Automatic transfer switches
An automatic transfer switch, or ATS, changes the source supplying connected electrical loads without requiring an operator to perform each transfer. In a typical home standby system, the sources are the electric utility and a permanently installed generator.
A typical transfer sequence
The control system watches the normal supply. When that supply becomes unacceptable, a compatible standby system starts the generator. Transfer occurs after the alternate source meets the required conditions. When acceptable utility power returns, the system transfers back and the generator follows its programmed shutdown sequence. [1][3]
Timing and control responsibilities vary by equipment. The sensing logic may be distributed between the generator controller and transfer equipment. Consult the documentation for both pieces rather than treating a general sequence as the operating instructions for a particular installation.
Open transition and source isolation
An open-transition switch breaks the connection to the first source before making the connection to the second. A brief interruption is part of that process. Cummins describes its GTEC switch as using mechanical interlocking and break-before-make action to prevent the two sources from being connected through its power contacts. [2]
Closed-transition equipment can briefly connect both sources in an engineered application. It is a different system design, with additional coordination requirements. Do not assume an ATS provides uninterruptible power to sensitive equipment; that question requires examining the entire backup system. [1]
What determines compatibility?
Selection must account for the electrical service and the generator together. A 200-amp switch does not mean that the generator can deliver 200 amps. The generator's fuel-specific output and the supported load calculation determine the available backup power. [1]
| Specification | Why it matters |
|---|---|
| Voltage, frequency, and phases | Must suit the sources and connected loads |
| Current rating | Defines the switch's intended current capacity |
| Service-entrance suitability | Depends on the equipment's role in the service |
| Neutral arrangement | Must match the grounding and system design |
| Withstand and closing rating | Must coordinate with available fault current and protection |
| Control compatibility | Determines how starting and transfer functions work together |
Owner information and service
Keep the switch model, generator model, and commissioning records with the system documents. Know which circuits receive backup and whether load management can temporarily shed appliances. Transfer cabinets contain hazardous electrical equipment; installation, internal diagnostics, and repairs belong to qualified professionals.
Sources and further reading
- Eaton: transfer sequence, transitions, and specification considerations ↗www.eaton.com
- Cummins GTEC: source interlocking and open transition ↗www.cummins.com
- Generac: residential automatic transfer equipment ↗www.generac.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.