What Is the Difference Between ATS and STS?
- ATS vs. STS: Key Differences at a Glance
- How an Automatic Transfer Switch Keeps the Power Running
- How a Static Transfer Switch Responds to Power Loss
- The Device That Fits Your Facility Depends on the Load
- Scenarios Where Pairing Both Devices Makes More Sense
- Make the Right Call Before the Next Power Failure
- FAQs
Power reliability is a major concern for any facility. When the primary source of power fails, it is necessary to have a backup source that can be switched on instantly. Two devices handle this job: the Automatic Transfer Switch (ATS) and the Static Transfer Switch (STS). They share the same goal but take very different approaches to achieve it.
ATS vs. STS: Key Differences at a Glance
Before diving into each device in detail, a side-by-side comparison helps frame the broader picture.
| Feature | Automatic Transfer Switch (ATS) | Static Transfer Switch (STS) |
| Switching speed | 10 ms to several seconds | Under 4 ms |
| Core components | Electromechanical (relays, contactors) | Solid-state (thyristors, SCRs) |
| Transfer interruption | Brief gap is possible | Near-seamless |
| Maintenance needs | Moderate (moving parts present) | Lower (no moving parts) |
| Typical cost | Lower | Higher |
| Best pairing | Standby generators | Dual utility feeds or UPS |
| Common applications | General commercial, industrial | Data centers, medical, broadcast |
This table reflects general patterns. Actual performance depends heavily on installation quality and how well the device matches the load.
How an Automatic Transfer Switch Keeps the Power Running
The ATS is one of the most widely used devices in backup power management. When it detects a loss of grid power, it automatically shifts the electrical load to a backup source. No one needs to flip a switch or make a call. The transition happens on its own.
The Mechanical Transfer Process
The components used in an Automatic Transfer Switch are electromechanical, which means they will mechanically switch from one power source to another. This means there will be a brief time in which power is lost as the switch happens. This time can be anywhere from 10 milliseconds to several seconds, depending upon the switch.
In most cases, this time is perfectly acceptable, and people will merely see lights blink as power is restored. In some situations, this time can be problematic, particularly for systems that require continuous power output. In this instance, even a half second can be problematic.
Common Applications
The generator automatic transfer switch is one common solution for buildings that utilize diesel or gas-powered generators as backup power sources. This includes hospitals, office buildings, warehouses, and factories, just to name a few examples. The automatic transfer switch for generator use is cost-effective, familiar to electricians, and can support large loads for long periods of time.
Modern home backup systems have made this pairing more accessible than ever. For instance, the EcoFlow DELTA Pro Ultra, with the EcoFlow Smart Home Panel 2, provides automatic switch over in 30 milliseconds or less as part of an entire system. This system provides from 6 kWh to 90 kWh capacity and supports generator charging as one of its five input options, making it suitable for homes that require multiple layers of power backup.
The main advantage to this solution is that it is simple to use, comes in all sizes, is easy to maintain, and has been proven for decades in all types of industries.
How a Static Transfer Switch Responds to Power Loss
Having covered the ATS, let's take a moment to consider the STS, which takes a fundamentally different approach. Instead of relying on moving parts, the STS makes use of solid-state electronic components. These include thyristors or silicon-controlled rectifiers.
Speed Through Solid-State Technology
The most immediate advantage is switching speed. A static transfer switch can complete a transfer in under four milliseconds. That is fast enough that many sensitive loads never register the event at all. There are no physical parts clicking into place, no mechanical delay, and no audible sound during the transition.
This speed is critical for equipment that cannot tolerate even a brief power gap. Servers, precision manufacturing tools, and certain medical devices all fall into this category. Any system where a fraction-of-a-second interruption could corrupt data or halt a process will benefit from the near-seamless response of an STS.
Fewer Moving Parts, Different Maintenance Needs
Because the static transfer switch has no mechanical components, it generally requires less maintenance over time. There is simply less to wear out. That said, solid-state components come with their own sensitivities. Heat and electrical surges can degrade them, so thermal management and surge protection are important parts of any STS installation.
