Page 232 - Electrical Installation in Hazardous Area

196 ElectriCal installations in hazardous areas

occur with a standard screw when the hole was a clearance hole with the
maximum tolerance permitted by medium fit H13 as defined in IS0 286-2
(1988)18.
BS 5501 part 1 (1977) included a requirement for shrouds on special
fasteners (see Fig. 8.3 and 8.4) to make it difficult to gain purchase on
them by such things as adjustable wrenches, etc. This requirement has been
dropped in the case of BS/EN 50014 (1993)l, except for mining applications
which are not dealt with here.
The objective of all of the above in the case of fasteners is to create a
situation where fasteners do not become loose or cannot easily be inad-
vertently loosened. The difference between normal fasteners and special
fasteners is the degree of security which is considered necessary by the
particular protection concept standard. It is worthy of note that in more
than one case the protection concept Standard requires even a higher level
of security as will be seen when the individual protection concepts are
discussed.

A

Shroud height at least
equal to thickness of
bolt head, screwhead or nut

~
~~
Thread size A in mm (normal)
M5 9.0
M6 9.5
M8 12.0
Mi0 14.0
M12 17.0
M14 19.0
M16 21 .o
M20 24.0
M24 30.0

Fig. 8.3 Typical shrouds for fasteners