Page 172 - Electrical Installation in Hazardous Area

138 Electrical installations in hazardous areas

considered possible from the application of Table 3.1 (if it is decided that
it is acceptable in any given circumstance to have a building which is all
Zone 1).
If it is not considered as acceptable for the entire building to be Zone 1,
then each primary grade source of release must have local ventilation
applied to remove the material released before it can access the generality
of the building. In no circumstances is it considered possible to allow any
continuous grade source of release to freely release into the building where
only general ventilation is present.
Having determined the maximum quantity of flammable material
released into the building, it is then necessary to ensure that the amount
of air supplied by the ventilation system is sufficient to ensure that, if a
release occurs when the ventilation is on, the mixture of air and flammable
material exhausted from the room is below the lower explosive limit and
it is wise to apply a safety factor here. Chapter 4 provides equations which
will allow this to be done. When the ventilation has been properly designed
it can be accepted that, provided that early repair of the ventilation system
is executed so that it is not off for a long time (say repair is to be completed
well within a shift), then a secondary grade release will not occur while
the ventilation is off and there is little likelihood of the building filling with
explosive atmosphere and then exhausting when the ventilation is switched
on, creating an explosive atmosphere outside the building. Unfortunately,
this does not hold for primary grade sources of release and these must be
expected to release at times when the ventilation has failed but, as failure
of the ventilation is abnormal, such releases can be considered as secondary
grade giving rise to Zone 2 within the building and not Zone 1. There
remains, however, the problem that when the ventilation is repaired and
switched on, an explosive atmosphere will be exhausted from the building.
This, together with the fact that if input forced ventilation is used there will
be some leakage of explosive atmosphere from within the building at all
openings, demands that any ventilation system used is extract ventilation
so that the pressure in the building will be very slightly lower than outside,
resulting in release of explosive atmosphere to the outside only at the
ventilation exhaust. Figs. 5.12 and 5.13 show how this situation exists in
practice.
Where only secondary grade sources of release exist with general extract
ventilation, the inside of the building will be Zone 2 and no external
hazardous area will exist. Even in these circumstances, however, it is
advisable to have an indirectly operated fan with the drive motor outside
the ducting and clear of the ducting end, as if the ventilation fails for a long
time the fan will be necessary to remove a possible explosive atmosphere
from within the building. Although this is so unlikely as not to be within the
area classification considerations it is clearly not acceptable to use a source
of ignition to try to clear an explosive atmosphere, however unlikely that
atmosphere may be.
Where primary grade sources of release exist without local ventilation,
as described earlier, the inside of the building will be Zone 1 and a Zone 2