An automatic transfer switch (ATS) is an electrical device that automatically shifts a load’s power source from the main utility supply to a standby source (typically a generator) when the primary supply fails or falls outside acceptable limits. Once the primary supply returns, the ATS transfers the load back and signals the generator to shut down. This guide explains how ATS works, the main types, applicable standards, and how to select the right ATS for your project.
What Is an Automatic Transfer Switch?
An automatic transfer switch is a self-acting, intelligent switching device installed between the normal power source (utility), the alternate power source (generator, secondary utility, or UPS), and the load. It continuously monitors the voltage and frequency of the primary source. When a fault, outage, brownout, or overvoltage is detected, the ATS starts the generator, waits for it to stabilize, and then transfers the load. When utility power returns and is stable for a defined period, the ATS retransfers and cools down the generator.
ATSs are required anywhere an unplanned power interruption is unacceptable — hospitals, data centers, fire-fighting systems, telecom base stations, banks, industrial processes, and high-rise buildings. A correctly specified ATS prevents equipment damage, data loss, production downtime, and safety incidents caused by sudden blackouts.
For B2B buyers, choosing the right ATS matters because transfer time, breaking capacity, bypass-isolation capability, and communication interfaces directly affect how safely and reliably critical loads are protected.
How Does an Automatic Transfer Switch Work?
An ATS works through four sequential steps. The exact timing depends on the controller settings and the type of ATS.
- Source sensing — The ATS controller monitors the primary source (voltage, frequency, phase loss, phase sequence). When parameters fall outside the preset thresholds, a transfer is initiated.
- Generator start — The ATS sends a start signal to the standby generator. Most generators reach stable voltage and frequency within 10–30 seconds.
- Transfer to emergency — After confirming the generator is healthy, the ATS disconnects the load from the primary source and connects it to the generator, either with a brief interruption (open transition) or with a momentary overlap (closed transition).
- Re-transfer to normal — When the primary source returns and remains stable for the set time, the ATS retransfers the load back. The generator runs unloaded for a cool-down period (typically 1–5 minutes) and stops.
What Are the Main Components of an ATS?
1. Power Switching Mechanism
- Contactor-based mechanism — Uses two mechanically and electrically interlocked contactors. Economical and widely used for LV ATS up to 1600 A.
- Motorized circuit breaker-based mechanism — Uses two motorized MCCBs or ACBs. Suitable for larger currents (up to 6300 A) and higher breaking capacities.
2. Controller
- Voltage sensing — Monitors both sources for under-voltage, over-voltage, and phase loss.
- Frequency sensing — Monitors frequency drift and the generator stability.
- Programmable timers — Set time delays for engine start, transfer, re-transfer, and stop.
- Manual override — Allows operator-initiated transfer or bypass.
- Communication — Modbus, RS485, Ethernet for remote monitoring and integration with BMS / SCADA.
3. Mechanical and Electrical Interlocks
- Mechanical interlock — Ensures both contactors / breakers cannot close at the same time.
- Electrical interlock — Adds a second layer of protection through auxiliary contacts in the control circuit.
4. Bypass-Isolation Feature (optional)
- Bypass switch — Allows the load to be fed directly from either source while the ATS is being maintained or tested, without interrupting the load.
5. Enclosure and Wiring
- Enclosure — Steel cabinet, IP54 indoor standard, IP65 outdoor, with proper ventilation.
- Terminals — For incoming and outgoing power cables, control wiring, and communication.
What Types of ATS Are Available?
1. By Transition Type
| Type | Description | Typical Application |
|---|---|---|
| Open transition (break-before-make) | Load is briefly disconnected during transfer. Simple and reliable. | Most commercial and industrial loads; motor loads that can tolerate a short interruption |
| Closed transition (make-before-break) | Both sources are momentarily paralleled for a few cycles during transfer. No interruption. | Sensitive loads where zero interruption is required (data centers, hospitals, continuous processes) |
| Delayed transition | Adds an adjustable OFF delay (typically 0–10 s) between disconnection of two sources, allowing motors to coast down. | Large motor loads where re-energizing while still spinning could damage equipment |
| Soft loading | Gradually ramps load onto the generator to avoid sudden step loading. | Generators with limited step-load capability |
2. By Switching Mechanism
| Type | Description | Typical Application |
|---|---|---|
| Contactor-based ATS | Two mechanically interlocked contactors | LV systems up to 1600 A, standard commercial use |
| Circuit-breaker ATS | Two motorized circuit breakers | LV systems above 1600 A, or MV systems; high breaking capacity required |
| Static ATS | Uses thyristors for sub-cycle transfer (no mechanical parts) | Critical loads requiring sub-cycle switching; data centers, semiconductor fabs |
3. By Installation
| Type | Description | Typical Application |
|---|---|---|
| Wall-mounted ATS | Compact design for indoor installation | Small commercial buildings, telecom sites |
| Floor-standing ATS | Larger cabinet for indoor electrical rooms | Industrial plants, hospitals, data centers |
| Outdoor ATS | Weather-proof enclosure, IP65 or higher | Utility substations, remote telecom towers |
Open Transition vs Closed Transition ATS — How to Choose
| Feature | Open Transition | Closed Transition |
|---|---|---|
| Load interruption | Brief (typically < 100 ms) | None (overlap of about 100 ms) |
| Risk of inrush | Yes — motors and transformers may inrush on re-energization | No — soft handoff |
| Cost | Lower | Higher (additional synchronization and protection) |
| Synchronization required | No | Yes — sources must be synchronized within set limits |
| Typical use | General commercial and industrial loads | Sensitive and critical loads where any interruption is unacceptable |
For most industrial applications where a brief interruption (less than 100 ms) is acceptable, an open-transition ATS is the most cost-effective choice. For data centers, semiconductor fabrication, and similar sensitive environments, a closed-transition ATS — or even a static ATS — is preferred.
