This arrangement ensures that the rectifier provides charge every time the exchange is taken into use. As the exchange often has a few consecutive days without use, the time clock is set to provide a charge for sufficient time to maintain the battery.
The 50 volt supply is also taken via a second rectifier to isolate the O/P wire from the exchange battery. An output only appears on the O/P wire when the rectifier is on. This output merely lights a lamp via a relay to say that the rectifier is on.
A "standby" rectifier is also provided and is very often used as its output tracks the battery usage very closely.