Automatic Transfer Switch (ATS)
An automatic transfer switch (ATS) transfers a load between a normal source, typically the utility supply, and an alternate source, such as a standby generator, when it detects loss or degradation of the normal supply. A controller monitors voltage and frequency; on failure it signals the generator to start, waits until the generator output is within acceptable limits, then transfers the load. When normal power returns and stabilizes, it retransfers after a time delay and allows the generator to run unloaded for a cool-down period. Transition modes include open transition (break-before-make), closed transition (make-before-break with momentary paralleling) and delayed transition. Relevant standards include IEC 60947-6-1 and UL 1008. Selection parameters include number of poles, rated current and voltage, withstand and closing current ratings, utilization category, transition type and switched-neutral requirements.
Examples
- Mains-to-generator ATS for a commercial building
- Closed-transition ATS for a hospital essential supply
- Dual-utility ATS feeding a critical distribution board