S-Units were used to provide full facility working UAX junction circuits to and from the parent exchange via open wire carrier systems, or on metallic junctions which did not meet the required signalling standard.

In the AC2 and AC3 signalling systems used by the New Zealand Post Office, only one frequency (2280 Hz) was used and signalling was effected by pulses of this frequency in either direction. Discrimination between signals for all different circuit conditions was provided by the direction of transmission, length of signal pulse, and time of arrival during the call sequence.

AC2 was normally used on the trunk network and AC3 on the junction network, for example between a UAX and its parent exchange.

N.Z. 13 S-UNIT - GENERAL DESCRIPTION.

An S-unit was a B-unit type cabinet and was installed at a UAX to accommodate AC3 signalling equipment.

There were two basic types of S-units, a Group 1 and a Group 2. The Group 1 unit contained the common equipment - e.g., oscillators, etc., in addition to the junction equipment. The Group 2 unit contained junction equipment only. These units were originally designed for valve type AC3 signalling equipment but the later versions were specifically designed for the transistorised equipment which superseded it.

The S1 units were designed to be equipped with AC3 relay sets for either four bothway junctions, eight unidirectional (four incoming, four outgoing) junctions, or a combination of both. They were also equipped with two 2280 Hz. Oscillator Relay Sets and the Positive Battery Supply Relay Sets.

The S2 units were designed to be equipped with AC3 relay sets for either six bothway junctions, 12 unidirectional (six incoming, six outgoing) junctions, or a combination of both.