Static Transfer Switch (STS)

STS (Static Transfer Switch) is an automatic static switching equipment designed to transfer critical loads between two independent AC power sources without interruption or with a transfer time of less than a cycle (20ms). Static Transfer Switches use semiconductor technology to detect even the slightest power fluctuations and instantly switch to a more stable power source.

Unlike mechanical switches, an STS has no moving parts and operates extremely fast—often switching within less than 4 milliseconds, even under extreme temperatures and demanding operating conditions. The system is built using Silicon Controlled Rectifiers (SCRs), advanced control logic, and sensing circuits that constantly monitor both power sources.

In case the primary power source fails or becomes unstable, the STS immediately diverts the critical load to the alternate source to ensure uninterrupted operation of essential equipment.

How STS Works

A Static Transfer Switch includes control logic, metering circuitry, thyristor circuits, breakers, and bus assemblies. The control logic continuously measures voltage and current from both the preferred and alternate sources.

If the preferred source experiences failures or disturbances such as over/under voltage or abnormal current levels, the STS evaluates the stability of the alternate source. If the alternate source is within acceptable limits, the load is automatically transferred.

When both power sources are synchronized with acceptable phase error, the automatic transfer can occur in as fast as 6 milliseconds.