Page 233 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 97

Thread Clearance
size hole size B
(mm) (mm)

M5 5.5 18
M6 6.6 19
M8 9.0 24
M10 11.0 28
MI 2 14.0 34
MI4 16.0 38
MI6 18.0 42
M20 22.0 48
M24 26.0 60
C = at least height of bolthead, screwhead or nut

Fig. 8.4 Typical recesses for shrouding purposes

Interlocking devices
Interlocking devices, may in some cases, be used instead of high security
fasteners in such a way that the interlock isolates the electrical equipment
and removes any residual sources of ignition (e.g., stored energy) before
the apparatus enclosure can be opened. There is no formal specification
for such devices in BS/EN 50014 (1993)' except indications to give some
idea as to the difficulty which should be encountered by those seeking to
circumvent them. The Standard requires that they are to be incapable of
being defeated by screwdrivers or pliers. In addition, as no specification
for them is given, they may be either mechanical or electrical.
The requirements are very much more relaxed then those in BS 5501,
Part 1 (1977) (the first edition of this Standard) as the requirements there
were that the interlock could only be defeated by a tool specifically designed
for the purpose and, by inference, it should not be susceptible to being
defeated by any tool in the normal range of tools used by personnel.