Choose a data center transfer switch by its position in the power system, the sources it connects and the interruption the downstream load can tolerate. Verify the actual transition method, electrical ratings, protection requirements and maintenance arrangement. An ATS or STS label alone does not establish compatibility with your UPS, generator or distribution system.
A purchase enquiry that asks only for amperage leaves the most important operating questions unanswered. What happens when one source fails? What happens when both sources are outside limits? Should the switch transfer during a downstream fault? The correct equipment depends on those answers and the approved electrical design.
SecondWatt's automatic transfer switch catalogue and static transfer switch catalogue provide equipment-family starting points. Evaluate a candidate against its own drawings, configuration and records before treating a family description as a purchase specification.
Key Takeaways
- Define the two sources, downstream load and required behavior for each event.
- Distinguish transition method from transfer speed and total load interruption.
- Verify continuous current, fault withstand, protection and neutral configuration separately.
- Identify the actual bypass, isolation, controller and communications equipment supplied.
- Require configuration-specific evidence and an agreed acceptance plan before commitment.
Start with the switch's job in the electrical design#
In equipment procurement, ATS commonly describes an automatic transfer assembly using an electromechanical switching mechanism. STS describes a static transfer switch using power semiconductors. An STS can also operate automatically; the familiar abbreviations are not a distinction between automatic and manual operation.
Locate the proposed equipment on the site's single-line diagram. An assembly selecting between utility and generator supply has a different operating context from a static switch feeding a load from two UPS-backed paths. Both types can appear in the same facility. Neither device creates an independent source of energy when its available inputs cannot supply the load.
Define the transfer application#
| Requirement | Information to put in the enquiry |
|---|---|
| Electrical position | Source and load connections on the current single-line diagram |
| Source identity | Utility, generator, UPS output or another defined supply |
| Load behavior | Permitted interruption, inrush and relevant operating states |
| Normal preference | Preferred source and policy for returning to it |
| Disturbance response | Required behavior for source loss, recovery and load faults |
| Maintenance | Required access, isolation and continuity during service |
Identify whether the downstream equipment has one supply connection or two and how those connections are used. A two-cord server arrangement and a single-input load supplied through an STS are different designs. Do not infer end-to-end independence from the presence of two switch inputs if they share upstream dependencies.
The existing UPS topology comparison helps place the transfer function in the wider power path. Carry the selected UPS operating mode into the switch assessment rather than assuming that a result in one mode covers every mode.
Separate transition method from interruption time#
Open transition breaks the original connection before making the alternate connection. Closed transition briefly overlaps acceptable sources under the required control conditions. Eaton's automatic transfer switch fundamentals distinguishes these methods and describes open delayed and open in-phase arrangements.
Closed transition is not a promise of uninterrupted transfer after an unexpected source failure. If the original source is already unavailable, a planned overlap between two acceptable sources cannot recreate the lost supply. The overall interruption also depends on source detection, any intentional delay, alternate-source readiness and the switching sequence.
Static Power describes its iMAX static transfer switches as solid-state, break-before-make equipment. The product page also identifies bypass and input-breaker arrangements. Confirm the particular model and supplied options; a family page does not establish the contents of every assembly offered for sale.
Transfer terms that should appear separately#
| Term | Purchasing question |
|---|---|
| Transition method | Are sources separated or briefly overlapped during the defined transfer? |
| Detection | What source condition starts the response? |
| Intentional delay | What configured waiting period applies? |
| Switching interval | What interval is specified, and how is it measured? |
| Source readiness | Is the alternate source already acceptable or still starting? |
| Load interruption | What does the connected load actually experience? |
Keep units explicit when an offer uses fractions of a cycle. At 60 Hz, one cycle is approximately 16.67 milliseconds and a quarter-cycle is 4.17 milliseconds. At 50 Hz, a cycle is 20 milliseconds and a quarter-cycle is 5 milliseconds. These are frequency conversions, not performance guarantees for an ATS or STS.
Request the conditions behind any transfer-time claim: voltage, frequency, phase relationship, load level and type, and the initiating event. Ask whether detection time is included. A short switching interval under one laboratory condition cannot by itself establish the maximum interruption across every site event.
Specify behavior when sources or loads are abnormal#
Turn the operating requirement into an event matrix that the electrical designer and equipment respondent can review together. Include source recovery and maintenance states as well as an initial failure. An appropriate outcome may be to remain on the present source or inhibit transfer rather than move the load.
