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It is possible to produce a more economic scheme than the one uniselector per customer's line so long as the customers connected to a 50 point linefinder scheme are all low usage. In the main, residential customers are suitable for connection to the scheme.

In the 50 point linefinder scheme the low calling rate customers are connected to line relays which in turn are connected to the outlets around a bank of five 50 point linefinder uniselectors. The line relays can be used by an ordinary customer's line or a pair of shared service lines. In practice this meant that up to 75 customers could be connected via the five linefinders in the group.

Each linefinder is then connected to a sub's uniselector inorder to gain access to a first selector.

The start conditions from the line circuits are connected into five groups of ten, each group starting a preferred control relay set. If the preferred relay set is already busy, the start signal is extended to the next free control relay set. There is no allotter to allocate the linefinder to be used.

Simultaneous calls in different line circuit groups start the linefinders hunting in those groups although the switching order will be at random.

The linefinders are non-homing so that if a rapid series of calls is made from one customer's line, then the mechanisms do not hunt.

Note : If a shared service customer provides a calling loop but not a calling earth (say on clear down on incoming calls) the SW relay will not be operated. This connects a circuit to energise the thermal relay LFB which operates after 30-60 seconds.
LFB1 operates relay PG.
PG2 and PG5 connect relay LFA to operate to line to monitor the loop.
PG3 removes the calling earth on the outgoing - wire to release the selector hunter and first selector and lights a PG alarm.
When the loop is removed relay LFA releases and restores the control relay set to normal.

Page provided by John Bathgate This page was last updated on 9th May 2008