The Device That Fits Your Facility Depends on the Load
Now that both devices are clear, the practical question is which one belongs in a given setting. Both protect against power loss, but they do so at different speeds, costs, and levels of complexity.
When ATS Is the Right Fit
If your facility can handle a short interruption during a power transfer, an automatic transfer switch is usually the more practical choice. It handles large electrical loads well, pairs naturally with generators, and is easier to service in most environments.
A generator automatic transfer switch works particularly well for general commercial buildings, retail spaces, and industrial facilities where brief outages do not cause lasting harm. The upfront and ongoing costs are also lower, which matters in large installations.
When STS Is the Better Option
If your equipment cannot afford even a brief gap in supply, the static transfer switch is worth the added investment. Data centers, broadcast facilities, and precision medical environments are common examples. These operations need power continuity that mechanical switching simply cannot guarantee.
The STS is also suited for dual-utility configurations, where two live power feeds are available simultaneously. The device shifts between them instantly if one feed degrades. This differs from the typical automatic transfer switch for generator setup, where one source is always a standby that must start up before delivering power.
Scenarios Where Pairing Both Devices Makes More Sense
There are a few situations that need to be looked at in more detail because the correct answer is not necessarily one device or the other.
The first scenario is that of a facility that has dual utility feeds and a generator backup. In this case, the STS is used to handle the high-speed transfer between the two live feeds, and the ATS is used to handle the transfer to the generator if both utility feeds fail. They function at different levels of the same protection strategy.
The second scenario is that of pairing the STS and the standby generator. While it is technically possible to pair the STS and the standby generator, it is not advisable. The generator takes time to come up to stable voltage and frequency, regardless of how quickly the static transfer switch can make the transfer. The UPS bridges this time delay. The STS is best used when both sources of power are live and stable.
The third scenario is that of a facility that overbuilds protection for a non-sensitive load. Installing an STS in a warehouse or retail facility is probably unnecessary and is overkill. The ATS for generator use handles those environments well and costs considerably less.
Make the Right Call Before the Next Power Failure
The ATS and STS are designed to ensure your power is always on even when the grid is out. The ATS is the right choice for a reliable and cost-effective backup power solution, especially when paired with a generator set. The STS is the smart choice when every millisecond counts and your equipment is critical. For homeowners looking for a convenient place to begin, the EcoFlow DELTA Pro Ultra offers a scalable, UL9540-certified system that integrates automatic switchover and supports multiple charging sources, including solar and generators, all in a single unit.
FAQs
Q1: Can an ATS and an STS Work Together in the Same System?
The answer is yes, as this is a viable and feasible option in some critical power systems. The ATS manages the generator backup power supply, and the STS manages the switching between dual utility power sources or a UPS supply. If they work together, they can operate at different tiers of the same protection setup, so as to complement each other rather than conflict.
Q2: Which Device Tends to Be More Reliable Over Time, ATS or STS?
Both are highly reliable if well-maintained and installed. STS has fewer mechanical parts, which makes it less prone to mechanical wear and tear. However, it is more sensitive to heat and voltage spikes, depending on how well thermal protection is implemented. On the other hand, the ATS has been well-proven in heavy-duty applications and is generally easier to maintain and inspect. Ultimately, reliability is more a function of installation quality rather than device type.
Q3: Is an STS Suitable for Use With a Generator?
Yes, it is technically possible, but it is generally not recommended. The reason for this is that generators take time to start and stabilize to a safe voltage and frequency, even though the STS switch itself takes only four milliseconds to switch over. In most applications, this delay is bridged by a UPS. However, for STS to work well, both sources must be alive and stable, which is normally not the case for a standby generator.
Q4: How Often Should an ATS Be Tested?
For most commercial and industrial applications, it is recommended that each ATS be tested at least once per month. Each test should involve simulating a power failure and verifying that the load transfers successfully to the backup source and then back to the main source once grid power is restored. More frequent tests may be necessary for mission-critical applications. It is also recommended that each test be recorded in order to identify any trends in performance and detect possible problems before they become actual failures.