Page 388 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 131363
atmosphere. As a switch, a contactor or a breaker produces
an arc during a switching operation, an explosion may
occur within the enclosure on a re-switching. It is also
possible that when the enclosure door is opened to check,
test or replace a component, contaminants may have
entered the enclosure. It is also likely that on a re-closing
of the feeder door, the closure is not perfect due to human
error and contaminants have leaked into the enclosure.
All this may lead to an explosion on a re-switching.
Since it is not practical to manufacture a flameproof
enclosure due to its size and bulk and the number of
knockouts and openings on the doors for switches,
metering, indicators, and pushbuttons (PBs) etc., it is
common practice to locate these assemblies some distance
from the affected area in a separate well-ventilated room.
Depending upon the location and intensity of contamina-
tion, it may be permissible to meet the requirement by
using a pressurized enclosure by maintaining a positive
pressure inside the enclosure similar to that for motors
(Section 7.13.3). When there are many switchgear
With front doors open assemblies, the room itself can be pressurized, which is
safer and easier. Small enclosures, however, such as a
PB station, switch or a switch fuse unit or an individual
starter unit etc., which can be easily made of MS plates
or cast iron, as discussed in Section 7.13, can be mounted
in the hazardous area while the main MCC can be installed
in the control room, away from the contaminated area
and from where the process can be monitored.
Note
Avoid soldering of cable and wire terminal joints at such installations,
which may loosen with temperature and time.
Excessive vibrations and seismic effects
Installations prone to heavy vibrations and shocks such
as on the mounting platform near a large power-generating
unit, on a ship, locomotive and similar installations may
require provisions to absorb shocks and dampen vibrations
to prevent loosening of components and wire connections
in a switchgear or a controlgear assembly. Some eco-
nomical methods to achieve this can be by providing:
Anti-vibration pads at the foundation
With front doors closed 0 A rubber pad between the bottom of the assembly and
its base or
Figure 13.28 A typical compartmentalized outdoor-type panel Mounting pads below the base frame.
(Courtesy: ECS)
which remain the same as for motors conforming to IEC But shocks of a seismic nature are different and may be
60079-1, are also discussed in Chapter 7. Some of the more violent. They may require a special steel structure,
vital requirements that a flameproof controlgear or forming the switchgear or the controlgear assemblies, to
switchgear assembly must fulfil are, however, as follows: sustain the shocks without damaging or loosening
components and wires mounted inside the assemblies. It
1 To prevent an electric arc or flame from the enclosure may be possible to achieve this by using thicker sheets
escaping to the atmosphere. for the enclosures and providing additional reinforcement,
2 To withstand, without distortion, an explosion, within wherever necessary, and exercising special care in
the enclosure. selecting and fixing the hardware. The degree of
reinforcement and other provisions to make the whole
An internal explosion is normally a consequence of assembly and inside mounts suitable for a particular
absorption during a switch-on after a delayed shutdown. intensity of seismic effects will depend upon the probable
When an enclosure is switched OFF after normal running, seismic conditions in that area. This subject is dealt in
the internal air cools and absorbs contaminants from the with more detail in Section 14.6.