Page 229 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 1 93

plastic surface is surrounded with a conductive frame which is earthed (see
Fig. 8.2), when the area of the particular surface can be increased to 400 cm2,
and for apparatus given Group IIC, limitation to 20 cm2 is necessary unless
the plastic surface is surrounded with a conducting frame which is earthed
(see Fig. 8.2), when the area of the particular surface can be increased to
100 cm2.
The Standard also permits the use of a warning label indicating how
charge is to be avoided if the above measures are not possible. Historically,
the words ‘not possible’ have been taken in the UK to mean ’have not been
taken’ because of the difficulty in defining what is and is not possible. In
practice, therefore, the two most common solutions for fixed apparatus have
been to either load the plastics material with some conducting medium,
such as carbon (although ths can give problems with insulation within the
apparatus) or to utilize the warning label. Portable apparatus has tended to
rely on an overall cover of non-plastics material, such as leather or, again,
loading the plastic with conducting material. BS 5501 part 1 (1977). (The
first edition of this Standard did not limit the requirements to non-fixed
apparatus and apparatus likely to be cleaned but the introduction of this
limitation is unlikely to have significant effect upon design as manufacturers
tend to design their apparatus for the widest possible market.
The final problem with plastic enclosures is the problem which can be
associated with using fastening screws (for example, to hold the lid, in
place). Plastic being somewhat softer than metal in most cases, it is more
subject to thread stripping and where threads for fixing screws, which hold
covers in place and other parts likely to be opened in service for adjustment
etc., are tapped directly into the plastic, the Standard requires the thread
form to be compatible with the plastics material. No further guidance is
given but fine or shallow threads should be avoided as these are more
likely to strip, particularly if plastic screws are used. Many manufacturers
solve this problem by casting into the plastic metal thread bosses and this
is probably the best way to solve the problem.

Specific requirements for light metal (aluminium and similar)
enclosures
While such enclosures offer a similar advantage to plastic enclosures in
weight and ease of casting, they mainly exhibit greater strength. They do,
however, have similar drawbacks in respect of threads for fastening screws
and such threads should be treated in the same way as those for plas-
tics enclosures. There is also a problem associated with so-called thermite
sparking, where magnesium content of alloy enclosures is high. This means
that where the enclosure comes forcibly into contact with oxidized iron or
steel (rust), the resultant spark is enhanced by the presence of the magne-
sium and the spark is much more powerful and is ignition capable in most
cases. To avoid this situation, alloys with more than 6 per cent magnesium
are not permitted by the standard.