Automatic Transfer Switch Selection Guide | ATS Explained

Automatic Transfer Switch (ATS) Selection Guide – How to Choose the Right Changeover Switch

An Automatic Transfer Switch (ATS) provides a reliable way to transfer electrical power between two supplies, helping maintain power to essential equipment when the primary source fails.

Commonly used in commercial buildings, industrial facilities, data centres, hospitals and critical infrastructure, automatic transfer switches can change between mains and generator supplies or between two independent mains supplies.

Selecting the correct ATS involves more than simply matching the current rating. Factors such as supply configuration, number of poles, switching capacity, load type and control functionality must all be considered.

In this guide, h.TE Western Automation Bristol explains how automatic transfer switches work, the different types available and how to select the correct solution for your installation.

1. What Is an Automatic Transfer Switch?

An automatic transfer switch continuously monitors the availability and condition of electrical supplies. If the preferred supply fails or falls outside defined voltage or frequency limits, the ATS initiates a transfer to an alternative supply when it is available and suitable.

When the normal supply returns and stabilises, the ATS can automatically transfer the load back after a configurable delay.

A typical mains-to-generator arrangement operates as follows:

1. The ATS monitors the normal mains supply.

2. If the mains fails, the ATS controller sends a start signal to the standby generator.

3. Once generator voltage and frequency are acceptable, the ATS transfers the load.

4. When mains power returns and stabilises, the ATS transfers the load back.

5. The generator completes its cooling-down cycle before stopping.

The ATS prevents unintended connection of the two supplies together. Standard open-transition transfer systems briefly interrupt the load during changeover.

2. Types of Automatic Transfer Switches

Different installations require different changeover arrangements.

Important: A motorised changeover switch is not necessarily a complete automatic transfer switch. Some models require an external ATS controller to monitor supplies and initiate switching.

3. How to Select the Correct ATS Current Rating

The current rating is one of the most important considerations when selecting automatic transfer switching equipment.

Common ATS ratings include:

An ATS must be rated for the design current and the type of electrical load being switched.

For example, an installation with a calculated load current of 180A might require a 200A or 250A ATS, depending on the available ratings, utilisation category and design requirements.

Consider the following when sizing:

• Maximum anticipated load and allowance for future expansion.

• Supply voltage and whether the system is single-phase or three-phase.

• Motor starting currents and inductive loads.

• Short-circuit withstand and making capacity.

• Upstream protective devices and coordination.

• Manufacturer operating temperature and enclosure derating requirements.

An ATS must also have the appropriate utilisation category, such as AC-33 for applications involving motor or mixed resistive and inductive loads, where applicable.

4. 3-Pole vs 4-Pole Automatic Transfer Switches

A common question when selecting an automatic changeover switch is whether a 3-pole or 4-pole device is required.

3-Pole ATS

Switches the three phase conductors while maintaining a solid neutral connection. Suitable for certain three-phase installations where the earthing and neutral arrangement permits a common neutral.

4-Pole ATS

Switches all three phases and the neutral. May be required where sources have different neutral-earthing arrangements or where neutral switching is necessary for the installation design.

The decision depends on the earthing system, generator arrangement, neutral bonding, protective devices and relevant wiring regulations.

A 4-pole ATS is not automatically the correct choice for every generator installation. Neutral and protective conductor arrangements should be confirmed by the electrical designer before equipment is selected.

5. Open Transition vs Closed Transition ATS

Automatic transfer switches can have different transfer operating characteristics.

Open-transition ATS (break-before-make)

The normal supply is disconnected before the alternative supply is connected. This is a common arrangement for industrial and commercial standby power systems, and it involves a brief interruption to the load.

Closed-transition ATS (make-before-break)

For suitable installations, the two sources are momentarily paralleled during a controlled transfer. This requires appropriate source synchronisation, protection and approval where applicable.

Closed-transition equipment is generally selected for specialist applications where interruption must be minimised.

