The STS-3 is a solid-state, 3-phase, 4-pole switch which connects critical loads to 1 of 2 separate power sources; 1 source is primary or priority and the other is an alternative power source to ensure power continuity.

The STS 3 is fitted with 2 internal maintenance bypass switches enabling load transfer to supply without interruption to load. Key-operating locks ensure 2 bypass switches cannot be closed simultaneously so as to avoid connecting both sources to each other. The complete bypass isolation allows for maintenance of all serviceable components without interruption to the load. Control logic ensures in the case of an accidental bypass closure on the passive line, the load is automatically transferred to the line.

This switch is designed to switch between different AC input lines according to priority criteria and the technical characteristics of the input voltages. If lines are synchronised, the transfer time from one power source to the other is less than 4ms.