Page 179 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 145

explosion is much greater as the persistence of an explosive atmosphere in
an underground room with no forced ventilation is theoretically infinite.

5.6 Rooms without any internal release but which abut
external hazardous areas

While no opening should exist in the part of a room which is adjacent to
an external hazardous area, such an ideal situation is not always possible.
Installations, such as cable ducts in walls, can be effectively sealed but
it is often necessary to have a door in a wall, the other side of which
is a hazardous area. In such circumstances the ideal solution would be
to produce a situation where the explosive atmosphere does not access
the room, as it is usually required to install electrical equipment which
may be a source of ignition. This is not always possible, however, and
as a result there are a variety of situations where partial or total inva-
sion of the room by the external hazardous area occurs. The principal
situation is where a room has a door or window which opens on to a
hazardous area.
Where a room has an opening which abuts a hazardous area then, even
if the opening is a door which is not used normally, the interior of the
room must be assumed to be contaminated with the explosive atmosphere
from the outside. As the ventilation inside the building is not normally
free ventilation, the entire room must normally be assumed to adopt the
external classification at least.
Where a room has a normal door or other opening which abuts a Zone 1
or 2 the entire room should be assumed to be itself a Zone 1 or 2 ( i.e.,
the external classification) at best. If ventilation within the room is unusu-
ally poor then the interior would be Zone 1, even if the external classifi-
cation is Zone 2, but this more severe situation would not be likely in the
case of normal above-ground rooms. It is not considered acceptable for any
room openings to abut Zone 0 as such zones are normally expected to be
contained.
There are several ways to mitigate the effects described above in the
Zone 1 and Zone 2 situations. If, for example, a door is present in a room
and is only used in emergencies, then an essentially gas-tight door could
be fitted and be made either self closing or to alarm if it is left open. Such
doors are assumed not to transmit the hazardous area to the inside of the
room. An airlock comprising of two self-closing doors is assumed to have
a similar effect, as is a single self-closing gas-tight door coupled with fresh
air inlet ventilation into a room which keeps its pressure slightly positive.
The argument in this latter case is that even if the door is used regularly
there will only be a problem if the ventilation fails and the time for which
the door would be open in these circumstances would be unlikely to create
a scenario where any significant amount of gas entered the room, bearing
in mind that personnel would be aware that the ventilation had failed.