Page 252 - Electrical Installation in Hazardous Area

21 6 Electrical installations in hazardous areas

this makes little difference as the differences between the British Standard
and the IEC Standard centre around the restrictive breathing concept which
does not lend itself to oil immersion in any event. Chapter 15 of this book
should be consulted for the basic apparatus construction requirements.

9.2.2 Containment of the oil
The oil is required to be contained within the apparatus by one of the
following two methods. It may be within an apparatus sealed at the manu-
facturing stage, in which case the enclosure must be capable of withstanding
the maximum internal pressure which occurs at maximum temperature
with the apparaw in operation and the oil at maximum level. The Standard
requires a pressure relief device installed at the time of manufacture which
operates at 1.1 times that pressure, and is sealed to prevent interference.
Sealing in this context does not necessarily mean fusion-type sealing and
gasketted enclosures are acceptable for this purpose. While sealing of enclo-
sures is not specifically identified in BS 5501, Part 2 (1977)14 as a means of
oil containment, the wording in that Standard is sufficiently vague to permit
it. Sealed enclosures are subject to overpressure tests at 1.5 times the setting
of the pressure relief device and leakage tests at reduced pressure to ensure
their strength and sealing capabilities. These tests were not required by BS
5501, Part 2 (1977)14.
Alternatively, it must be in an enclosure which has at least an IP rating
of IP66 to BS/EN 60529 (1991)17 (see also Chapter 8). In this case, there
must be a device to permit breathing of vapours from the oil without any
pressurization of the enclosure and the breathing device will have to have
a desiccant to prevent the ingress of water or water vapour which will
degrade the oil. The Standard requires the breathing device to achieve at
least IP 23 to BS/EN 60529 (1991)17. While this is a fairly low level of ingress
protection, care needs to be taken to ensure that any breathing device is
suitable for intended locations of installation. If, for example, the apparatus
is in normal outdoor process plant locations hosing of those locations must
be expected and the breathing device will need some form of downward
facing outlet to stop ingress of water which would overburden the desiccant.
A typical arrangement is shown in Fig. 9.1.
Non-sealed apparatus is required to have a trip which isolates the
electrical supply to the apparatus in conditions of fault (internal arcs or
excessively high temperatures caused by significant overcurrent due to
internal faults) where oil decomposition is likely. Because the circumstances
giving rise to this problem are almost certain to produce either transient
or continuous overcurrent, an overcurrent trip is the most likely method
of achieving this, but it must also be noted that the trip must not be self
resetting or capable of being reset remotely. Such a trip must only be capable
of being reset locally and manually. Non-sealed enclosures need no leakage
test but must be capable of withstanding a pressure test at 1.5 x 105 N/m2
absolute to ensure they have sufficient strength. This test was not required
by BS 5501, Part 2 (1977)14.