Page 231 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 95

8.7.4 fasteners, interlocking devices, bushings and cements
Once again the Standard identifies those facets of fasteners, interlocking
devices, bushings and cements; the performance of which are important
to the protection concept and these apply to all the subsequent individual
protection concept Standards, unless the particular protection concept Stan-
dard specifically excludes them.

Fasteners (including special fasteners)

The only general statement made about fasteners which fix parts necessary
to maintain the protection concept (apart from the obvious one that the
fastener in material and construction must be compatible with the enclo-
sure it fastens) is that such fasteners must be releasable only with the aid
of a tool. The term 'tool' is not defined but in a subsequent Clause dealing
with interlocking devices, screwdrivers and pliers are specifically defined
as not being acceptable as a method of defeating an interlock. Some certi-
fication bodies have used this wording or wording of similar import in
BS 5501 part 1 (1977) as justifying the non-use of screwdrivers and pliers
for fasteners in general, and it is wise to utilize such things as hexagon
and socket-headed bolts and screws in this regard, unless prior knowledge
of the attitude of the chosen certification body is obtained in advance of
completion of the particular apparatus design.
Some of the protection concept standards call up 'special fasteners' which
are intended to give a greater degree of security than the norm. To ensure
this, the screws, nuts and threads used are carefully defined, as are the
holes, (either clearance or tapped through) which the screws or studs pass.
The basic requirements for threads are as: all threads shall be coarse pitch
metric in accordance with IS0 262 (1973)8; tolerance of fit of threads shall
be 6g/6H in accordance with IS0 965 (1980)9.
The heads of screws or the nuts, where studs and nuts are used to secure
a cover or similar part essential for the protection concept, are required to
be one of the following:
1. Screws with hexagon heads to IS0 4014 (1988)lO or IS0 4017 (1988)".
(Note: only the head must comply with the named Standard.)
2. Hexagon nuts to Is0 4032 (1986)'*.
3. Cap screws with hexagon socket heads to IS0 4762 (1989)13. (Note: only
the head must comply with the named Standard.)
4. Hexagon socket set screws to IS0 4026 (1977)14, IS0 4027 (1977)15, IS0
4028 (1977)16 or IS0 4029 (1977)17 provided that the set screw does not
protrude from the threaded hole in which it is fitted when it is tight.
In the case of screws with heads, the Standard also requires that the land
under the head from the edge of the clearance hole, or the threaded hole
in the case of a reduced shank fastener, is no more than that which would