Page 104 - Electrical Installation in Hazardous Area

80 Electrical installations in hazardous areas

grow until the release stops or the evaporation rate exceeds the release
rate. A Zone 2 will be produced by this pool in accordance with Table 3.7,
the dimensions of the pool being defined by either the containment of the
pool where this exists or, where no containment exists by calculation (see
Chapter 4).

3.4.8 Trenches

There are two types of trench normally encountered. The first is a trench
which contains pipework and there is the possibility of a leak within it due
to pipe joints or valve seals, etc. This means that the leak could occur in
normal operation (a primary grade release) or only abnormally (a secondary
grade source of release). Such trenches should be given the same type
of treatment as drains based upon the type of leak and the quantity of
flammable material leaking (if the leak is liquid). If the release is of gas or
vapour then types of, and the extent of, hazardous areas produced should
be those produced by the leak occurring outside the trench but should
extend throughout any length of trench between blockages. (It is normal to
put deliberate blockages in trenches to prevent transmission of flammable
materials from one part of a plant to another.) In these cases the interior of
the trench will be Zone 1 even if the release is only abnormal due to the
persistence introduced by the trench.
The second type of trench is that which is not intended to contain any
flammable materials such as a trench used for a cable route. Such trenches
are only important insofar as they may be in areas defined as hazardous
areas for another reason. The hazardous area inside the trench will in such
cases be as follows:

External Zone 0 - Internal Zone 0
External Zone 1 - Internal Zone 1
External Zone 2 - Internal Zone 1
The hazardous are a within the trench will again exist for the entire
length between blockages. The increase in hazard in external Zone 2 is
to take account of the persistence occurring within the trench. The above
relationship will remain true in the alternative case where flammable liquid
leaks into the drain from an external pool. The presumption in the case of
liquids is that action is taken in a reasonable time to remove the flammable
material from the drain (which is considered as essential).

3.4.9 Sampling points

The problem with sampling points is that most are manual and, apart from
the doubtful wisdom of deliberately creating a significant hazardous area
by a routing action, there is the problem as the personnel sampling at the