Previous page: The lines enter the building via the Main Distribution Frame

Subscribers line circuits

Shelf of line circuits (50 lines)

Line relays are arranged in 'cans' of 10 lines (1 L & 1K relay per line). The uniselectors are arranged above and below the relay cans in 2 rows of 25

Subscribers line circuits

What happens when you lift the handset?

A customers line is controlled by its Line Circuit. This is two relays and a uniselector. The uniselector is a semi circular arc of contacts (typically 24) and a set of wipers which rotate round the arc to make contact with any of the 24 outlets.

The wipers are metal arms carrying the incoming line. When someone lifts the handset the line circuit relays detect this and operate the uniselector. The uniselector hunts around its 24 outlets looking for one that is not in use. Each outlet leads to a 2 motion selector.

A line needs to be connected to a 2 motion selector before dialling can commence as the 2 motion selector is the basic building block of the exchange which processes the dialled digits.

Although lines could be wired directly to their own 2 motion selector this would be inefficient as most lines are idle most of the time. 2 motion selectors are complex and costly pieces of kit so there are only enough to handle the anticipated number of simultaneous calls. It's the job of the subscribers line circuit & uniselector to find a free 2 motion selector when a call is made.

Subscribers line circuits

Side view of uniselector

The wiper carries the incoming line from the line circuit relays and the outlets lead to the first group of 2 motion selectors

Next: Selecting the dialled number