Page 403 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/377
6 For a positive ground connection and to provide and eliminate arcing between them. Otherwise it
total safety for the operator, a spring-loaded scraping- may develop hot spots and result in corrosion of the
type grounding assembly may be provided on each contacts in the normal operation. It may also lead to
trolley so that the moving part can slide on the fixed an eventual failure of the joint/contact.
grounding strip fitted in the assembly, and make
before and break after the power contacts have
engaged or disengaged respectively. Figures 13.2 N(Jtc
and 13.35 illustrate such an arrangement. The The main incoming male contacts are generally made of copper or
grounding contact may be of brass and silver plated brass and are either bolted or clamped on the vertical bus. Since the
bus is generally of aluminium. the contacts may form a bimetallic
(a good practice). It may be fitted on the tray of joint with the bu\bars and cause corrosion and pitting of the metal.
each module and permanently connected to the This may result in a failure of the joint in due courTe. To minimize
ground bus running vertically through each vertical metal oxidation and bimetallic corrosion, the contacts must be silver
panel and connected to the main horizontal ground plated.
bus. If the main incoming male contacts are made of aluminium
A positive grounding arrangement will mean alloy, which ic normally a composition of aluminium-magnesium
automatic grounding of the trolley as soon as it is and silicon, they must be provided with a coat of bronre. copper
mounted on the tray for racking-in and then remaining and tin to give it an adequate mechanical hardness and reriqtance to
corrosion. For more details refer to Section 29.2.5.
in permanent contact with the ground contact when
the trolley is fully racked into its service position.
7 In a draw-out MCC, it is recommended that shrouds 10 Each trolley is recommended to possess three distinct
are provided at the main incoming power contacts, positions of movement, as shown in Figure 13.36.
from where the vertical bus is tapped, to feed the i.e. ‘service’, ‘test’ and ‘isolation’:
draw-out module. Manufacturers may adopt different Service: this is the position of the trolley when it is
practices to achieve this. The most common is a fully inserted into its housing (module) and the power
DMC/SMC (Bakelite, which was more common and control contacts are fully made.
earlier, is now being discarded, being hygroscopic Test: This is the position of the trolley when the
and inflammable) gravity-operated drop-down shutter, power contacts are isolated but the control circuit is
which lifts automatically while the trolley is being still connected, because it is lapped directly from
racked in and slides down as the trolley is being the auxiliary bus. This condition is essential to
racked out. Refer to Figure 13.35, showing a feeder facilitate testing of control circuits with functional
with the shroud lifted. Figure 13.2 also illustrates a interlocks, without energizing the connected load.
few feeders with shrouds fitted. One of the feeders Isolation: This is the position of the trolley when
is shown with a lifted shroud. the power and the control circuits are both isolated.
8 Indicating instruments, lamps, pushbuttons, selector Depending upon the site requirements, sometimes
switches and reset knobs etc. are mounted on the the control circuit may be required to be still energized
trolley on a hinged auxiliary door, as shown in Figures for some test requirements.
13.2 and 13.12. 11 The feeder door should not close unless the trolley
The main outer door on the frame may have either is racked-in, up to the test position at least.
an opening to seat the auxiliary door of the trolley 12 The trolley should not permit its withdrawal when
on it or telescopic knock-outs for all such door mounts its switch is in the ON position.
to provide a peep-through type of aperture. On it 13 While racking-out the trolley, it should not be possible
are mounted the light and pushbutton tops. Figures to completely withdraw it unless it has reached the
13.2 and 13.12 illustrate this type of arrangement. ‘isolation’ position. In the isolation position there
The latter alternative provides a better arrangement, must be a ‘holding-on’ latch arrangement, to prevent
for it ensures a greater degree of protection. It has an abrupt fall of the trolley from its module. Figure
the most significant advantage when the trolley is 13.36 illustrates this feature.
removed from its module and the outer door can 14 Interchangeability of a module with another module
still be securely shut on the module to provide total of the same type and size is the basic requirement of
protection for the empty module from dust, vermin such panels.
and rodents as well as inadvertent human contact 15 Provision of an extra interlocking facility (peg and
with the live incoming terminals. In the other design, hole system) may also be essential to prevent
the outer door has a large knock-out to seat the interchangeability between two similar trolleys when
auxiliary door. which may remain open when the the circuits or the functions of the two otherwise
trolley is removed for repairs or replacement, and identical trolleys are different, and it is undesirable
expose the interiors. to interchange these trolleys.
9 Low contact resistance is desirable at all current- 16 Withdrawal of a draw-out circuit breakcr or a
carrying contacts, such as the busbar joints, between withdrawable switch or contactor will not be possible
busbars and the incoming fixed power contacts, unless they are in an OFF position.
between incoming fixed power contacts and incoming 17 Operation of withdrawable equipment, such as a
moving power contacts on the trolley and between breaker or components on a withdrawable chassis,
the outgoing moving and fixed power contacts etc. will not be possible unless it is in service, test. isolated
This is to ensure proper surface-to-surface contact or totally removed positions.