The telephone equipment connected to the exchange must be powered.
   The switching equipment in the exchange also has to be powered.
 

    The exchange is therefore to the mains electricity network, and rectifiers
    etc. , ensure that the correct voltage is maintained. For the sake of security,
    the exchanges always have to have some form of reserve power in case the
    main power fails. This is often provided by a bank of accumulators, which
    normally receive a maintenance charge from the mains. Particularly important
    exchanges may also have diesel-driven generators in reserve.
 
 
 
 
 

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