STS can be automatic
Static names the power-switching technology. Many STS units monitor inputs and transfer without an operator.
Critical power selection guide
Static Transfer Switch vs Automatic Transfer Switch: Which Fits Your System?
An STS uses electronic parts to move a load between two AC sources that are already ready. An ATS can watch two sources and run a utility-to-generator change. Compare the whole power path, not only the switch time.
STS INPUTTwo ready sourcesBoth inputs must meet the exact product's source limits.
ATS ROLEAutomatic source controlMay monitor, request generator start, check sources, transfer and retransfer.
TIME RULEMeasure the whole eventSource startup can matter more than switch motion.
DESIGN RULEOne-line firstLoad, sources, neutral, fault duty and bypass drive selection.
Generic switchgear room, not an STS or ATS product photo. Image: P199, public domain, Wikimedia Commons. Cropped for presentation.
Direct answer
Choose from source readiness and load ride-through
The words “static” and “automatic” describe different things.
Use an STS when two acceptable AC sources are already present and the load needs a very short, model-specific transfer.
Use an ATS when the system must monitor normal and alternate sources and manage the transfer sequence. A generator-backed ATS may issue a generator-start signal and then check the alternate source output. If the load cannot ride through that start time, add UPS, storage or another ride-through layer.
ATS is not one mechanism
ATS products may use contactor, molded-case or power-frame mechanisms. Transition modes also vary.
STS is not stored energy
It cannot create an alternate source. A missing or out-of-range source gives it nowhere safe to transfer.
Closed ATS is not STS
Closed transition briefly overlaps suitable sources. A fast STS may use break-before-make with no overlap.
Do not buy from “sub-cycle versus several seconds.” Those phrases can compare a static switch between two live UPS buses with a generator system whose alternate source must start. They describe different events.
Terminology boundary
The acronyms overlap, but the product categories do not
First identify the function. Then identify the switching technology, transition mode and exact product standard.
ATS monitors and controls the source change
An ATS keeps a load on the chosen source when conditions are safe. Its controller checks voltage, frequency and other set limits.
In a utility-generator system, it may issue a start signal, wait for stable power, transfer the load, return it later and run cooldown. The ATS name alone does not tell you the switch type or power gap.
STS uses solid-state power switching
An STS often uses silicon-controlled rectifiers (SCRs). These electronic parts move the load between two AC inputs without moving main contacts in the normal path.
It is often used between two UPS-backed buses and a sensitive load with one power cord. Both sources and the load must fit the limits of the exact model.
ATS and STS products do not use interchangeable product standards. UL 1008 excludes solid-state static transfer equipment, while UL 1008S has a separate scope for solid-state transfer switches. IEC 60947-6-1:2026 also excludes static transfer systems covered by IEC 62310. An STS may operate automatically, but the exact product file and intended use must match the application.
Two different event paths
How ATS and STS typically work
These flows explain the logic. Exact sensing, timing and operating states come from the selected product and system design.
Typical generator-backed ATS event
The alternate source is not ready when the outage begins.
Detect an unacceptable normal sourceThe controller applies its sensing limits and time delay.
Request generator startThe engine starts, builds voltage and reaches stable frequency.
Accept the alternate sourceThe controller confirms that the source stays within its set window.
Command the power transferThe mechanism completes its stated open, delayed or other transition.
Retransfer after recoveryNormal power must recover and remain acceptable before the load returns.
Typical dual-source STS event
Both sources are normally live and acceptable.
Monitor preferred and alternate inputsThe controller checks each source against model-specific limits.
Detect a preferred-source problemThe cause and source phase relation can change the response.
Validate the alternate sourceNo transfer occurs if the other source is outside the allowed window.
Move the load through the static pathThe cited STS2 example uses fast break-before-make with no overlap.
Log, alarm and manage returnRetransfer and preferred-source rules remain product-specific.
Whole-system rule: a switch can transfer only to a usable source. It does not replace a generator, UPS, battery or flywheel.
Automatic transfer equipment
The cabinet is only one layer of the backup-power system
A generator transfer switch may contain sensing, control and a physical switching assembly. Its real duty also depends on the generator, upstream protection, neutral design and connected loads.
A photograph cannot prove its transition type, transfer time, ampere rating or standard. Read the nameplate, one-line diagram, manual and certification file.
