When power outages strike, automatic transfer switches (ATS) are the unsung heroes that keep critical systems running. They seamlessly switch power from a primary source (like the grid) to a backup generator, ensuring minimal downtime. But not all ATS systems work the same way. The two primary types of automatic transfer switches are Open Transition ATS and Closed Transition ATS—each designed for specific needs, budgets, and levels of power reliability. In this guide, we’ll break down how they work, their key differences, and which one is right for your home, business, or industrial facility.
1. Open Transition Automatic Transfer Switch (Open Transition ATS)
Open Transition ATS, also called “break-before-make” switches, are the most common and affordable type. They work by first disconnecting the primary power source, then connecting the backup generator—creating a brief (50–200 millisecond) power gap during the switch.
How It Works:
- Normal Operation: The ATS monitors the grid voltage. When it detects a failure (e.g., voltage drops below 80% of normal), it sends a signal to start the generator.
- Switching Sequence: Once the generator is stable, the ATS breaks the connection to the grid, waits a split second, then makes the connection to the generator.
- Return to Grid: When grid power is restored, the ATS reverses the process: disconnects the generator, reconnects the grid, and shuts down the generator.
Best For:
- Non-critical loads: Homes, small businesses, retail stores, or warehouses where a tiny power blip won’t cause major issues. Examples include lighting, HVAC systems, or refrigerators (which can tolerate a momentary outage).
- Budget-sensitive projects: Open Transition ATS costs 30–50% less than closed transition models, making it ideal for residential backup systems or small commercial setups.
Pros & Cons:
| Pros | Cons |
|---|---|
| Affordable and widely available | Brief power gap may disrupt sensitive electronics (e.g., servers, medical equipment) |
| Simple design, low maintenance | Not suitable for “mission-critical” systems |
| Works with most standard generators | May cause voltage spikes if the generator isn’t properly synchronized |
2. Closed Transition Automatic Transfer Switch (Closed Transition ATS)
Closed Transition ATS, or “make-before-break” switches, eliminate the power gap entirely. They briefly connect both the primary and backup power sources during the switch, ensuring seamless, uninterrupted power flow.
How It Works:
- Synchronization: Before switching, the ATS checks if the generator’s voltage, frequency, and phase match the grid. If they align, it “bumps” the generator into parallel with the grid.
- Zero-Downtime Switch: The ATS connects the generator before disconnecting the grid, so power flows continuously.
- Safety Features: Built-in protections prevent backfeeding (sending generator power back to the grid, which risks linemen’s lives) by ensuring the grid is stable before reconnection.
Best For:
- Critical infrastructure: Hospitals, data centers, airports, or manufacturing plants where even a millisecond of downtime costs thousands. For example, a data center hosting cloud servers can’t afford a power blip, as it would crash systems and lose customer data.
- Sensitive electronics: Labs with scientific equipment, medical facilities with life-support machines, or financial institutions with real-time trading systems.
Pros & Cons:
| Pros | Cons |
|---|---|
| Seamless power transfer (no downtime) | Expensive (2–3x the cost of open transition ATS) |
| Protects sensitive equipment from voltage drops | Requires specialized installation and generator synchronization |
| Reduces wear on equipment (no sudden power surges) | Higher maintenance costs (complex electronics) |
Open vs. Closed Transition ATS: Key Differences at a Glance
| Feature | Open Transition ATS | Closed Transition ATS |
|---|---|---|
| Power Gap | 50–200 milliseconds (brief outage) | 0 milliseconds (seamless) |
| Cost | $500–$2,000 (residential/commercial) | $3,000–$10,000+ (industrial) |
| Best For | Homes, small offices, retail | Hospitals, data centers, factories |
| Generator Requirement | Standard generators (no sync needed) | Synchronized generators (inverter-based) |
| Maintenance | Low (simple mechanical parts) | High (electronics, sync sensors) |
How to Choose the Right ATS Type
- Evaluate Your Loads: If you’re powering lights, TVs, or a home fridge, Open Transition ATS is sufficient. For servers, MRI machines, or production lines, invest in Closed Transition.
- Check Your Budget: Closed Transition ATS is a premium upgrade—only worth it if downtime costs exceed the extra expense.
- Generator Compatibility: Most portable generators work with Open Transition ATS, but Closed Transition requires a generator with synchronization capabilities (e.g., inverter generators).
Related ATS Terms You Should Know
- Manual Transfer Switch (MTS): Not automatic—requires manual flipping to switch power. Cheaper than ATS but only useful for hands-on users.
- Bypass Isolation ATS: Adds a manual bypass for maintenance, allowing you to service the ATS without shutting down the backup system. Critical for 24/7 operations like airports.
- ATS Controller: The “brain” of the system, which monitors grid voltage and sends signals to the generator. Advanced controllers (used in Closed Transition ATS) include sync-check and load management features.
Final Thoughts
Choosing between Open and Closed Transition ATS comes down to downtime tolerance and budget. For most homes and small businesses, Open Transition ATS offers reliable, affordable backup power. For critical systems where every second counts, Closed Transition ATS is non-negotiable. Whichever you choose, ensure it’s sized for your load (measured in amps) and compatible with your generator—this ensures safe, efficient power transfer when you need it most.
Post time: Aug-14-2025
