STS – Static Transfer Switch are automatic static switching equipments designed to transfer electric loads between two independent AC power sources without interruption.
The static transfer switch (STS) is an electrical device that allows instantaneous transfer of power sources to the load. This superior switching time means that if one power source fails the STS switches to the back-up power source so quickly that the load never recognizes the transfer made.
Fast-acting STS are increasingly being applied to protect large loads and entire facilities from short-duration power disturbances. These products use solid-state power electronics or "static" switching.

The basic STS unit consists of three major parts :
• Controls and metering
• Silicon-controlled rectifiers
• Breaker/bus assembly
The STS monitors two power sources and automatically shifts to the better one on sensing the failure or degradation of either source. There is no wear and tear associated with this process.
STS are applied to protect loads and entire facilities and can be used as a stand-alone system or in conjunction with power distribution units or uninterruptible power supplies (UPS).
STS is applied to the critical equipment or to groups of loads or at the service entrance of a building.
A STS is often employed synergistically with UPS and other power quality equipment types for applications. For example :
• Data centers typically isolate the critical load and may employ STS with dual-redundant UPS as the power sources
• An industrial plant may choose to use STS with dual-redundant UPS as the power sources
• An industrial plant may choose to use STS on isolated groups of critical process equipment with UPS as the power sources
The same industrial user may instead choose a facility-scale system to protect the entire plant, particularly if the critical process cannot be easily isolated from other independent manufacturing steps.

Some application areas where STS are used:
• Critical IT operations
• Internet data centers
• E-commerce businesses
• Data centers
• Telecommunication centers
• Network operations centers
• Optic Fibre nerve center
• Process control
• Airport security system
• Commercial buildings
• Power generation plants
• Power quality and industrial parks
• Automated manufacturing
• R&D labs
• Semiconductor industry
STS has the design goal of protecting an entire building or facility from power disturbances originating from utility feeders.
Surveys have demonstrated that the greatest number of power disturbances is the result of faults on utility systems that are cleared in less than 10 seconds.
Fast switching of an STS to an alternate feeder will solve many of these problems especially if the alternate feeder is on a different substation transformer.