For SENTOP product matching, review the automatic transfer switch range and the generator transfer switch solution.
Photo: Robert.Harker, Wikimedia Commons, CC BY-SA 3.0. Shown for context; no performance claim is inferred.
Side-by-side comparison
STS vs ATS: compare the system assumptions
This table shows common roles. The exact catalog number and complete equipment design always control.
| Decision point | Static transfer switch (STS) | Automatic transfer switch (ATS) | What to verify |
|---|---|---|---|
| Basic meaning | Solid-state power-transfer technology that can work automatically. | Automatic source sensing and transfer control; power mechanism varies. | Exact technology, transition mode and standard. |
| Normal source setup | Often two live, compatible UPS or AC buses. | Utility to generator, one utility to another, two generators, or another planned source pair. | One-line diagram and true source independence. |
| Alternate-source readiness | Alternate must already be acceptable for a fast transfer. | Controller may issue a generator-start signal and then check its output before transfer. | Source acceptance limits and outage sequence. |
| Typical power path | Solid-state parts carry the load in the normal switch path. | The path may use contactors or other listed moving parts. | Heat, bypass, poles and switch type. |
| Transition behavior | Depends on the model. Some break before they make, with no overlap. | May use open, delayed or closed transition. | Gap time, overlap, source match and utility rules. |
| Generator-start gap | Does not bridge it without a ready UPS or stored-energy path. | Can control the generator sequence but cannot remove engine start time. | Load ride-through and full restoration time. |
| Load focus | Often used for sensitive IT, control or process loads with one power cord. | Often used for building, process, standby and emergency loads. | Starting current, motors, transformers and restart behavior. |
| Maintenance | No moving main contacts, but the electronics, cooling, controls and bypass need checks. | Contacts, moving parts, controls, case and bypass need checks. | Safe service state, spare parts and access. |
| Fault duty | Use the fault and protection marks for the exact model. | Use its marked withstand and closing rating (WCR). | Fault current from both sources and upstream protection. |
| Best buying question | Can the exact STS transfer this real load between these two real sources? | Can the exact ATS manage this source sequence and distribution duty? | Evidence from drawings, ratings, files and acceptance tests. |
Compare the full installed cost. Include the switch, bypass, cooling, protection, tests, spare parts and the cost of downtime. Do not assume every ATS costs less.
Read the diagram, not the acronym
Source sensing and interlocked control shape the changeover
This teaching diagram separates the control logic from the power path. A real project can add generator start signals, time delays, neutral switching, bypass, load shed and remote monitoring.
Do not use a web diagram as a construction drawing. Follow the approved design and the exact equipment manual. SENTOP's transfer switch wiring guide gives a broader planning overview.
Teaching diagram: Dmitry G, Wikimedia Commons, CC BY-SA 3.0. Not a project wiring plan.
Transfer time without the marketing shortcut
Measure from source failure to acceptable load power
A device transfer interval is only one part of the load's outage budget.
Detection
How long must voltage or frequency stay outside the set window before the controller acts?
Source readiness
Is the alternate already live, or must a generator start, build voltage and stabilize?
Transfer action
What break, overlap or delay occurs under the exact source and load condition?
Load recovery
Does the load ride through, reset, draw inrush or need a controlled restart?
Model-specific example: the cited Liebert STS2 specification states no more than one-eighth cycle for a manual transfer inside its allowed sync window. It states one-quarter cycle typical for an emergency transfer when the alternate source is available. The result depends on load, source phase difference and the type of source failure. At 60 Hz, those intervals are about 2.1 ms and 4.2 ms. They are not universal STS guarantees.
Closed transition does not fix generator start time. It uses make-before-break only when both sources are present, acceptable and inside the synchronism window. If one source has failed, the transfer normally occurs open transition. Closed transition mainly removes a planned transfer or retransfer gap between live sources.
Five design gates
Fast transfer still needs electrical coordination
These issues can reject an otherwise attractive product.
Source phase and sync
Some STS units need matching nominal voltage, phase rotation and frequency. The allowed phase difference is model-specific.
A static device does not make arbitrary sources compatible.
Backfeed and overlap
Open transition breaks before it makes. Closed transition briefly parallels qualified sources.
Parallel operation can need utility approval, sync checks and fail-to-disconnect protection.
