Transfer switch resources
How static and automatic transfer switches work, how to choose and size one, and how to install it. Written by the engineers who build them.
What Is a Static Transfer Switch?
A static transfer switch moves a critical load between two independent AC sources in milliseconds, using SCRs, not contacts. How it works, what to check.
STS vs ATS
Static transfer switches use SCRs; automatic transfer switches use moving parts. Compare transfer time, out-of-phase transfers, bypass and uses.
How to Size a Static Transfer Switch
Convert kW or kVA to amps, add margin and choose a 100% rated static transfer switch. Includes a sizing calculator.
Rack Mount vs Wall Mount STS
Choose a static transfer switch enclosure by where the load is and how much current it draws. Compare rack-mount, wall-mount and free-standing STS options.
Single Phase vs Three Phase STS
Match a static transfer switch to the supply: 2-pole single phase, 3-pole three phase, or 4-pole with a switched neutral. When each applies and why.
Static Transfer Switch Bypass Overview
Why every static transfer switch needs a bypass, maintenance bypass versus bypass isolation, and how the TwinSource 7-position selector works.
Transfer Time and Source Quality Basics
What a load can ride through, what makes up transfer time, and why out-of-phase performance matters. Includes an interactive waveform and the ITIC curve.
Installation
What to check before installing a rack-mount or wall-mount static transfer switch, the installation sequence and the commissioning tests.
Implementation
How to connect static transfer switches to single-cord, dual-cord and super-critical loads, and why a point-of-use switch covers more than a central one.
Where to Start
Seven decisions before buying a rack-mount transfer switch: rating, power factor, criticality, fleet MTBF, bypass, verification and fault protection.