By S. Hussain Ather
Updated Mar 24, 2022

Automatic transfer switches (ATS) play a vital role in maintaining power continuity during emergencies. When a public utility fails—such as during a natural disaster—an ATS automatically redirects the load to a backup generator, preventing downtime for critical facilities like hospitals.

Key Principles of Automatic Starting Control

ATS systems are designed to detect a loss of utility power and initiate a generator start sequence. The process typically follows these steps:

1. Utility power drops, triggering the ATS to start the generator.
2. Once the generator reaches operating speed, the ATS transfers the load from the utility to the generator.
3. When utility power returns, the ATS switches back to the main supply and signals the generator to shut down.

By isolating the generator from the distribution panel until it is fully operational, ATS protects the generator from overload and prevents back‑feeding into the utility grid.

Load Management and Soft‑Load Techniques

Advanced ATS models include load‑shedding and priority‑adjustment functions. These features allow critical circuits to retain power while less essential loads are temporarily disconnected, optimizing generator use and preventing overheating.

Soft‑load transfer—also known as “make‑before‑break” switching—minimizes voltage ripple and allows the generator to take over seamlessly, which is essential in environments where even a brief interruption can be hazardous.

Building Your Own ATS‑Controlled Circuit

Designing an ATS requires a solid background in electrical engineering. While hobbyists can create basic control panels using DIN rails, busbars, and LEDs, safety protocols and proper grounding are non‑negotiable. The typical steps involve:

1. Mounting circuit breakers on a DIN rail enclosure.
2. Installing neutral and ground busbars with appropriate insulation.
3. Connecting the busbars to the load and generator inputs.
4. Integrating an ATS module and an AC meter to monitor voltage and current.
5. Testing for overheating, voltage drops, and proper isolation before deployment.

Using Multiple Generators with a Single ATS

Large facilities may employ several generators to support distributed loads. Each generator is linked to its own ATS, yet the system can coordinate them as a single virtual source. Synchronization—ensuring voltage differences stay below 5% and frequency differences under 0.2 Hz—is critical for smooth transitions.

Types of Automatic Transfer Switches

There are several ATS configurations, each suited to different applications:

• Open‑Transition (Break‑Before‑Make) – Disconnects from the utility before connecting to the generator, preventing back‑feeding.
• Closed‑Transition (Make‑Before‑Break) – Maintains power during the switch, ideal for sensitive equipment.
• Static Transfer Switches – Use semiconductor devices (e.g., silicon‑controlled rectifiers) for instant power transfer.
• Soft‑Load ATS – Gradually ramps generator power to match the utility, reducing voltage spikes.

Motor Starters and Inrush Current Considerations

When sizing an ATS, engineers evaluate the inrush current of motor starters. The ATS must accommodate the peak surge that occurs when an AC motor is energized, ensuring the generator can supply sufficient current without tripping protective devices.

Commercial and Residential ATS Solutions

Residential systems often use 100 A or 200 A ATS units from manufacturers like Generac, with prices starting around $600. Commercial installations may involve larger, multi‑bus ATS designs capable of serving data centers, hospitals, or industrial plants.

Installation Overview for Generator‑Powered ATS

Installing an ATS involves:

1. Integrating the ATS into the building’s main service panel.
2. Connecting the generator to the ATS input and configuring synchronization settings.
3. Programming the ATS logic for automatic start, transfer, and shutdown sequences.
4. Implementing safety interlocks, alarms, and monitoring for load loss.
5. Conducting thorough testing and commissioning to certify reliability.

Reputable manufacturers—APC, Dell, Cummins Power Generation, General Electric, and Western Telemetry—provide ATS solutions tailored to specific industry needs, along with post‑installation support.

By deploying an ATS, facilities can guarantee that critical operations continue uninterrupted, even when external power sources fail.