Neutral and grounding
Phase transition and neutral transition are separate choices. Solid, switched and overlapping neutral schemes affect ground-fault sensing and current paths.
Fault duty
WCR and SCCR are not the same label. Use the exact marked rating and named upstream fuse or breaker conditions.
Check fault current from both sources and any motor contribution.
Load behavior
Transformers, motors, rectifiers and power supplies can react badly to a phase jump or transfer inrush.
Test the real load or a valid model during acceptance.
Bypass state
A bypass can improve service access, but it creates more operating states and possible feeds.
Interlocks, labels, procedures and testing must cover each state.
Architecture-first selector
Open the transfer switch selection guideMatch the device to the power path
These are planning patterns, not final designs. A qualified engineer must confirm the project and adopted rules.
Facility standby
Core fit: ATS.
The controller can issue the generator-start signal and then check the alternate source output. Add UPS or storage for loads that cannot ride through startup.
Single-cord critical load
Core fit: evaluate STS.
Confirm source independence, synchronization, load inrush, bypass, alarms and real-load tests.
No-gap retransfer goal
Core fit: closed-transition ATS may fit.
Both sources must be live and inside the sync window. Utility and protection rules can apply.
Service without a planned outage
Core fit: bypass-isolation ATS or STS service bypass.
Validate interlocks, access, test state, training and every possible feed.
Layered critical power
Core fit: combined architecture.
Generator and ATS cover long outages. UPS or storage covers the start gap. STS or dual cords can protect the load tier.
Map failure domains. Two source labels do not prove redundancy. The feeds may share a transformer, feeder, generator, control supply, room, maintenance path or human error risk.
Dual-bus load tier
Two feeds are useful only when the downstream load can use them
Many servers accept two power inputs. A true A/B design can connect each input to a separate path. A single-cord device needs another source-selection method.
An STS can serve that role when both inputs are compatible and the exact load is accepted. The STS does not prove that the two upstream paths are independent.
The pictured A/B rack PDUs are downstream distribution units, not a static transfer switch. Photo: Schleifenbauer, Wikimedia Commons, CC BY-SA 4.0.
Maintenance and evidence
Solid-state does not mean maintenance-free
Availability comes from tested operating states, clear records and safe service access.
Mechanism and contacts
Follow the maker's plan for the main contacts, moving parts, controller, sensors, case, bypass and safety locks.
Electronics and heat
An STS has no moving main contacts, but its power parts, fans, heat sinks, controls and bypass still need checks.
Build a timeline
Record each source, the timer settings, the generator, the switch command, alarms and what the load did.
Test every state
Test loss of each source, transfer, return, bypass, alarms, remote signals and loss of both sources.
Do not use one service interval for every switch. Follow the current manual and adjust the plan for the site, duty, history and local rules.
Multi-source safety
“Off” on the screen is not isolation
Transfer equipment can remain energized from either source, a bypass, backfeed or stored energy. A selector switch, HMI command or interlock is not an isolating means.
Before service, qualified people must identify and isolate every source. Apply the required lockout/tagout process, release stored energy and verify de-energization with suitable test equipment.
Generic distribution-panel inspection, not an ATS/STS service procedure or permission for live work. U.S. Forces Iraq / U.S. Army courtesy photo, public domain, Wikimedia Commons.
Standards boundary
Ask for the exact product file and intended use
A standard has a set job. It does not make every switch fit a hospital, data center, generator or emergency system.
Transfer switching equipment
This standard covers transfer switching equipment. It excludes static transfer systems covered by IEC 62310. Check the national adoption and exact product file.
Static transfer systems
Parts 1, 2 and 3 cover safety, EMC, and performance/test methods for static transfer systems.
Separate North American scopes
UL 1008 covers transfer-switch equipment within its stated scope and excludes solid-state static transfer equipment. UL 1008S has a separate scope for solid-state transfer switches.
System and service context
NFPA 70, NFPA 110 and NFPA 70B may set site, standby-power and service rules. Use the locally adopted edition and meet AHJ and utility requirements.
A hospital or “critical” claim is not a product file. Check the exact listing, use group, ratings, use limits and full system approval.
Procurement-ready input
Send a switch requirementBuild the RFQ from the one-line diagram
SENTOP can review automatic transfer switch models, ratings, documents, samples and supply needs. Critical-power architecture and site approval remain with the qualified project team.
