Page 247 - Electrical Installation in Hazardous Area

Protection concepts 'm', '0' and '9' 21 1

necessary if the encapsulated apparatus is intended for use in a moist
environment. In reality the water absorption, test will be necessary in all
cases as manufacturers would find the limitations to dry environments very
restrictive for general usage purposes.
Both tests require discs of encapsulant of 50mm diameter and 3mm
thickness. For the electric strength test the disc has two 30mm diameter
electrodes placed centrally one on each side. The disc is then heated to a
temperature equal to the maximum temperature of its specified operating
range and 4000 V at nominally 50 Hz is applied between the electrodes. Any
flashover or insulation breakdown within 5 minutes means that the encap-
sulation is not suitable for use in association with BS 5501, Part S3 (1988).
For the water absorption test, dry samples are weighed and then
immersed in tap water at laboratory temperature (23 "C) for 24 hours. After
drying and reweighing, the weight must not have increased by more than
1 per cent.
Encapsulation may be hard or soft but in the latter case it is likely that it
would need to be in a mould which formed part of the apparatus.

9.1.3 Types of apparatus for encapsulation

Examination of the requirements of BS 5501, Part 83 (1988) leads imme-
diately to the conclusion that the concept is intended for 'light current'
electrical equipment and instrumentation only.
Although the Standard specifically only limits apparatus to that which
operates below 11 kV, it also states that apparatus must be capable of oper-
ation within the terms of the Standard from a supply of 4000 A Prospective
current capability. This prospective is typical of the requirements which
were placed on mains-powered instrumentation in the protection concept
in BS 5501, Part 7 (1977).6 This requirement was reduced to 1500 A in BS/EN
50020 (1995)7 (the second edition of the Intrinsic Safety Standard) because
of the use of standard IEC fuses to BS/EN 60127, (1991) Parts 1,2 and 3' of
which are identified and deal with miniature fuses. Thus, the requirement is
typical of one which is applied to instrumentation and low-power electrical
apparatus.
It is highly unlikely that switchgear could satisfy the prospective current
limits specified above, or the 4000 A specified in BS 5501, Part 8 (1988) and,
in any event, it is not considered likely that switchgear which has to be
destroyed to gain access would be economical.
Luminaires which have to be destroyed to change lamps or tubes would
not be economical and only small low-voltage lamps in such things as
annunciators are likely to be suitable for protection in accordance with this
concept .
Although the Standard contains requirements for encapsulation of stator
windings of rotating machines, the magnetostrictive forces involved do not
lend the protection concept to rotating machines of significant size, and
other more historically applied techniques are proving adequate.