Page 273 - Handbook of Electrical Engineering

258 HANDBOOK OF ELECTRICAL ENGINEERING

Note: Until 1999 the practice in the USA did not recognise type ‘n’ or the certification of equipment
for Zone 2 (Division 2) areas. In these areas good quality, standard industrial equipment may
be installed. It is worth noting, however, that the NEC, Reference 2, was revised in 1999 and
Article 500 now includes the IEC system of zones and the types of protection ‘d’, ‘e’, ‘m’,
‘n’, ‘o’, ‘p’ and ‘q’ in Article 505. Type of protection ‘N’ is not included.

10.5.6 Type of Protection ‘o’

With this type of protection the active and sparking parts of the equipment are immersed in mineral
oil. The concept is similar to that used in the manufacture of bulk oil immersed and small volume
oil immersed switchgear (both of which are seldom encountered nowadays). Oil immersion finds
application with electronic and telemetering equipment.
Type ‘o’ protection is only permitted in Zone 2 and non-hazardous areas. Oil immersed
switchgear is not normally specified in the oil industry because there are far better insulating media
available in modern designs e.g. SF6 and vacuum.

10.5.7 Type of Protection ‘p’

This is also known as pressurisation or continuous dilution. It is mainly applied to large motors,
control panels, display panels, and occasionally special purpose generators. Type ‘p’ protection is
suitable for Zone 1 and Zone 2 areas.

Type ‘p’ protection allows well-designed standard industrial equipment to be used in hazardous
areas, provided that the enclosure is suitable for pressurisation by air or an inert gas. The enclosure
should be reasonably airtight so that the pressurisation can be maintained by a modest throughput of
air or gas.

The pressurisation process is carried out in two parts, the first part when the equipment is
ready to be energised and the second part to cater for the running and shutdown of the equipment.
The first part is called ‘purging’. Air or inert gas is passed into and vented from the enclosure,
to purge out any gas-air mixture that may be present. The equipment is prevented from being energised
until the purging cycle is complete. The purging cycle will need to pass a prescribed volume of air or
inert gas through the equipment. Measuring devices will be incorporated into the purging equipment
to ensure that the necessary volume of air or gas has been passed. If the purging equipment fails
then the enclosure cannot be energised. The purging equipment maintains a throughput of air or gas
to balance the leakage to atmosphere from joints, bearing seals, gaskets and the like, and to maintain
a prescribed pressure inside the enclosure.
The purging air must be drawn from a non-hazardous area source e.g. through suitable ducting
or from a plant air compressor. If the enclosure is large, as in the case of high voltage motors, then
the use of plant air may present problems of air consumption. The purging gas for a small enclosure
may be taken from high-pressure storage cylinders, using a suitable pressure reduction regulator.

Wherever the purging medium is derived from, it should be filtered and dried so that the
enclosure is not contaminated or dampened, and the insulation of the internal components degraded.