01 ApplicationName the site, panel or machine. State how long the load can lose power and where the system will be sold.
02 One-line diagramShow both sources. Add the UPS, generator, bypass, breakers, ground points and loads.
03 Source dataGive voltage, phase and frequency. State the source acceptance limits and generator-start sequence.
04 Load dataGive normal and peak current. List motors, transformers and large starting loads.
05 Transfer goalState the allowed power gap. Add transfer, return, test, bypass and two-source-loss needs.
06 Fault and neutralGive fault current from each source, including motor contribution. State the marked WCR or other short-circuit rating, required upstream breaker or fuse, poles and neutral plan.
07 EnvironmentState indoor or outdoor use, heat, altitude, case rating, access and data links.
08 Supply evidenceList target files, quantity, destination, labels, packing, spare parts and delivery needs.
Use the transfer switch sizing tool as a starting point. Then send the result with the one-line diagram. Browse the SENTOP product catalogue for model references.
Continue the decision
Related SENTOP resources
Frequently asked questions
Static transfer switch vs automatic transfer switch FAQ
Is a static transfer switch the same as an automatic transfer switch?
Not exactly. ATS describes an automatic change between power sources. STS describes a switch that uses solid-state parts and may also operate automatically. Check the one-line diagram, model data and product standard.
Which is faster, an STS or an ATS?
An STS is often faster when both sources are already live and ready. But no one speed fits every STS or ATS. A generator-backed system must detect an unacceptable normal source and request generator start. It must then wait for voltage and frequency to build and qualify the alternate source. Compare this full time with the time the load can stay on.
Can an STS transfer directly from utility power to a generator?
Do not assume it can. Many STS units need two live sources within set voltage, phase and frequency limits. A stopped generator is not ready. A project may use an ATS at the generator and an STS farther downstream.
Does an ATS always interrupt the load during transfer?
No. Open transition turns off the first source before it turns on the second, so there is a gap. Closed transition can join two ready, matched sources for a short time. It may need utility approval and extra protection.
Does an STS always provide zero interruption?
No. The result depends on the model, source quality, phase angle, fault, settings and load. Some STS units switch very fast but still break before they make. Treat “zero interruption” as a claim that needs test data.
What is the difference between a closed-transition ATS and an STS?
A closed-transition ATS joins two ready sources for a short time before it opens the first path. An STS uses solid-state parts and may break before it makes. Neither choice removes generator start time when the second source is off.
Do I need a switched neutral on a transfer switch?
Maybe. It depends on how each source is grounded, how ground faults are sensed and whether current can use another neutral path. A qualified designer should set the pole count and neutral action on the one-line diagram.
What should I send a transfer-switch vendor?
Send the one-line diagram, source and load ratings, allowed power gap, transfer plan, fault current and upstream protection. Add the grounding and neutral plan, site conditions, target standards, bypass needs, tests, quantity and destination.
Primary technical references
Sources used for the technical boundaries
Product examples show why ratings and transfer behavior must stay tied to an exact model. Standards links show scope, not approval of a project.
- Eaton — Automatic Transfer Switch Fundamentals: ATS controls, open/closed transfer, switch types, neutral, bypass and WCR.
- Cummins — Transfer Switch Application Manual: generator start, source acceptance, transfer, retransfer and cooldown sequence.
- Vertiv — Liebert STS2 Guide Specification: exact STS source rules, SCR path, timing and bypass.
- Eaton — PDI Static Transfer Switch: model-specific source checks, timing and sync.
- IEC 60947-6-1:2026: current transfer switching equipment scope and STS exclusion.
- IEC 62310-1:2005, IEC 62310-2:2006 and IEC 62310-3:2008: static transfer system safety, EMC, performance and test scopes.
- UL 1008 and UL 1008S: separate scopes for conventional and solid-state transfer equipment.
- NFPA 70 (2026), NFPA 110 (2025) and NFPA 70B (2026): site, standby-power and service rules where adopted.
- OSHA 1910.333: rules to shut down, lock out and test for no voltage in covered U.S. work sites.
From comparison to a usable RFQ
Send the source diagram, load duty and target ATS evidence
SENTOP can help match an ATS model, poles, ratings, controls, documents, samples and supply needs. For an STS or a layered power plan, use the system designer and the maker of the exact gear.
