Page 248 - Electrical Installation in Hazardous Area

21 2 Electrical installations in hazardous areas

Taking the view above then, the requirements of the concept become
more understandable and realistic.

9.1.4 Encapsulated circuits and components
The Standard permits a wide range of electrical and electronic components
to be encapsulated, provided that the operating temperature of such compo-
nents does not exceed their rating at the maximum ambient temperature
envisaged for operation (normally 40 "C). Even components such as relays
and switches are permitted, provided that they are themselves enclosed
before being encapsulated. In the case of these components, however, the
maximum volume of their enclosure is limited to 100cm2 and, if they switch
more than 6 A, their enclosure is required to be inorganic to prevent combus-
tion of its material on arcing or sparking. The rating of the components
used needs to be sufficient to ensure that they remain within it in case of a
single fault elsewhere, or their failure in the worst possible way assumed
to be a part of that fault. The types of fault which are considered include,
but are not limited to, short or open circuit of any component, or faults
in printed circuitry such as open circuit failures or shorts between tracks.
The apparatus is not permitted anywhere to exceed its specified operating
temperature range in normal operation or fault conditions, and this may be
ensured by an internal thermal trip provided such a trip, is not self-resetting
which, in effect, means that if it operated the apparatus would be at the
end of its useful life.
Some components are considered as not subject to faults in the protec-
tion concept if operated at less than two third of their voltage and power
ratings in normal operation, and less than their voltage and power ratings
in a fault condition and these are: carbon or metal film type resistors; wire
wound resistors with the wire in a single helical layer; plastic foil, paper
and ceramic capacitors.
In addition, the following components are considered as not subject to fault
when they satisfy the requirements applied to them in normal operation and
do not exceed their ratings in a single fault situation.
First, optocouplers supplying isolation between separate circuits if the sum
of the voltages of the separated circuits is less than 1000 V rms and the rated
voltage of the optocoupler between these circuits is at least 1.5 times that
voltage sum.
Second, transformer and other windings complying with BS 5501, Part 6
(1977)9 (this will include its second edition BS/EN 50019 (1994)'O the
requirements of which are not dissimilar). In addition, windings which
use wire of less than 0.25mm diameter which are not permitted by BS
5501, Part 6 (19m9 or its successor BS/EN 50019 (1994)lO are permitted,
provided they otherwise comply with those standards and, in addition, are
protected against inadmissible internal temperatures by embedded thermal
cut-outs or similar devices.
Third, transformers complying with the requirements for infallible mains
transformers in BS 5501, Part 7 (1977)6 (and its successor BS/EN 50020