Page 184 - Electrical Installation in Hazardous Area

1 50 Electrical installations in hazardous areas

Zone Z (by definition) appears similar to Zone 1 for gases and Zone Y to
Zone 2. This is further reinforced by the fact that BS 6467, Part 2* excludes
the interior of vessels from its scope. This exclusion was clearly based upon
the understanding that the inclusion of electrical apparatus inside vessels
and silos, etc., was generally only acceptable in special circumstances. There
has now been a change of view in this regard as IEC 1241-33 recognizes
three Zones of risk in respect of dusts. These definitions are much more
synonymous with those for gases and vapours, giving the relationship
shown in Table 6.1. In this chapter the new three-Zone approach will be
used as the document referred to is a draft British Standard, is based upon
a draft IEC (International Electrotechnical Commission) Standard and will
become a British Standard in the near future. In addition, this three-Zone
technique has been used in one large company since 1973 with considerable
success and, because of its relationship with the gas and vapour classifica-
tion, the approach appears more logical.

Table 6.1 Approximate relationship between Zones

BS 5345, Part 2 (1983) BS 6467, Part 2 (1988) IEC 1241-3 (1994)
Zone Zone (Draft)
Zone

0 20
1 Z 21
2 Y 22

IEC 1241-33 also envisages dust releases outside buildings, which is
an unusual situation as the effect of the environment is such that areas
where dust is handled, stored or processed are normally indoors, or at
least protected. The use of materials producing combustible dusts outdoors
should be treated with caution. While in some cases (e.g., the outlets of
extract ventilation systems where, in abnormal cases, the possibility of dust
release cannot be ruled out) the presence of a loading facility (particularly a
manual one) where dust is subject to normal exterior air currents provided
by wind is difficult to justdy as the distribution of any dust release becomes
much more difficult to determine. Some external situations may be accept-
able but the control which can be exercised within buildings in the case of
dust releases is so advantageous it is hard to ignore. Even if outside oper-
ations are considered, partial enclosure to give some degree of confidence
as to the performance of dust layers and clouds should be addressed.

6.1 Properties of dusts

Two properties of dusts are important to area classification and these are the
ignition temperature required for a dust cloud and the ignition temperature