In real engineering environments, a power outage is never just “a few minutes of darkness.” For data centers or hospitals, even a few seconds of power interruption can mean disaster. On industrial production lines, an unexpected power dip may cause an entire batch of raw materials to be scrapped and lead to a long equipment restart process. Even in high-end commercial buildings, the failure of fire protection and security systems is an unacceptable risk.

An Automatic Transfer Switch (ATS) is the “last line of defense” in this power supply lifeline. As the core hub of a dual-power system, when the main utility experiences undervoltage, power loss, or even phase loss, the ATS must make an accurate decision within a very short time, safely transfer critical loads to the backup power source, and switch them back smoothly only after the utility supply has become fully stable again.

Many projects make a fatal mistake during procurement: they assume that an ATS is just an ordinary “large switch,” and that choosing the correct rated current is enough. In reality, it is the “traffic controller” between two power sources. From generator voltage build-up delay, to neutral switching strategy (pole selection), to short-circuit protection coordination with upstream and downstream circuit breakers, any wrong decision in one of these details may cause the entire system to fail at the critical moment.

If you are still at the equipment evaluation stage of your project, it is recommended that you first browse our [Automatic Transfer Switches (ATS) core product section] or [ATS selection guide by application], so that you can quickly identify the basic architecture most suitable for your system.

In international projects, ATS selection should not rely only on product catalogs. It should also be evaluated against the applicable standards and codes behind the product, including IEC 60947-6-1 for transfer switching equipment used to ensure continuity of supply, NFPA 70 (NEC) for safe electrical design and installation in US projects, and NFPA 110 for emergency and standby power systems in buildings and facilities.

What Is an Automatic Transfer Switch (ATS)?

An Automatic Transfer Switch (ATS) is a critical switching device used in dual-power supply systems. It is typically installed between the primary power source and the backup power source, allowing the load to be transferred from one source to the other when required. Common source combinations include utility-to-generator, utility-to-utility, and utility-to-energy storage or inverter output.

Unlike a conventional circuit breaker, the main purpose of an ATS is not to interrupt fault current directly. Its core function is to ensure reliable, controlled, and properly interlocked transfer between two available power sources. For this reason, ATS design is usually centered on the switching mechanism, control logic, source monitoring, mechanical and electrical interlocking, and the continuity of the overall power supply system.

From an engineering perspective, an ATS is essentially a device used to maintain continuity of supply. IEC also provides a clear standards framework for transfer switching equipment and automatic transfer switching equipment. Therefore, in export projects, EPC projects, and cross-border engineering applications, the applicable standards, test documents, and operating limits of the ATS are all critical considerations.

How an ATS Works Between the Main and Backup Power Sources

The working process of an ATS between the main power source and the backup power source mainly includes real-time monitoring, fault judgment, reliable switching, and automatic return transfer. The whole process is fully automatic and equipped with safety interlocking, and it can be specifically divided into four stages:

1. Normal Operating Stage (Main Power Supply Active)

By default, the ATS supplies the load from the main power source (utility power), while the controller continuously monitors key parameters of the main source 24/7, such as voltage, frequency, and phase condition. The backup power source remains in hot standby mode, and the main and backup switching devices are mechanically and/or electrically interlocked to strictly prevent simultaneous closing, thereby avoiding a short circuit between the two power sources.

2. Main Power Failure Stage (Transfer Triggered)

When the main power source experiences power loss, undervoltage, overvoltage, phase loss, or abnormal frequency, the ATS will first apply a configurable fault confirmation delay (typically 0.5–5 seconds) to filter out momentary fluctuations. Once the fault is confirmed to be real, the following sequence takes place:

• The controller sends a command to drive the mechanism to open the main power switch;
• After the main power switch is fully opened, the backup power switch is then closed. In some applications, the system must first wait until the backup power source becomes stable, such as when a generator has started and reached its rated voltage and frequency;
• The load is then transferred to the backup power source. The entire process is usually completed within milliseconds, so critical loads can experience little to almost no noticeable interruption.

