Parkend Exchange Final Selector Traffic Distribution Circuit
The traffic on the Parkend exchange is very low. Should a final selector become faulty, then all calls will try to use that selector and the exchange will be effectively isolated. This circuit distributes calls over the five final selectors so that should a call encounter a faulty final, the next try will use a different final.
The circuit uses a type 3 uniselector which "moves on" whenever the H relay releases due to there being no circuit via a busy final selector. The 250 ohm resistor acts as the battery testing condition for the final selector being allocated.
It was necessary to make a slight modification to the final selectors.