of release

Where a particular source of release is provided with individual venti-
lation then, provided that such ventilation is properly conceived, the size
of the hazardous area around that source may be described, even though
the area in which it is sited is not generally freely ventilated (e.g., is in
a building). The ventilation provided here must be local to the source of
release and must be sized to ensure that the airflow in the area of the release
is greater than the release velocity. It must also ensure that the quantity
of ventilation air provided is sufficient to dilute the maximum release of
the source in question to some fraction of the lower explosive limit of the
flammable material released, so as to ensure that the gas vapour/air mixture
exhausted from the area is below the lower explosive limit. Chapter 4
provides a method of defining the quantity of air necessary, but deter-
mination of the velocity of airflow depends upon detailed knowledge of
the geometry around the release and the release velocity. This approach is
generally only effective for extract point source ventilation as in other cases
the effect of the ventilation would be to push the released gas/vapour into
the effectively enclosed area. Also, the nature of secondary grade sources
of release, in that they are often large and at high velocity, means that
the approach is only effective in limiting Zones 0 and 1 caused by contin-
uous and primary grade sources of release. In such cases, however, it has
significant value in that it can be used to maintain a general Zone 2 clas-
sification in such places as buildings containing continuous and primary