Static Transfer Switch (STS) vs Automatic Transfer Switch (ATS) for Data Centers
A static transfer switch (STS) and an automatic transfer switch (ATS) both move a load from a failed power source to a healthy one. The difference is speed and position. An STS uses solid-state SCRs to switch between two live sources, such as UPS A and UPS B, inless than 4 milliseconds, or about a quarter cycle. An ATS uses mechanical contacts to move facility loads from utility power to a standby generator, a sequence that NFPA 110 Level 1 systems must complete within 10 seconds.
So what happens when power drops for a few milliseconds? With an STS in the path, the IT load keeps running because the transfer finishes before servers lose power. During the longer ATS transfer, the UPS batteries carry the IT load until the generator takes over. That is why many Tier III and Tier IV facilities use both.
Understanding Transfer Switch Technology in Data Center Power Systems
An STS is a solid-state device that transfers a critical load between two independent, live power sources in less than a quarter cycle. It has no moving contacts, so the switching happens electronically through silicon-controlled rectifiers (SCRs).
How an STS Works
The STS feeds the load from a preferred source, usually the output of UPS A.
It monitors both sources continuously for voltage, frequency and phase angle.
When the preferred source sags or fails, the controller turns off its SCRs and fires the SCRs on the alternate source.
The load moves to UPS B in under 4 ms, well inside the 20 ms zero-voltage ride-through that the ITIC (formerly CBEMA) curve defines for IT power supplies.
Phase Synchronization and Inrush Protection
The two sources must sit within a set phase synchronization window before the STS allows a fast transfer. If they are too far out of phase, a well-designed STS delays or blocks the transfer. This protects downstream PDU transformers from inrush current, which can trip upstream breakers.
Where an STS Is Used
An STS is the standard way to give single-corded equipment the benefit of two UPS systems. Dual-corded servers already draw from both A and B feeds, but many network switches, legacy storage arrays and specialty devices have only one power cord. The STS lets those loads survive a failure of either UPS path.
Single AI racks such as NVIDIA's GB200 NVL72 are now designed around 120 kW, so one interruption affects far more compute per rack than before. The STS, its current rating and its downstream PDUs must be sized for these densities from the start.
Understanding Automatic Transfer Switch (ATS) Technology
An ATS is an electromechanical switch that moves a load from utility power to a standby generator when the utility fails, then back again when it returns. It is built for long outages, not for millisecond disturbances.
How an ATS Works
The ATS monitors utility voltage and frequency.
When the utility fails, it waits through a short time delay that filters out momentary dips, then signals the generator to start.
Once the generator reaches rated voltage and frequency, the ATS moves its contacts to the generator side.
When the utility is stable again, the ATS retransfers the load and sends the generator into a cool-down run.
In a data center, the UPS bridges the gap between steps 2 and 3. For life-safety loads,NFPA 110 Type 10 systems must restore power within 10 seconds, and data center generators are commonly designed to the same benchmark. UPS batteries or flywheels carry the IT load for that time.
Open, Delayed and Closed Transition
Open transition (break-before-make): The load is briefly disconnected during transfer. This is the most common and lowest-cost design.
Delayed transition: The switch pauses in a neutral position so motor and transformer fields decay before reconnection. This reduces inrush on large mechanical loads such as chillers.
Closed transition (make-before-break): Both sources connect for a brief overlap, usually under 100 ms, so re-transfer back to utility causes no outage. It usually requires approval from the local utility because the generator briefly runs in parallel with the grid.
Bypass-Isolation ATS
A bypass-isolation ATS adds a second switching path. Technicians can isolate and service the main ATS while the load stays powered, which supports the concurrent maintainability that Tier III facilities require.
ATS units for emergency and standby power are listed to UL 1008 in the United States.
Static Transfer Switch vs Automatic Transfer Switch: Key Differences
A UPS static bypass switch protects one UPS, while a standalone STS chooses between two separate UPS systems. Buyers often confuse the two because both use SCRs and both switch in milliseconds.
Feature
UPS static bypass switch
Standalone STS
Location
Inside the UPS cabinet
Separate unit downstream of two UPS systems
What it switches between
The UPS inverter and a raw utility (bypass) feed
Two independent, conditioned sources (UPS A and UPS B)
When it acts
UPS fault, overload or maintenance
Failure or disturbance on either upstream path
What the load gets after transfer
Unconditioned utility power
Conditioned power from the second UPS
Protects against
Failure inside one UPS module
Failure of an entire UPS, its switchgear or its distribution path
In short, the static bypass is a safety valve for a single UPS. It cannot give a single-corded load two independent power paths. If the design goal is 2N redundancy for single-corded equipment, a standalone STS is still required.
How STS and ATS Work Together in Data Center Power Systems
A UPS static bypass switch protects one UPS, while a standalone STS chooses between two separate UPS systems. Buyers often confuse the two because both use SCRs and both switch in milliseconds.
