Page 239 - Electrical Installation in Hazardous Area

General requirements for explosion protected apparatus 203

Switchgear

Switchgear is a potential source of ignition if failure occurs or if it is
operated inadvertently. For these reasons, the Standard contains limitations
on the types of switchgear (i.e., no oil-filled switchgear is permitted because
of the possibility of discharge of burning oil if the gear fails). In addition, it
requires locking capability to prevent inadvertent operation and, in the case
of disconnectors, all poles (including the earth) are required to be discon-
nected to prevent sparking due to circulating earth currents during such
activities as maintenance.
Warning labels are also required for such things as off-load isolators
to prevent their operation on-load, which could cause an ignition, and
for doors and covers giving access to automatic switchgear, unless those
doors or covers are interlocked. Where some parts need to remain ener-
gized within cabinets containing switchgear after an interlock has operated,
those parts must remain protected by a recognized protection concept or
be protected to IP20 (BS/EN 60529').

Fuses

Enclosures containing fuses are required to be either interlocked, to prevent
opening while the fuses are energized, or are required to be fitted with
a warning label to identify the danger of removing fuses live in a
hazardous area.

Plugs and sockets

In general, plugs and sockets should not be used in a hazardous area but
where they are, the ideal situation is one where they are interlocked to
ensure that they are de-energized before separation. BS/EN 50014 (1993)'
recognizes this as not always possible and allows as the alternative a
label with a warning against energized separation. If de-energization is
not possible, a warning not to separate in a hazardous area is permitted,
although this is felt to be of little value for fixed equipment. It does,
however, clearly identify the acceptance of energized separation at times
when the absence of explosive atmosphere can be guaranteed.
In addition, it is recognized that a socket can be designed so that during
removal of the plug, the arc is in an enclosure which has protection
by one of the recognized protection concepts and, after separation, the
energized conductors in the socket remain so protected. Typically Ex-d
during separation and Ex-e after separation is complete is possible. It
should be noted that this relaxation applies to the socket (fixed part of
the arrangement) only and not the plug.