3. Backup Power Supply Stage

The ATS continues to monitor the recovery status of the main power source while maintaining stable power supply from the backup source to the load. Some models also include backup power fault monitoring, helping reduce the risk of both power sources failing at the same time.

4. Main Power Recovery Stage (Automatic Retransfer)

When the voltage and frequency of the main power source return to the acceptable range and remain stable for a certain period of time (configurable, for example 1–30 minutes), the ATS performs an automatic retransfer from backup to main power:

• It first opens the backup power switch, and then closes the main power switch;
• The load is transferred back to the main power source, while the backup source returns

Key safety design: the ATS uses a mechanical and/or electrical interlocking mechanism to ensure that the main power switch and the backup power switch can never be closed at the same time under any circumstances, completely preventing fatal risks such as circulating current and short circuit. This is also the most fundamental difference between an ATS and an ordinary dual-switch arrangement.

Main Components of an Automatic Transfer Switch (ATS)

An ATS is not a single switching device, but a complete system that integrates monitoring, control, operation, and safety protection. Its core components are mainly divided into four major modules, which together enable the full process of power source status monitoring, fault detection, safe transfer, and automatic retransfer, as follows:

1. Dual-Power Switching Body (Actuating Mechanism)

This is the core execution unit of the ATS, responsible for the physical transfer between the main power source and the backup power source. It is commonly divided into two main forms:

• Integrated PC Class ATS: a single integrated transfer switch with a dedicated mechanical interlocking structure, designed to achieve a “break-before-make” operation. It can withstand relatively high short-circuit current and is widely used in critical-load applications.

• CB Class ATS: composed of two circuit breakers (or isolating switches) together with mechanical and/or electrical interlocking. It provides both protection and transfer functions, usually at a lower cost, and is commonly used for generally important loads.

2. Intelligent Controller (Control Brain)

This is the “decision-making center” of the ATS, responsible for power status monitoring and transfer logic control:

• Real-time monitoring of parameters of both power sources, such as voltage, frequency, phase loss, overvoltage, and undervoltage;

• Execution of logic such as delay confirmation (to prevent false operation), fault transfer, retransfer from backup to main supply, and generator start/stop coordination;

• Provision of functions such as local/remote control, status indication, fault alarm, and communication interfaces (such as Modbus).

3. Driving and Interlocking Mechanism (Safety Core)

• Driving mechanism: may be motor-operated, electromagnetic, or spring energy-storage type, providing the switching force for the switch body and ensuring reliable operation;

• Interlocking mechanism: adopts both mechanical and electrical interlocking to strictly prevent the main and backup power sources from being closed at the same time, completely avoiding short circuits and circulating current accidents between the two sources. This is the key safety design that distinguishes an ATS from an ordinary dual-switch arrangement.

4. Auxiliary and Supporting Components

• Power sampling unit: provides the controller with voltage and current signals from the two power sources

• Status feedback contacts: feed back the opening/closing position of the switch and fault status to the external system

• Generator control interface: some ATS units can directly output signals to control generator start and stop, enabling the generator to start automatically and supply power when the utility power fails;

• Operating panel / display screen: used for parameter setting, manual/automatic transfer control, and operating status display.

Supplementary Note:

The differences in the core components of different types of ATS are essentially different implementation forms under IEC standards. For example, the differences between PC Class ATS and CB Class ATS are mainly reflected in the structural design of the switch body and the level of protection capability, while the design logic of core safety modules such as the controller and interlocking mechanism must all comply with the mandatory requirements of the applicable standards.

If you would like to learn more about these two types of products, you can visit our PC Class ATS and CB Class ATS product pages.