Feature
UPS static bypass switch
Standalone STS
Location
Inside the UPS cabinet
Separate unit downstream of two UPS systems
What it switches between
The UPS inverter and a raw utility (bypass) feed
Two independent, conditioned sources (UPS A and UPS B)
When it acts
UPS fault, overload or maintenance
Failure or disturbance on either upstream path
What the load gets after transfer
Unconditioned utility power
Conditioned power from the second UPS
Protects against
Failure inside one UPS module
Failure of an entire UPS, its switchgear or its distribution path
In short, the static bypass is a safety valve for a single UPS. It cannot give a single-corded load two independent power paths. If the design goal is 2N redundancy for single-corded equipment, a standalone STS is still required.
Two layers of the same defense: the ATS keeps the building powered, and the STS keeps each IT load on a healthy UPS path. A typical 2N power path runs in this order:
Utility service and medium-voltage switchgear
ATS, which selects utility or generator
UPS A and UPS B, which condition power and ride through the generator start
STS, which selects UPS A or UPS B for single-corded loads
PDUs and remote power panels
Racks and IT equipment
During a utility outage, the ATS starts the generator, and the UPS carries the load for those seconds. During a UPS or distribution failure, the STS moves the load to the other UPS path in milliseconds. Neither device can do the other's job.
How to Choose Between STS and ATS
Use an ATS wherever a generator backs up utility power and add an STS wherever a single-corded critical load needs two UPS paths. The questions below decide the details.
1. How Long Can the Load Tolerate an Interruption?
IT power supplies are designed to ride through about 20 ms at zero voltage, according to the ITIC curve. An STS transfers well inside that limit. An open-transition ATS does not, so IT loads must never sit directly on an ATS without a UPS in between.
2. Are the Loads Single-Corded or Dual-Corded?
Dual-corded servers can connect to A and B PDUs directly and may not need an STS. Single-corded devices need an STS, or a rack-mounted ATS for small loads, to benefit from 2N redundancy.
3. What Tier Are You Designing For?
Tier III requires concurrent maintainability, so bypass-isolation ATS units and dual UPS paths are common. Tier IV requires fault tolerance, which usually means 2N distribution with STS units for any single-corded equipment.
4. How Dense Will the Racks Get?
Size the STS and downstream PDUs for future rack density, not today's. With AI racks now designed around 120 kW, an STS sized for legacy 10 kW racks can become the bottleneck within one hardware refresh.
Procurement Checklist
Continuous current rating and short-circuit withstand rating
Transfer time and phase synchronization window
Transformer inrush control (primary-side vs secondary-side STS)
Transition type for the ATS (open, delayed or closed)
Bypass-isolation and maintenance access
Monitoring, event logging and BMS/DCIM integration
UL 1008 listing and local code compliance
Spare capacity for planned expansion
Common Mistakes When Selecting Transfer Switches
Most transfer switch problems come from choosing a device in isolation instead of as part of the full power path.
Treating the UPS Static Bypass as a Substitute for an STS
The static bypass only switches one UPS to raw utility power. It does not give single-corded loads a second conditioned source.
Putting IT Loads Directly on an ATS
Even a fast open-transition ATS interrupts the load for longer than the 20 ms IT equipment can tolerate. IT loads need a UPS between the ATS and the rack.
Ignoring Transformer Inrush Downstream of an STS
A primary-side STS that transfers between out-of-phase sources can trigger transformer inrush and trip upstream breakers. Specify inrush control or a secondary-side design.
Skipping Bypass-Isolation to Save Cost
An ATS without bypass-isolation must be de-energized for service. That forces a planned outage and breaks concurrent maintainability.
Buying on Initial Price Alone
The cheapest unit can cost more over its life through downtime, service limits and early replacement. Compare reliability, maintenance needs and spare capacity alongside price.
Frequently Asked Questions About Static Transfer Switch and ATS
What is the main difference between STS and ATS?
STS uses semiconductor technology to provide rapid transfer between power sources for critical loads. ATS typically uses mechanical switching to transfer power between utility and backup sources.
Can ATS replace a Static Transfer Switch?
No. ATS and STS serve different purposes. ATS manages generator and utility transfer, while STS supports fast transfer between redundant power sources for sensitive equipment.
Where is a Static Transfer Switch installed in a data center?
STS is commonly installed between redundant UPS systems and critical distribution equipment supplying sensitive IT loads.
Does every data center require an STS?
Not every facility requires STS technology. The need depends on uptime objectives, redundancy requirements, and equipment sensitivity.
Why do data centers use both ATS and STS?
Large data centers often use both technologies because ATS protects facility power availability, while STS improves continuity for critical IT systems.
Choosing the Right Transfer Switch for Your Facility
Choose an ATS to keep the facility running through utility outages, and an STS to keep single-corded IT loads on a healthy UPS path. Most high-availability data centers need both, sized for the rack densities they will run five years from now.
Need help specifying STS or ATS equipment for your data center? eINDUSTRIFY supplies critical power equipment for data centers and industrial facilities. Call us at 1 888 7747632 or email info@eindustrify.com to discuss your project with our team.
Tags: static transfer switch automatic transfer switch data center power redundancy UPS static bypass switch NFPA 110 transfer time
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