Page 272 - Handbook of Electrical Engineering

HAZARDOUS AREA CLASSIFICATION 257

10.5.4 Type of Protection ‘m’

Type ‘m’ enclosures are encapsulations, for example an electronic circuit encapsulated in solid epoxy
resin or fire-resistance solid material. There are very few examples in electrical power equipment
used in the oil industry.

10.5.5 Type of Protection ‘N’ and ‘n’

This type of protection does not have any particular title description. It also has some mixed conno-
tations with type of protection ‘N’ which is very similar but not identically the same.
The subject of type of protection ‘N’ attracted much debate in the 1980s and 1990s, as
explained in Chapter 6 of Reference 1 and Reference 5, much of which centred around whether or
not sparking equipment could be included in an enclosure.

The ‘N’ was originally developed in the UK and became covered by BS4683 Part 3. BS4683
has been superseded by BS6941, which has been updated in 1997. BS5000 part 16 covers non-
sparking motors with the type of protection ‘N’. The use of type of protection ‘N’ in zone 2 areas
may not be universally assumed to be completely satisfactory, as described in Reference 5 which
recommends that some action should be taken to reduce the hazardous situation when a release of
gas occurs. Taking appropriate action manually may not be achievable on a highly reliable basis in
practice. A form of automatic action will be needed such as a ‘fire and gas detection’ scheme, with
alarms and tripping functions.
References 1 and 5 give good descriptions of the background to the development of the type
‘N’ concept.
Type ‘n’ was not covered by the early editions of IEC60079, it became included as Part 15
in 1987.
The basic concept of type ‘n’ protection was to have an enclosure design that would be suitable
for Zone 2 areas. The application to Zone 1 areas was deliberately excluded. Consequently it should
be possible to design an enclosure which is ‘better’ than standard industrial designs of good quality
and which has some similarity with type ‘e’ designs. The intent was to have non-sparking components
inside a suitable enclosure, and to have a certifiable item of equipment for Zone 2 use. Inherent in
the concept of a good quality industrial design for use in a Zone 2 area is the need for a robust
water and dust resistant enclosure. Hence the IEC and BSI standards require a certain high level of
‘ingress protection’, see sub-section 10.6. In most cases the minimum ingress protection is IP54, but
fully insulated conductors IP44 may be used e.g. motors.
As far as motors are concerned, the emphasis on hot surfaces, high starting currents and
extended run-up times is not as great as with type ‘e’ for Zone 1 areas, due to the inherently lower
risk of hazard in a Zone 2 area. Special protective relays are not required and a ‘system’ approach is
not used. Similar design features in the mechanical part will be found e.g. clearances of fan blades,
length of the air-gap between the rotor surface and the stator inner surface. Oil company specifications
often call for non-sparking materials for the construction of the fans.
Other types of equipment than motors are often chosen with type ‘n’ enclosures, e.g. luminaries,
junction boxes, terminal boxes, if the designer can be sure that they will be located in a Zone 2 or
non-hazardous area.