Page 205 - Electrical Installation in Hazardous Area

Design philosophy for electrical apparatus for explosive atmospheres 1 69

all energy storage in the equipment is closely controlled to ensure that
stored energy is limited. As with pressurization, this equipment relies to
a large extent upon other equipment, which feeds it with electricity and
the technique is somewhat complex. It is also limited in its application to
instrumentation because of the energy limitations imposed, but within this
limitation it is very flexible.
There are two grades of intrinsic safety, the higher grade ‘ia’ being suit-
able for Zone 0, Zone 1 and Zone 2, but the lower grade ’ib’ being limited
to Zone 1 and Zone 2.

7.2.5 Special situations
While the methods of equipment protection given in Sections 7.2.1 to 7.2.4
cover most situations normally encountered, there is an additional type of
protection for use where the types already described are not appropriate
because of advances in technology or special circumstances.

Special protection (symbol ‘s’)

Special protection is not a fixed type of protection, as in the above but is
used to describe equipment which is suitable for use in explosive atmo-
spheres on the basis that its individual type of protection is equivalent to
one of the more classic types, although different. Its typical use is to permit
advances in technology where such advances do not easily fit into standard
types of protection. It has been typically used for encapsulated equipment
’m’ prior to the publication of the construction Standard for that type of
protection, and for sintered flame arresters prior to those being included in
type of protection ‘d‘ (flameproof enclosure). It has also been used where
techniques contrary to existing standards such as bi-pin tubes in type ‘e‘
(increased safety) fluorescent fittings are used on the basis of equivalent
safety. It can also be used where equipment which has two independent
types of protection, each of which is complete in itself (e.g., an increased
safety ’e’ terminal chamber in a flameproof enclosure), is used provided
that the two protection concepts are each independent of the other. This
latter approach must, however, be treated with caution.
Type ‘sf equipment is usable as specified in each particular case, and is
only usable in Zone 0 and 1 where this is specifically stated.

7.3 Situation in respect of Zone 2 apparatus
While the techniques described in Sections 7.2.1 to 7.2.5 are all suitable for,
at least, Zone 1 and 2 (with the exception of the current situation in respect
of protection concepts ’0’ and ’q’) they offer a standard of protection which
is higher than is necessary for Zone 2. For this reason a further type of
protection has been developed.