Page 203 - Electrical Installation in Hazardous Area

Design philosophy for electrical apparatus for explosive atmospheres 1 67

encapsulation of the electrical circuits, and pressurization of the enclosure
containing the electrical circuits with non-flammable gas.

Oil immersion (symbol ‘0’)
In this protection concept (type of protection) the electrical components
are immersed in oil within their enclosure, thus preventing access of the
explosive atmosphere. It is dealt with in detail in Chapter 9. There are limi-
tations on the type of oil which may be used and requirements to ensure
the security of the oil within the enclosure, so that the electrical circuits
remain immersed. The technique is considered as suitable for Zone 2 only,
at present, within the UK but is used in more hazardous areas in other coun-
tries. This is likely to change in the not-too-distant future (see Chapter 9)
and will be the protection concept permitted in Zone 1.

Encapsulation (symbol ‘m’)

This protection concept is dealt with in detail in Chapter 9. Again, the explo-
sive atmosphere and electrical circuits are separated from one another, this
time by encapsulating (potting) material. The types of material used are
defined and the encapsulated block is arranged so that surfaces presented
to the outside are hard enough to give protection. The technique is consid-
ered as suitable for both Zone 1 and 2.

Pressurization (symbol ‘p’)

This protection concept is dealt with in detail in Chapter 11. In this tech-
nique the explosive atmosphere is kept out of the equipment enclosure by
pressure of air or inert gas. Air is normally used as there are asphyxiation
problems with inert gas. Because the protection is by a gas (air) it is almost
always necessary to provide ancillary equipment to ensure that the air pres-
sure is maintained in spite of small leakages, which are almost certain to
occur. Because of this the technique almost always requires a pressure-
control system, together with a purge system to ensure that on start-up any
internal explosive atmosphere is removed, and this gives added compli-
cation which must be set against its added flexibility because of the ease
of depressurizing for repair. The technique is considered as suitable for
both Zone 1 and 2 or only Zone 2, depending upon the pressure-control
systems used.

7.2.2 Prevention of sparking (criterion b)

There is only one formal protection concept which is suitable for both
Zone 1 and 2 in this area.