Page 204 - Electrical Installation in Hazardous Area

168 Electrical installations in hazardous areas

Increased safety (symbol ‘e’)
This protection concept is dealt with in detail in Chapter 12. The technique
accepts that there is ignition-capable energy within the equipment but relies
upon the quality of construction to ensure that the energy is not released
in a way which could cause ignition. This is achieved by the quality of
construction of the equipment. The technique is considered as suitable for
both Zone 1 and 2.

7.2.3 Containment of explosions (criterion c)
There are two protection concepts which use this technique.

Flameproof equipment (symbol Id’)
This is the oldest protection technique. The explosive atmosphere is
permitted to enter the equipment enclosure and to be ignited by the
components within. The equipment is, however, provided with a very
special strong enclosure which will withstand the internal explosion without
sustaining damage and, in addition, will prevent the flames associated with
the internal explosion from exiting the enclosure in a way which would
permit them to ignite any surrounding explosive atmosphere. The technique
is considered as suitable for both Zone 1 and 2.

Powder filling (symbol ‘q’)

Here the equipment enclosure is filled with quartz, sand or some similar
inert small-grained filling. The technique is an extension of an old technique
involving the use of stone chambers to prevent the passage of flame up-
ducting. These chambers served to quench the flame, or so it was thought.
The filling does not prevent the explosive atmosphere from accessing the
equipment and coming into the proximity of a source of ignition. The fine-
grained filling, however, serves to quench any ignition, thus preventing an
explosion. The technique is considered as suitable for Zone 2 only in the
UK, but is used in more hazardous areas in other countries and is expected
to be acceptable for use in Zone 1 in the UK in the not-too-distant future
(see Chapter 9).

7.2.4 Energy limitation (criterion d)
There is only one protedion concept in this area but it is the protection
concept offering the highest level of security.

Intrinsic safety (symbol ‘i’)
In this technique the electrical energy fed to the equipment is below that
which will cause ignition if released in a spark or as a hot surface, and