Static Power's iSTS G user manual, sections 2.1 and 3.2, illustrates two configuration-specific concerns: inhibition of transfer during a downstream load fault and the consequences of source phase displacement for transformer loads. Use the manual for the actual supplied revision; do not transplant this model's settings into another product.
Events to resolve in the purchase specification#
| Event | Response that needs agreement |
|---|---|
| Preferred source outside limits | Transfer eligibility and alternate-source conditions |
| Both sources outside limits | Defined behavior without assuming a healthy destination |
| Sources out of phase | Permitted transfer behavior and load consequences |
| Downstream load fault | Transfer inhibition and protective-device coordination |
| Preferred source recovers | Stability requirement and return policy |
| Maintenance bypass selected | Available protection, monitoring and transfer capability |
The event matrix is a requirements document, not a sequence for operating energized equipment. Qualified parties should develop the approved settings and test procedures from the actual design and manufacturer instructions. The buyer should receive the resulting configuration record and evidence of the agreed behavior.
Pay particular attention to downstream transformers, motors and other loads with transfer-sensitive behavior. Ask how their characteristics were included in the assessment. A switching device that operates within its own specification can still interact poorly with a load if the combined arrangement was not evaluated.
Define alarm and event-record requirements. A useful handover identifies source status, transfer cause, time stamps and the state of the equipment after the event. Confirm who can retrieve those records and whether time synchronization is part of the delivered communications scope.
Verify ratings against the actual load and fault duty#
Record voltage, frequency, phase arrangement, continuous current and environmental conditions before comparing offers. Keep short-circuit performance separate from the normal load rating. A nameplate current tells you neither the available fault current at the receiving bus nor whether a particular protective-device combination is acceptable.
Consider a hypothetical balanced three-phase load of 800 kW at 480 V line-to-line and 0.90 power factor. Its calculated line current is:
800,000 ÷ (√3 × 480 × 0.90) = approximately 1,069 A.
This is a load-current calculation under stated assumptions. It does not select a switch size, establish a continuous-loading allowance or verify fault duty. Actual loading, waveform, phase imbalance, ambient conditions and the applicable equipment ratings still need review. Use the UPS kVA, kW and power-factor guide for the related power-basis distinctions.
Electrical compatibility record#
| Field | Evidence to request |
|---|---|
| Normal current | Nameplate and applicable operating-condition limits |
| Voltage and frequency | Exact assembly rating and receiving-site requirements |
| Fault performance | Applicable withstand or withstand-and-close rating and conditions |
| Protection | Required upstream device, settings and coordination review |
| Poles and neutral | Actual switching arrangement and grounding-design compatibility |
| Terminations | Conductor accommodation, entry arrangement and supplied connection hardware |
| Environment | Enclosure suitability, ventilation, access and installation limits |
Ask for the rating's stated voltage, duration and protective-device conditions. Do not substitute a circuit breaker's interrupting rating for a transfer assembly's withstand rating. A large current figure without its associated conditions is incomplete purchasing evidence.
The neutral arrangement also belongs in the design review. Confirm whether the neutral is switched or solid and how that configuration fits the sources and grounding arrangement. Do not select pole count solely because the previous installation used the same nominal voltage.
Where a transformer changes the distribution interface, include its impedance and protection implications in the engineering assessment. The data center transformer sizing guide provides adjacent planning context; it does not replace the project fault and coordination studies.
Inspect the assembly, controller and maintenance path#
A used transfer switch is an identified assembly with a service history. Request the nameplate, serial number, drawings, controller identification, firmware information, maintenance records and modification history. Reconcile the photographed equipment with the proposed bill of materials and any supplied test report.
ABB's TruONE ATS description presents the switching mechanism and controller as an integrated product. That product architecture illustrates why controller identity and fitted accessories belong in the purchase scope. A controller designation alone does not fully identify an assembly's electrical ratings or included functions.
Used-equipment condition and scope checks#
| Area | Evidence to obtain |
|---|---|
| Main assembly | Identity, rating labels, condition photographs and drawings |
| Switching components | Relevant inspection, maintenance and replacement history |
| Controller | Model, firmware, settings access and supported functions |
| Bypass and isolation | Actual supplied arrangement and maintenance limitations |
| Communications | Fitted hardware, protocols, access rights and required licenses |
| Storage | Location, duration, preservation and receiving inspection scope |
| Support | Parts availability and service confirmation for the offered revision |
Ask what can be isolated for maintenance while the load remains supplied, and what capability is unavailable in that condition. The word bypass does not establish that every internal component is safely isolated or that automatic transfer remains available. Obtain the manufacturer's drawings and a site-specific maintenance assessment.
