Page 178 - Electrical Installation in Hazardous Area

144 Electrical installations in hazardous areas

In an open air situation near the ground the extent of the hazardous
area would be given by Equation 4.7 multiplied by 1.5 in accordance with
Fig. 3.3.
This gives:

X = 1.5 x 10.8[GT/ME]o.55 m
X = 16.2[2.5 x 10 - 03 x 295/50 x 2]0,55 m

X = 1.1 (in an open air situation) m
If we assume the minimum amount of air is used in the ventilation system
of the indoor area then Equation 4.31 can be used and the following is the
case:

Q = 0.03 x 2.5 x x 2952150 x 2 m3/s
Q, = 0.065 m3/s

If the room is 100 m3 in volume then the number of air changes per second
is 6.5 x At this low flow there will be badly ventilated parts and so a
low airflow efficiency is expected and factor f (see Chapter 4) will be 5 and,
if the source of release is a primary grade source of release, then k will be
0.25. If we now use Equation 4.32 from we have the volume of explosive
atmosphere:

Q = 0.065 x 516.5 x x 0.25 m3

Q = 2000111~
This would give a linear extent of hazardous area in the best case of:

X = 7.8m

Therefore, the extent of the hazardous area is around 8 m if the volume was
a sphere but as it will not be due to the directional nature of ventilation it
will be considerably larger.
In addition the volume of the room would need to be much greater than
the explosive atmosphere or it would result in the entire room being a
hazardous area.

5.5 Rooms below ground

Rooms below ground should be treated as rooms above ground but,
as natural ventilation via openings is not possible, only the forced
ventilation solutions are possible. Provision of mechanisms to remove the
gas vapour/air contents of the room become much more important as
there is no other mechanism for changing the air in the room. Unless it
is forcibly changed before energization of electrical equipment the risk of