Page 251 - Electrical Installation in Hazardous Area

Protection concepts ‘m’, ‘0’ and ‘4’ 21 5

accommodated by surrounding the cells or batteries in soft encapsulant
before the overall encapsulation is applied. In such cases it is also necessary
to carefully define the charging conditions necessary to achieve this objec-
tive if the charger is not a component part of the encapsulated apparatus.

9.7.7 Type testing

Encapsulated apparatus is subject to type tests to ensure that the protection
concept is not breached within the specified operating conditions.
Temperature testing is carried out to ensure that internal heating does
not cause the maximum temperature of the encapsulant, or the maximum
operating temperature of the apparatus, to be exceeded.
The apparatus is also subjected to a temperature cycling test (see Fig. 9.1)
to ensure that no deterioration takes place. These tests are carried out at
the worst possible input voltage in the range of the apparatus. For mains
supplies this is assumed to be plus or minus 10 per cent. There are also
electric strength tests between separate circuits, discharge tests at above
maximum ambient temperature for cells and batteries, and pull tests on
cables which exit the encapsulant to ensure their integrity. These are all
detailed in BS 5501, Part 8 (1988)3.

9.2 Oil immersion-‘0’
(BS/EN 50015, (1 994))13

This type of protection is again one where the explosive atmosphere is preven-
ted from accessing the electrical circuits by their immersion in oil. Originally
a wide range of apparatus was covered by the Standard including switchgear
(see BS 5501, Part 2 (1977))14 but the second edition, BS/EN 50015 (1994)13,
limits the protection concept to apparatus which is not capable of causing an
ignition of an explosive atmosphere in normal operation (see Chapter 15) and
would be usable in Zone 2 without oil immersion. Apparatus in accordance
with BS 5501, Part 2 (1977)14 will not be discussed as no apparatus other than
that which would comply with BS/EN 50015 (1994)13 has been used in the
UK and it is not recommended that any is in the future. If any apparatus
is constructed for oil immersion before Directive 76/117/EEC1 it is recom-
mended that it complies with the requirements in BS/EN 50015 (1994)13 in
respect of construction of the electrical apparatus itself.

9.2.7 Construction of the apparatus
The basic constructional requirements for the apparatus itself do not appear
in this Standard. Instead, it refers to apparatus complying with IEC 79-15
(1987)15 which covers apparatus with type of protection ’n’. This standard is
not yet fully accepted in the UK and the constructional requirements should
be those given in the UK equivalent which is BS 6941 (1988).16 In reality,