Quick ATS Selection Checklist

✅ Load category and application scenario confirmed

✅ Rated current and frame size meet load and short-circuit requirements

✅ Number of poles (3P/4P) matches the system configuration

✅ ATS type (PC Class / CB Class) matches the required load reliability level

✅ Control mode and delay parameters can be adjusted according to site conditions

✅ Additional functions (interlocking / communication / bypass) configured as needed

✅ Coordination logic with upstream and downstream protection devices, generator, and fire protection system is clearly defined

Start with the Application: Define the Load Level and Project Requirements First

Confirm the Core Parameters: Prioritize the Key Indicators

1. Rated Current and Frame Size

• Rated current (Ie): should be at least 1.25 times the calculated load current, taking into account starting current and long-term operating margin. For example, if the load current is 160A, a 200A or 250A ATS should be selected.
• Frame size: should be selected according to the system short-circuit current and must meet the requirement for rated short-time withstand current (Icw), in order to avoid burnout under fault conditions.

2. Pole Selection (P)

• 3P ATS: suitable for three-phase three-wire systems, where the neutral conductor does not participate in switching. It is applicable to TN/IT systems and generally offers lower cost.
• 4P ATS: suitable for three-phase four-wire systems, where the neutral conductor is switched simultaneously. It is applicable to scenarios involving neutral shift, severe harmonic distortion, or non-common neutral between two power sources (such as hospitals and data centers), helping avoid neutral circulating current.

Type Selection: PC Class vs CB Class (Core Differences)

Control and Transfer Modes

• Control modes: automatic (priority), manual, and remote control (Modbus / dry contact), to be selected according to operation and maintenance requirements.

• Transfer modes:
  Automatic transfer and automatic retransfer: after the main power source recovers, the load is automatically transferred back to utility power. This is suitable for applications where utility power is the preferred source.
  Automatic transfer without automatic retransfer: after switching to the backup power source, the ATS remains in that state and requires manual reset. This is suitable for applications where utility power quality is poor or fluctuates frequently.
• Delay settings: fault confirmation delay (to filter out momentary fluctuations), generator start delay, and utility power recovery delay should all be adjusted according to actual site conditions.

Additional Functional Requirements

• Generator linkage: can directly output start/stop signals to enable automatic generator startup when utility power fails.

• Bypass function: supports online maintenance of the ATS without interrupting the load power supply, suitable for high-reliability applications.

• Communication function: Modbus/RS485 interfaces allow integration with building automation or power monitoring systems.

• Fire protection linkage: supports forced fire control logic and disconnects the power supply to non-firefighting loads in the event of a fire.

Key Pitfalls to Avoid During ATS Selection

1. Neutral handling: for 4P ATS, special attention should be paid to the switching sequence of the neutral conductor to avoid load overvoltage caused by opening the neutral before closing it again. In TN-S systems, if the neutrals of both power sources share a common grounding point, a 3P+N solution (neutral makes first and breaks last) may be considered first.

2. Short-circuit protection coordination: the interrupting capacity of the circuit breakers used in CB Class ATS must be higher than the prospective short-circuit current of the system, and selective coordination with the upstream circuit breaker should also be achieved to avoid nuisance tripping at higher levels.

3. Transfer time coordination: for critical loads, such as those located downstream of a UPS, it is necessary to confirm whether the ATS transfer time is within the acceptable range of the load, so as to avoid UPS battery discharge or load restart caused by the transfer process.

4. Environmental adaptability: in high-temperature, humid, or dusty environments, models with an IP protection rating of at least IP40 should be selected, and cabinet ventilation or dehumidification devices should be added when necessary.

Environmental adaptability: in high-temperature, humid, or dusty environments, models with an IP protection rating of IP40 or above should be selected, and cabinet ventilation or dehumidification devices should be added when necessary. For projects operating continuously in 50°C–60°C environments, it is recommended to further refer to our dedicated article: ATS derating guide for 50°C–60°C environments.

PC Class vs CB Class ATS: What’s the Difference?

PC Class Automatic Transfer Switch

Standard definition: a transfer switching device that is only capable of making, carrying the rated operating current, and withstanding the rated short-time current, but does not have the ability to interrupt short-circuit current.