For electromechanical equipment, request relevant operating and contact-condition records. For static equipment, request the applicable power-module, cooling and electronics service history. Have the inspection scope set for the actual design rather than assuming the same wear mechanisms or test methods apply to both.
Confirm that settings and monitoring access can be transferred to the buyer. Preserve the accepted configuration, remove obsolete access through the approved handover process and identify who supports future changes. A functioning display is useful inspection evidence, but it is not a complete functional acceptance test.
Agree acceptance around the delivered configuration#
Separate inspection of the offered equipment from acceptance within the new installation. An off-site report may verify identity and selected functions. It cannot demonstrate every source, protection, load and control interaction at the receiving facility.
NETA's public Acceptance Testing Specifications overview describes acceptance testing for suitability for initial energization. Here, ATS in the standard's abbreviated title means Acceptance Testing Specifications. It is not a product classification for an automatic transfer switch. Have the testing party identify the applicable standard edition and equipment-specific requirements.
Acceptance evidence for the purchasing file#
| Stage | Record the responsible party should supply |
|---|---|
| Before shipment | Identity, condition, completed work and remaining exceptions |
| On receipt | Shipping-condition inspection and included-equipment reconciliation |
| Before energization | Approved installation and equipment checks |
| Functional testing | Results against the agreed event matrix and configuration |
| Integrated testing | Results for required source, UPS, generator and load interactions |
| Handover | Final settings, drawings, records, access and service contacts |
Define the acceptance criteria before the purchase becomes unconditional. State which deficiencies prevent acceptance, which can be closed later by agreement and who pays for the corrective work. Record limitations if a test setup cannot reproduce a required source or load condition.
Maintain traceability between the result and the delivered equipment. A report for the right model but a different serial number, controller revision or protection arrangement may not answer the purchase question. Any material change after testing should trigger an explicit review of whether the evidence still applies.
Build a comparable transfer-switch enquiry#
Provide the electrical position, both source types, load requirements and required event behavior with the sourcing request. Attach the current drawings where available and identify assumptions that still need engineering resolution. Specify acceptable equipment condition and the destination requirements.
Request an itemized scope covering the main assembly, controller, enclosure, bypass or isolation equipment, connection hardware, communications accessories, inspection, testing and documentation. Ask respondents to identify deviations rather than silently substitute a different transition method or omit a maintenance function.
For delivery responsibilities, use a named rule and place where appropriate. The ICC's Incoterms 2020 guidance provides the trade-term framework. Electrical installation, functional acceptance, training and support need their own explicit contractual scope.
SecondWatt acts as an independent intermediary connecting equipment buyers and sellers. To request transfer-switch equipment sourcing, provide the required type, voltage, frequency, current, source arrangement, transition requirements, acceptable condition, destination and target date. Include bypass and testing requirements so the response addresses the complete assembly.
FAQ: Buying ATS and STS equipment#
Does an STS replace an automatic transfer switch everywhere?#
The selection depends on the electrical position, sources, load requirements and operating design. Both types can appear in one data center. Identify the function required at each location and verify the actual equipment's ratings and behavior. A general preference for faster switching is not a complete design specification.
Does closed transition eliminate every power interruption?#
No. Closed transition relies on the required conditions for briefly overlapping acceptable sources. It does not recreate a source that has already failed or remove the time needed for another source to become ready. Evaluate the load's experienced interruption for each specified event and the complete supporting power system.
Can an STS supply the load if both sources fail?#
A transfer switch does not provide stored energy or generation by itself. If neither input can supply the load, another part of the system must provide the required continuity. Confirm the UPS, storage, generation and distribution behavior as an integrated design rather than attributing those functions to the switch.
Is the ampere rating enough to select a used switch?#
No. Verify voltage, frequency, phase arrangement, environmental limits, fault performance, protective-device conditions and neutral configuration as well as current. Confirm the actual controller, bypass equipment and condition records. The receiving installation must be reviewed against the supplied assembly's documented ratings and configuration.
Does maintenance bypass preserve all normal functions?#
That depends on the specific assembly and bypass arrangement. Establish what remains energized, what can be isolated, and which transfer, protection or monitoring functions remain available. Obtain the relevant drawings and approved maintenance assessment. Do not infer a complete isolation or continuity capability from the word bypass alone.
What records should accompany a transfer-switch purchase?#
Request identity and rating records, drawings, controller and settings information, maintenance and modification history, included-equipment details and applicable test results. Include support and access arrangements. For the receiving site, agree the functional and integrated acceptance evidence needed to show that the delivered configuration meets the specified operating requirement.