Page 211 - Electrical Installation in Hazardous Area

Design philosophy for electrical apparatus for explosive atmospheres 175

It has now been largely taken over by the second editions of the European
Standards, but equipment certified to its requirements is still available and
used in the UK.

7.5.3 Zone 2 compatible apparatus for gases, vapours and mists
In the case of Zone 2 apparatus there is no long history of third-party
certification as in the more hazardous zones, and industry has historically
selected its own using a British Standard Guide1 for assistance. Within
the UK, however, industry has always been slightly uneasy at this and
has sought third-party certification for Zone 2 equipment also. Historically,
there was no avenue for this and as no detailed certification requirements
existed it was difficult to arrange one. This unhappy situation led to HM
Factory Inspectorate issuing ‘Letters of no Objection’ for some items of
equipment, which effectively said that the Factory Inspectorate had no
knowledge or evidence to suggest the equipment was unsuitable. This diffi-
cult state of affairs led to the production in the UK of a Standard for Type
’N’ equipment6 to permit formal Certification. The reason for the upper case
‘N was that there was an international symbol for this equipment ‘n‘ and
it was hoped that the use of the upper case letter would avoid confusion
with the international concept, for which there was no Standard. There still
remains only a National Standard for Type ’N equipment and, unlike the
situation in Section 7.4.1 where certification is valid across Europe, type ’N’
certificates issued within the UK are valid only in the UK. The Standard in
question is:

BS 6941 (1988) - Electrical apparatus for explosive atmospheres with type
of protection ‘N’ (see Chapter 14)
This Standard covers all of the requirements for all of the methods used for
protection of electrical equipment which is intended for Zone 2. It is based
upon an international document published by the International Electrotech-
nical Commission - EEC 79/15 (1987), but because of the fact that the UK
did not agree with some of the contents of IEC 79/15 there are significant
differences .

IEC 79-1 5 (1987) - Electrical equipment for explosive gas atmospheres
Pari 15: electrical apparatus with type of protection ‘n’ (see Chapter 14)

This international document was the subject of considerable controversy as
several countries objected to some parts of it. The principal objection of the
UK was to the restricted breathing concept being used for sparking contacts
and this concept does not appear in BS 6941. Because of these objections it
was only possible to issue IEC 79-15 as an IEC Report not a Standard. This
means that it is only a guidance document despite having the appearance
of a Standard.