Common Applications of Automatic Transfer Switches
1. Hospitals and Healthcare Facilities
Operating rooms, intensive care units, and life-support systems must remain energized without interruption. ATSs work together with standby generators to provide Tier-1 and Tier-2 power reliability.
2. Data Centers
Server rooms, network switches, and cooling equipment require sub-cycle or no-break transfer. Closed-transition or static ATSs are commonly used in combination with UPS systems.
3. Fire-Fighting and Life-Safety Systems
Fire pumps, smoke control fans, emergency lighting, and evacuation lifts must operate during a power outage. ATSs for life-safety loads must comply with local fire codes and standards such as NFPA 110 in North America.
4. Telecom Base Stations
Outdoor cabinet ATS combined with small diesel generators keep remote base stations running through utility outages. Outdoor IP65 ATSs with built-in controllers are common.
5. Industrial Plants and Manufacturing
Continuous processes — petrochemical, paper, glass, semiconductor — cannot tolerate sudden shutdowns. ATSs protect upstream utility failure while UPS and battery systems handle short-duration events.
6. Commercial High-Rise Buildings
Fire-fighting systems, evacuation lifts, and emergency lighting rely on ATS-controlled standby power to maintain life-safety functions.
What Specifications Should Buyers Check?
| Parameter | Typical Range / Value | Why It Matters |
|---|---|---|
| Rated current | 63 A to 6300 A | Must be greater than the maximum load current |
| Rated voltage | 400 V / 690 V (LV) or 6 kV / 11 kV / 33 kV (MV) | Must match the system voltage |
| Number of poles | 3-pole / 4-pole | 4-pole is needed for systems where the neutral must be switched |
| Transition type | Open / closed / delayed | Determines transfer behavior and load compatibility |
| Breaking capacity | 25 kA to 100 kA | Must be greater than the available short-circuit current at the ATS terminals |
| Transfer time | < 100 ms (open); < 50 ms (static) | Defines whether the load sees an interruption |
| Bypass-isolation | With / without | Required for critical loads to allow ATS maintenance |
| Communication | Modbus / RS485 / Ethernet | For remote monitoring and integration with BMS / SCADA |
| IP rating | IP54 indoor / IP65 outdoor | Defines environmental protection |
| Applicable standard | IEC 60947-6-1 / GB / UL 1008 / NFPA 110 | Defines quality and safety baseline |
| Ambient temperature | -5 °C to +40 °C standard | Derating may be required for higher temperatures |
How to Choose the Right Automatic Transfer Switch
- Define the load. Identify whether the load can tolerate a brief interruption (open transition OK) or requires no-break transfer (closed transition or static required).
- Calculate the rated current. Add up the full-load currents of all downstream circuits; select an ATS with at least 25% margin.
- Confirm the system voltage and number of poles. LV or MV; 3-pole or 4-pole depending on neutral switching requirements.
- Check the short-circuit level. The ATS breaking capacity must be greater than the available short-circuit current at the installation point.
- Decide on bypass-isolation. For critical loads, choose an ATS with a bypass switch so maintenance can be performed without shutting down the load.
- Verify the communication interface. If the ATS will be connected to a SCADA / BMS system, confirm the supported protocol.
- Match the IP rating to the environment. Use IP54 in indoor electrical rooms; IP65 or higher for outdoor installations.
- Confirm applicable standards. IEC 60947-6-1 for international projects, UL 1008 for North America, GB for China.
