Page 228 - Electrical Installation in Hazardous Area

192 Electrical installations in hazardous areas

International Standards Organization Standard as, in cases where
a manufacturer ceases production of a particular plastic, the search for a
substitute will be easier.
The Temperature Index (TI) (taken from the 20000 hour point on the
thermal endurance graph for the plastic) needs to demonstrate that at 20 "C
above the maximum temperature achieved by any part of the enclosure at
maximum operating temperature for the apparatus, the plastic has lost less
than 50 per cent of its flexing strength to demonstrate that the plastic is
of sufficient strength for the purpose. (While demonstration of this for the
purposes of BS/EN 50014 (1993) is by the manufacturer of the apparatus
specifying the particular value, the older Standard BS 5501, part 1 (1977)
required that the entire graph be produced to confirm this situation.) IEC
216-14, IEC 216-25 and IS0 1786 or IS0 E277 will need to be consulted
for the methods of producing this figure.
In addition to being chosen to comply with this figure, enclosures made
of plastics need to be tested for their resistance to heat, cold and, where
the enclosure or parts thereof are not protected from normal incident
light which is the general situation, their resistance to light. Likewise
the impact tests and, if appropriate, drop tests normally carried out at
laboratory temperature, will need to be carried out at the maximum and
minimum service temperatures, with safety factors of around 10°C for
plastics enclosures.
Most plastics also have the propensity to hold charge when subjected to
friction, such as being rubbed with a dry cloth, and this static charge is often
capable of igniting an explosive atmosphere. It is necessary, therefore, to
ensure that such the material cannot support such charge or, if its generation
is possible, precautions are taken to ensure that it is not generated.
In general terms, electrostatic charges only become a problem where an
enclosure, which is capable of sustaining such a charge is rubbed with a
dry cloth or similar. BS/EN 50014 (1993)' identifies these conditions as the
only conditions needing to be addressed by limiting the requirements for
prevention of, or dealing with, electrostatic charge to plastics surfaces of
portable and transportable (non-fixed) apparatus, and apparatus which is
likely to be rubbed or cleaned on site. In the latter case, only apparatus in
clean rooms or such places as pharmaceutical plants, where cleanliness is
of great importance, is likely to be affected. On typical chemical plants or
oil refineries, for example, it is more likely that equipment will be hosed
down rather than cleaned with dry cloths.
There are two methods of dealing with plastics enclosures to prevent the
build up of ignition-capable electrostatic charge. First, selection of material
so that it has a surface resistance of less than lGohm when measured
between two 1OOmm conducting lines, lOmm apart, which are painted on
the plastics enclosure surface or the surface of a flat plate of the same plastic,
or by design of the enclosure such that development of charge is unlikely.
Second, by limitation of the surface of the enclosure projected in any
direction (any single surface) to the following maximum sizes. For appa-
ratus given Group IIA or IIB, limitation to 100cm2 is necessary unless the