Product positioning: a device dedicated purely to power source transfer. It does not include overload or short-circuit trip protection mechanisms. Its core function is only to ensure reliable transfer between two power sources, while system fault protection is provided by upstream devices such as circuit breakers and fuses.

CB Class Automatic Transfer Switch

Standard definition: a transfer switching device equipped with an overcurrent trip device, whose contacts are capable of making and carrying current, and which can also directly interrupt short-circuit fault current.

Product positioning: composed of two circuit breakers together with an interlocking mechanism, it combines both power source transfer and overload/short-circuit fault protection functions, and can itself interrupt fault current in the circuit.

Typical Applications of PC Class ATS

1. Critical Level 1 loads such as hospitals, data centers, and financial server rooms, where power interruption may lead to major losses;
2. Emergency power supply circuits for fire pumps, smoke exhaust fans, and other firefighting systems;
3. Dual-power supply systems used together with generator sets;
4. Precision industrial control production lines with extremely high requirements for outage duration and transfer stability.

Typical Applications of CB Class ATS

1. Level 2 power supply loads in office buildings, shopping malls, and general commercial buildings;
2. Small equipment rooms, emergency lighting, and residential backup power supply;
3. Conventional power distribution projects with limited budget and relatively low short-circuit current in the circuit.

Key Considerations for Selection

1. PC Class ATS must be coordinated with upstream protective devices and cannot be used alone as a fault protection switch;
2. CB Class ATS usually has a longer transfer time, so it should be used with caution for sensitive electronic loads to prevent unexpected shutdown or restart of equipment;
3. In many locations, fire protection codes require the use of PC Class ATS to ensure that emergency power supply remains effective.

If your project involves upstream protection coordination, you may also refer to our MCCB product page and ACB product page for more information on protection coordination and system matching.

ATS Installation and Wiring Considerations

Key Installation Requirements

Installation environment:
Avoid humid, dusty, corrosive gas, and strong vibration areas. The ambient temperature should be controlled within -25°C to +40°C, the cabinet should have good ventilation and heat dissipation, and the protection rating should match the actual site conditions.

Installation fixing:
Install the ATS vertically in the upright position. The cabinet should remain level and stable, with sufficient space reserved for inspection and operation. Inclined or inverted installation is strictly prohibited. When multiple units are arranged together, adequate spacing for heat dissipation should be reserved.

Safety clearance:
Standard electrical clearance should be maintained between different phase conductors and between incoming and outgoing lines. The insulation withstand requirements must be met to prevent creepage and phase-to-phase discharge.

Grounding requirements:
The equipment enclosure and cabinet must be reliably grounded. The grounding conductor specification should comply with the applicable electrical code requirements to eliminate potential electric leakage hazards.

Core Wiring Requirements

Phase sequence verification:
Strictly distinguish the incoming phase sequence of the normal power source and the backup power source, and ensure that the phase positions of both power sources are consistent. Incorrect phase sequence may cause transfer failure or equipment damage.

Separation of incoming and outgoing lines:
The incoming terminals are connected to the main and backup power sources, while the outgoing terminals are uniformly connected to the downstream load. Reverse wiring or mixed wiring is not allowed. Wiring must follow the terminal layout and identification marks of the equipment.

Cable selection:
The conductor current-carrying capacity must be greater than the rated current of the switch, and should meet the requirements for voltage drop and temperature rise. Wiring terminals must be firmly tightened to prevent false contact and overheating caused by looseness.

Neutral conductor handling:
3P/4P switches must be wired according to the system configuration. The neutral conductors of the two power sources must never be mixed or incorrectly connected, so as to avoid neutral circulating current and voltage shift problems.

Control circuits:
Secondary control wiring should be arranged neatly, with strong-current and weak-current circuits routed separately to reduce electromagnetic interference. Interlocking, communication, and generator start/stop circuits must be wired according to the drawings.

