Q Using a combination of Uni-selectors and two motion selectors, draw a diagram of thousand line exchange and illustrate its working.

Ans:

The diagram for such an exchange is given in Figure below.  Every subscriber  is connected  to  a  single  rotary  pre-selector  switch at the exchange,  outputs  fromthis  switch  being  linked  to  a  bank  of  two-motion switches  are called 'group selectors'.  Out-puts  from  the  pre-selector  switches  of  a  whole  group  of subscribers  are  connected  together  in  parallel  as  that  group  of  subscribers  share  a single  bank  of  group  selectors.  Whenever  a  subscriber  lifts  his  telephone,  cradle switch causes a  circuit  to  be  completed  back to telephone exchange,  indicating that subscriber wishes to make a call. This causes pre-selector switch to step around till it finds a free group selector. Pre-selector switch then stops in this position and group selector is 'seized' by subscriber wishing to make a call.

In a 1000 line exchange, every subscriber has a 3 digit identification number which starts from 000 to 999. On seizing group selector, subscriber dials his first digit and  selector  switch  moves  up  to  the  appropriate row  on  switch  contact array. Every final selector has possibility of connection to 100 lines. 1000 lines are hence divided into 10 groups of 100 each, group being identified by first digit in subscriber's number. Vertical motion of the group selector thus selects a final selector in the group associated with first digit dialled. Every individual row of contacts or levels, of the  group selector is connected to a bank  of  final  selectors  associated  with  a  specific group  of  100  line  numbers. Having dialled first digit to select the appropriate group, group selector arm then automatically rotates in vertical direction till it finds a free final selector. In  final  selector,  both  directions  of  motion  are  under  the  control  of  subscriber's  dial  and,  after  dialling  two  further  digits  connection  is  established, providing called subscriber's to answer his telephone.