How to Inspect ATS Quality Before Delivery
- Visual inspection — Check paint finish, door alignment, label clarity, and mechanical interlock operation.
- Mechanical operation test — Manually operate the switch to confirm smooth transfer between positions.
- Electrical operation test — Simulate primary source failure and confirm the controller starts the generator, transfers the load, and retransfers when power returns.
- Interlock test — Confirm both sources cannot be connected simultaneously.
- Voltage and frequency setting test — Confirm under-voltage, over-voltage, and frequency thresholds match the specification.
- Dielectric test — Hi-pot test on power circuits per IEC 60947-6-1.
- Temperature rise test — For high-current ATS, verify the temperature rise at rated current stays within limits.
What Information Should Be Included in an ATS RFQ?
- Project name and location — Including country and applicable standard (IEC / UL / GB).
- Supply system — Voltage, frequency, number of phases, neutral arrangement.
- Rated current and number of poles — 3-pole / 4-pole; rating in A.
- Transition type — Open / closed / delayed.
- Breaking capacity — Required short-circuit withstand current.
- Source configuration — Utility + generator / utility + utility / utility + UPS.
- Bypass-isolation requirement — Yes / no.
- Controller and communication — Programmable timers, Modbus / Ethernet, dry contacts.
- Enclosure — Wall-mounted / floor-standing / outdoor; IP rating.
- Documentation and spare parts list — Drawings, manuals, test reports, recommended spares.
How to Choose a Reliable ATS Supplier
- Type-tested design — Confirm the ATS has been type-tested per IEC 60947-6-1 or UL 1008 for the specified rating.
- Wide product range — The supplier should cover LV and MV ATS, contactor- and breaker-based designs, open and closed transition.
- Engineering support — Look for in-house engineering teams that can size the ATS, integrate with generators and switchgear, and customize controllers.
- Quality system — ISO 9001 or equivalent certification.
- Reference projects — Hospitals, data centers, telecom operators, or industrial plants with verifiable reference lists.
- After-sales support — Local technical support, spare parts availability, and warranty terms.
- Compliance with project standards — IEC, UL, CCC, or other local certifications as required.
Frequently Asked Questions
1. What is the difference between an ATS and an STS?
An automatic transfer switch (ATS) transfers between two AC sources, typically utility and generator, with a brief interruption (or no interruption in closed-transition designs). A static transfer switch (STS) uses semiconductor switches for sub-cycle transfer between two synchronized sources and is typically used between two utility feeds or two UPS outputs in data centers.
2. How long does an ATS take to transfer?
An open-transition ATS typically transfers in 50 to 200 ms, depending on the controller settings. A closed-transition ATS transfers in less than 100 ms with both sources momentarily paralleled. A static ATS transfers in less than 1/4 cycle (less than 5 ms at 50 Hz).
3. Can an ATS be installed outdoors?
Yes. Outdoor ATSs use IP65 or higher enclosures, anti-condensation heaters, and corrosion-resistant paint. Stainless steel or hot-dip galvanized enclosures are common for outdoor use.
4. Does an ATS start the generator automatically?
Yes. The ATS controller sends a start signal to the standby generator when it detects primary source failure. Once the generator reaches stable voltage and frequency, the controller initiates the transfer.
5. Can an ATS handle 100% of the load from the start?
Most open-transition ATSs can transfer the full rated load, but the generator must be sized to accept the step load. For generators with limited step-load capability, a soft-loading or delayed-transition ATS is preferred.
6. What is bypass-isolation, and when is it required?
Bypass-isolation is an additional switch that allows the load to be powered directly from either source while the ATS is being maintained or tested. It is required for critical loads such as hospitals and data centers, where ATS maintenance must not interrupt the load.
7. What standard applies to ATSs?
The main standards are IEC 60947-6-1 (international) and UL 1008 (North America). For life-safety applications, NFPA 110 applies in North America. Many countries accept GB 14048.11 as the equivalent of IEC 60947-6-1.
Source Automatic Transfer Switches from BANGE Electric
BANGE Electric supplies automatic transfer switches for commercial, industrial, and infrastructure projects. Our ATS range covers contactor-based and breaker-based designs, open and closed transition, with currents from 63 A to 6300 A. All ATSs comply with IEC 60947-6-1 and can be configured for utility-generator, utility-utility, and UPS-generator applications.
For critical applications, we offer ATSs with bypass-isolation, programmable controllers, and Modbus / Ethernet communication. Contact us with your single-line diagram, load list, and project specifications — our engineering team will propose the right ATS configuration within 24 hours.
To explore related products, see our guides on withdrawable vs fixed switchgear, distribution boxes, and busway systems.
Requirements may vary by application, market, and applicable standard. Final specification should be confirmed with the engineering team.