Inspection and Commissioning After Wiring

1. Before energizing, check whether there is any short circuit, incorrect wiring, or missed connection, and tighten all wiring points.
2. Test the automatic transfer function and manual transfer function in sequence, and verify that the actions of fault power-off and utility power recovery transfer-back are normal.
3. Verify the delay parameters, protection logic, and interlocking signals to ensure they are consistent with the design scheme.
4. During trial operation, observe the temperature rise of the switch, abnormal noise, and indicator light status. The ATS can only be put into formal operation when no abnormality is found.

Prohibited Actions

It is strictly forbidden to dismantle the mechanical and electrical interlocking mechanism without authorization; it is prohibited to manually close the two power sources in parallel; overload connection to the load is not allowed; and it is strictly forbidden to open the cover for wiring or disassembly while energized.

Common ATS Selection Mistakes and Troubleshooting Resources

The most common selection mistakes include choosing the ATS only according to load current without leaving a margin, mistaking an ATS for a UPS, failing to verify the pole configuration, overlooking generator start signal requirements, ignoring generator voltage build-up delay, failing to consider high-temperature derating, and directly applying a conventional generator-based solution to solar PV or energy storage systems.

If you encounter issues in actual applications such as “ATS not switching to generator” or “ATS instability in renewable energy systems,” you may further refer to our troubleshooting articles on the website.

Key Takeaways:

If ATS is treated simply as “just another ordinary switch to buy,” it will almost certainly create hidden risks later, such as wiring confusion, incorrect transfer actions, and even improper upstream tripping. A selection logic that can truly stand up to real-world testing must always follow this principle: define the system boundaries first, and then define the product parameters. Only after the system structure and application scenario are clearly understood should the pole configuration, control strategy, and protection coordination be determined, so that the final product model can be selected accurately.

Different types of projects have fundamentally different technical demands for ATS:

Residential buildings and commercial complexes: the core requirement is absolute reliability of basic switching action together with a balanced budget.

Large industrial facilities and critical infrastructure: priority must be given to upstream and downstream protection coordination, as well as the ability to withstand extreme short-circuit fault conditions.

Renewable energy and microgrid systems: low-level control logic compatibility and inverter coordination have become new technical priorities.

Therefore, it is meaningless to discuss ATS selection without reference to the actual system architecture. That is the essence of engineering design: there is no “one-size-fits-all” model here, only a solution tailored specifically to your system.

There is no universal ATS solution for every project. If you need help evaluating your system architecture, pole configuration, or protection coordination, you are welcome to contact our engineering team for professional support.

Frequently Asked Questions About ATS:

Q: If a PC Class ATS is used, is an upstream circuit breaker such as an MCCB or ACB still required?

A:Yes, it is absolutely necessary. A PC Class ATS is like the “railway switch operator” in a power distribution system: it is only responsible for changing the track, that is, switching the power source, but it is not responsible for braking, that is, interrupting short-circuit faults.

When a severe short circuit occurs, it can only withstand the extremely high rated short-time withstand current (Icw) for a short period, while waiting for the upstream MCCB or ACB to trip and clear the fault.

Without a circuit breaker acting as its protection, a PC Class ATS is very dangerous under short-circuit conditions.

Q:How should the rated current be selected? Is it enough to simply add up the load power shown on the drawings?

A:Never select it right at full-load current. Real engineering conditions are far more complicated than theoretical calculations. If the temperature inside the distribution cabinet rises to 50°C or even 60°C in summer, the equipment must operate under derating conditions. In addition, inductive loads such as pumps and fans can produce high inrush current during startup. For this reason, we usually recommend leaving at least a 20% safety margin above the actual maximum operating current.

Q:Can two power sources with different voltage levels share the same ATS?

A:This is strongly not recommended unless a specially customized controller is used. The ATS controller usually needs to sample the voltage of both power sources at the same time for comparison and decision-making. If the two power sources are not at the same voltage level, the control logic may become completely disordered.

In standard engineering practice, the proper solution is to first unify the voltage reference through a transformer and then perform the transfer.