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       13.7.4 Requirements other than constructional
       features
           Mechanical and electrical interlocks
             An  industrial  load  having  a  connected  load
             requirement of more than 2000 kVA may normally
             call  for more than  one feeding  transformer  for
             limiting the fault level of the system, as discussed
             in Section  13.4.1(5). It may also have a standby
             emergency  source  of  supply. The two  feeding
             transformers,  although  they  may  be  identical
             electrically  and  suitable  for parallel  operation,
             are not supposed to run in parallel with a view to
             limiting  the fault  level. The emergency  source,
             as a result  of  different  electrical parameters,  is
             not run in parallel with any of the two incoming
             sources. To achieve the required safety by avoiding
             a parallel  operation,  it is essential to provide  a
             mechanical  or  an  electrical  interlock  or  both
             between  all  the  incoming  feeders.  Schemes to
             achieve the required safety interlocks are described
             in Section  13.7.5.
             When there are more than one sources of supply,
             it is recommended to distribute the loads also in
             as many sections as the incomers, and provide a
             tie-circuit between every two sections, to obtain
             more  flexibility.  Now  fault  on  one  section  or
             source  of  supply  will  not  result  in  the  loss  of
             power  to  the  entire  system.  Figures  13.16 and
             13.17 illustrate this type of distribution.                or straight through joints
                        transformers must be provided with   Figure 13.37(a)  Pressfit SMC/DMC shrouds for  busbar joints
           current limiting fuses at both ends.
           Control wiring  Wiring from supervisory or annun-
           ciator devices to the terminal blocks may be carried
           out with smaller wires, as may be recommended for
           such  devices.  However,  they  should  run  through
           separate wire bunches, and not through the bunches
           of control wires for easy identification and to remain
           unaffected by heat of control wires.
           For easy identification  and prompt maintenance  it
           is  mandatory  to  segregate  all  control  wires  when
           they  are  carrying  more  than  one  control  supply
           (e.g. at  different  voltages  and both  a.c. and  d.c.),
           and  run  them  in  separate bunches.  The  control
           wires must also be of different colours for different
           control  supplies.  The  colour  codes  have  been
           standardized for different control sumlies (refer to
           IEC 60445).
           Space heaters with temperature control  These
           are recommended with a view to eliminate condensa-
           tion of  moisture, particularly  when the switchgear
           is idle and the atmosphere is humid. The space heaters
           are normally rated for  Vr/fi,  40 W, single phase.
           They are located appropriately such as in the cable
           alley, and are switched ON when the switchgear is
           likely to be idle for a long period. The number of
           space heaters will depend upon the size and type of
           the  switchgear.  For  a  cubicle-type  panel,  it  is
           recommended  that  at  least  one  space  heater  be
           provided  in  each  vertical  panel.  They  should  be
           mounted at the lower portion of the panel for better
           heat  Circulation through  natural  heat  Convection.   Figure 13.37(b)  SMCIDMC shrouds for bus joints and tap-offs
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