Page 289 - Introduction to Marine Engineering
P. 289

Electrical equipment  263

          neutral  systems have occasionally been  installed. The  insulated  neutral
          system can suffer  from  surges of high voltage as a result of switching or
          system faults which could  damage  machinery.  Use of the earthed system
          could result in the loss of an essential service such as the steering  gear as
          a  result of an earth  fault.  An earth  fault  on  the  insulated system would
          not, however, break the supply and would be detected  in the earth  lamp
          display.  Insulated  systems have therefore  been  given  preference  since
          earth  faults  are  a common  occurrence  on  ships and  a loss of supply in
          such  situations cannot be  accepted.


                                                             D.C.
                                                             emergency
                                                             supply
              From shore supply
               CD

               Emergency
               supply




              Step down transformer


          Main
          suppty"TT IT"""

              Lighting  Power
              loads     loads
                              Turbo  Diesel
                              alternator  alternator
         Figure  14.8  A.C. distribution  system


           In  the  distribution  system there  will be circuit breakers  and  fuses,  as
         mentioned  previously for  d.c.  distribution systems. Equipment  for  a.c.
         systems  is  smaller  and  lighter  because  of  the  higher  voltage  and
         therefore  lower  currents.  Miniature  circuit  breakers  are  used  for
         currents up  to about  100 A and act as a fuse  and a circuit  breaker.  The
         device  will  open  on  overload  and  also  in  the  event  of  a  short  circuit.
         Unlike  a  fuse,  the  circuit can  be  quickly  remade  by simply  closing  the
         switch.  A  large  version  of  this  device  is  known as  the  'moulded-case
         circuit breaker' and can handle currents in excess of  1000 A. Preferential
         tripping  and  earth  fault  indication  will  also  be  a  part  of  the  a.c.
         distribution  system. These  two items have been  mentioned previously
         for  d.c. distribution  systems.
   284   285   286   287   288   289   290   291   292   293   294