Where even a brief interruption cannot be tolerated, a UPS or other appropriate no-break system may also be required.

6. Choosing the Correct ATS for Mains-to-Generator Applications

For generator backup systems, select an automatic transfer switch with suitable generator management functionality.

Important features can include:

• Automatic generator start and stop signals.

• Adjustable mains failure and restoration delays.

• Voltage and frequency monitoring.

• Generator warm-up and cooling-down delays.

• Manual and automatic operating modes.

• Test functions and remote status indication.

• Suitable electrical and mechanical interlocking.

Check that the generator start interface and control sequence are compatible with the generator controller.

7. Automatic Transfer Switches from Leading Manufacturers

Several established electrical manufacturers offer automatic transfer switching solutions for commercial and industrial applications.

Socomec ATyS Automatic Transfer Switches

The Socomec ATyS range includes dedicated mains-to-mains and mains-to-generator automatic transfer switches, with different levels of monitoring, control and communications.

Suitable options include ATyS t M, ATyS g M, ATyS g and ATyS p.

Socomec

+1

Schneider Electric TransferPacT

The Schneider Electric TransferPacT range offers integrated and configurable transfer switching solutions, including automatic systems for smaller commercial installations and larger power distribution applications.

TransferPacT All-in-one covers ratings up to 1600A, with larger configurable solutions available.

Schneider Electric UK

ABB TruONE ATS

ABB TruONE automatic transfer switches integrate switching and control functions into a compact arrangement, making them suitable for numerous critical and standby power applications.

Different configurations provide varying levels of monitoring, automation and connectivity.

ABB Library

8. ATS Selection Checklist

Before ordering an automatic transfer switch, confirm the following details.

9. Automatic Transfer Switch Standards and Safety

Automatic transfer switching equipment should be selected in accordance with the relevant electrical installation requirements.

IEC/BS EN 60947-6-1 is a key standard covering transfer switching equipment.

Designers must also consider relevant requirements of BS 7671, earthing arrangements, fault protection, short-circuit ratings and the installation's operational requirements.

An ATS is not necessarily a substitute for circuit protection, and additional protective devices may be needed.

Equipment selection, installation and commissioning should be undertaken by suitably competent electrical professionals.

10. Automatic Transfer Switch Frequently Asked Questions

What is the difference between an ATS and a changeover switch?

An ATS incorporates or works with automatic control to detect supply conditions and initiate transfer. A manual changeover switch requires an operator, while a motorised changeover switch may require a separate controller.

Can an automatic transfer switch start a generator?

Yes. Many mains-to-generator ATS controllers provide a generator start signal, but the exact interface and generator compatibility must be checked.

Can an ATS be used with two mains supplies?

Yes. Mains-to-mains ATS systems can monitor two electrical supplies and transfer loads when the preferred source is unavailable.

How fast does an ATS switch?

Switching times vary by device, control settings, source availability and transfer arrangement. Generator systems will usually take longer because the generator must start and stabilise before accepting the load.

Does an ATS provide uninterrupted power?

Not necessarily. Most conventional ATS installations allow a brief interruption. Critical electronic systems may need a UPS or specialist power continuity equipment.

11. Automatic Transfer Switches from h.TE Western Automation Bristol

At h.TE Western Automation Bristol, we support electrical contractors, panel builders, OEMs and industrial customers with the selection and supply of electrical control and power distribution equipment.

Whether you require an automatic mains-to-generator changeover switch, a mains-to-mains transfer solution or a larger industrial ATS arrangement, our experienced team can help you identify suitable equipment based on your application and technical requirements.

We supply customers throughout Bristol, the South West and across the UK, offering competitive pricing, technical product support and nationwide delivery.

For advice on selecting an automatic transfer switch, contact our Bristol sales team or explore our range of industrial electrical products online.

View out (ATS) Automatic Transfer Switch range here