Page 249 - Electrical Installation in Hazardous Area

Protection concepts ‘m’, ’0’ and ‘9’ 21 3

(1994)7 which is not dissimilar), except those which have one winding
wound over the other and are not separated by an earthed screen (type 2a
transformers).

9.1.5 The encapsulation process

The standard recognizes two types of encapsulation which are, first, embed-
ding, which is the situation where normally the encapsulant is poured over
the electrical circuits in a mould which is later removed, leaving the encap-
sulant as the outer part of the apparatus. (A mould may not be necessary,
the important fact being that the encapsulant is the outer part of the appa-
ratus and is in contact with the environment.
Second, potting, which is a similar process except that the mould remains
adhered to the encapsulant and forms the external surface of the appa-
ratus protecting the encapsulant from the environment. Where the mould
does not have a lid, it is acceptable to cover the encapsulant with another
encapsulant which adheres to the main encapsulant but protects it from the
environment.
The Standard defines the minimum thickness of encapsulant between
conductors and the surface of the encapsulant as 3mm but reduces this
requirement to lmm where the encapsulated apparatus has no single
surface greater than 2cm2 or is potted in a metallic mould or housing.
Where the mould is an insulator there is no minimum limit, provided the
mould is at least 1 rrun thick. In addition, the thickness of encapsulant round
a void, such as a relay box within the encapsulant, is required to be 3mm
unless the void is less than 1 cm3 when the thickness is reducible to 1 mm.
Anything exiting the encapsulant is a possible route for entry of explo-
sive atmosphere and must be effectively sealed. The Standard suggests that
5 mm of bare conductor within the encapsulant is acceptable to ensure effec-
tive sealing of connection cables which exit the encapsulant.
There are also minimum requirements for separation of bare live parts be
they within the same circuit, in two separate circuits or one circuit and the
frame (earth) of the apparatus and these are given in Table 9.1. Obviously
they only apply, as do all other requirements, where failure will affect the
protection concept.

9.1.6 Particular component problems
As in many of the other Standards, certain types of component are singled
out as having particular problems insofar as the protection concept is
concerned.

Fuses
To avoid problems with ingress of encapsulant into the fuse cartridge, fuses
are required to be enclosed types to BS 4265 (1977) (1984)’l or IEC 269.” It is
recognized that during rupturing such fuses might exceed the temperature