Servicing Your Automatic Transfer Switch

The Short Answer: An automatic transfer switch (ATS) automatically moves a building’s electrical load from utility power to a backup generator when the utility fails, then switches it back once power returns. It works without anyone on site, which keeps critical systems running through a power outage.

The ATS is one of the most important parts of a backup power system, yet it usually gets less attention than the generator it controls. If the switch fails, even a perfectly good generator may never pick up the load, and the facility stays dark.

This guide covers the difference between automatic and manual switches, how the unit works step by step, the transition types you will run into, the components inside it, and why maintenance matters. It closes with where these switches are used across commercial and industrial sites. All of it draws on what Duthie Power has learned servicing transfer switches and generators across Southern California since 1965, on equipment of every make and model.

Automatic Transfer Switch Basics

Every standby power system has two possible power sources: the utility grid and an on-site generator. Only one can feed the building at a time. The automatic transfer switch is the equipment that sits between them and controls which source is connected. It is wired between your incoming utility service and the generator, with the facility’s electrical load on the other side. When the utility power goes out, the ATS detects the disruption and switches the electrical load from the utility to the generator, providing a swift transition of power supply. Once the utility power is restored, the ATS automatically switches the load back, ensuring the transition is seamless.

In a code-driven system, the ATS is part of the emergency power supply system (EPSS) that protects life-safety equipment such as fire alarms and emergency lighting. That is why the switch is treated as critical hardware in hospitals, data centers, and industrial plants.

 

Automatic vs. Manual Transfer Switches

The two switch types differ in one key way: who moves the load between the primary power source and backup power and how fast it happens.

• Manual transfer switch: A person moves the load by hand, usually with a change-over switch or handle. Manual operation costs less and works where staff are on site, and a short delay is acceptable. Duthie installs and services manual transfer switches for sites like these.
• Automatic transfer switch: The unit senses the outage and transfers power with no human intervention, often using a motorized change-over switch to move the load. This is the fit for sites that need power back in seconds and cannot count on someone being there to throw a switch.

 

Key Features of an Automatic Transfer Switch

• Automatic operation: Moves the load from utility to generator, then back, on its own.
• Fast, reliable transfer: Restores power quickly so critical loads stay covered during a power outage.
• Hands-off convenience: Needs no manual steps, making it ideal for facilities with steady, high power demands.

 

How Does an Automatic Transfer Switch Work?

An ATS works in a continuous loop, watching the utility supply and acting the moment something changes. The process happens in four steps.

1. Monitor: The switch constantly checks the incoming power from the utility company, watching voltage and frequency on the normal utility power feed.
2. Signal: When it detects an outage or a supply that drifts outside safe limits, the ATS sends a start command to the standby generator.
3. Transfer: Once the generator is up to speed and producing stable power, the switch disconnects the load from the utility and connects it to the generator. Your electrical system now runs on backup power.
4. Return: When the ATS confirms the utility has come back and held steady, it transfers the load back to the primary power source and signals the generator to shut down and cool off.

This full cycle runs without anyone touching the equipment, which is what makes an ATS the reliable hand-off point between your two power sources.

For a closer look at the internal logic and the different switch designs, see our guide on how an automatic transfer switch works.

Transfer Switch Transition Types

Not all switches move the load the same way. The transition type describes what happens to the power during the hand-off, and it affects both cost and how sensitive equipment reacts. There are three common types.

Transition type	How it moves the load	Power interruption	Common use
Open transition	Disconnects one source before connecting the other (break-before-make)	Brief interruption during the switch	Standard for most commercial and emergency backup systems
Closed transition	Briefly connects both sources at once after syncing, then drops the utility (make-before-break)	None; a seamless power transition	Sites that cannot drop the load even for a moment, and routine test transfers
Delayed transition	An open transition that pauses in a neutral, off position for a set time before completing the transfer	Interruption plus a deliberate pause	Facilities with large motor loads that need residual voltage to settle

A quick correction to a common misconception: closed transition is the type that avoids any interruption, not the one that causes a delay. To do this, the generator must synchronize its voltage, frequency, and phase with the utility before the two briefly connect in parallel. If the generator cannot sync within the switch’s programmed limits, a closed-transition unit will fall back to an open-transition transfer, which produces a momentary break in power. An open transition transfer switch never parallels the two sources, so it is simpler and more common, at the cost of that short interruption.

 

Key Components of an ATS

Understanding what is inside the enclosure makes maintenance and troubleshooting easier. These are the main parts that make the switching mechanism work.

• Electrical contacts and contactors: These carry the current and make or break the connection between the power source and the load. Constant electrical load wears them over time, so they need to stay clean and free of corrosion.
• Switching mechanism: The heart of the unit, this is what physically transfers the load between utility and generator using the open or closed transition method described above.
• Control and transfer circuits: Relays and sensors monitor the health of the utility supply, detect outages, and command the transfer. A failed relay or sensor can cause an improper transfer or no transfer at all.
• Mechanical components: The manual handle and motorized change-over switch let an operator move the load by hand if needed. These parts need periodic inspection and lubrication to move freely.

Because relays, sensors, and contacts are the parts most likely to wear or fail, they are the focus of a good inspection routine.

 

Why ATS Maintenance Matters

An ATS spends most of its life waiting, then has to perform flawlessly the one time it is needed. Regular maintenance is what makes that possible, and it pays off in three ways.

• Reliability: A well-maintained switch transfers power without hesitation when the utility fails, preventing avoidable downtime.
• Safety: A failing ATS can create electrical hazards or leave life-safety systems like fire alarms and emergency lighting without power. In a hospital, where ventilators and other patient-care equipment cannot lose power even for a moment, a failed transfer can put lives at risk.
• Cost savings: Catching a worn contact or a weak relay early is far cheaper than repairing a failed transfer and the damage or lost operation that follows.

Environmental exposure is a frequent and underrated threat. Dust, moisture, and corrosion can all reach the electrical parts inside the enclosure. One coastal facility learned this the hard way: salt air slowly rusted its aging switch until a hole formed, rain worked its way in, and the resulting fault took down the entire backup power system, leading to costly repairs and extended downtime.

 

Visual Inspections & Cleaning

A professional ATS service goes beyond what an in-house team can catch on its own. It typically includes:

• Visual inspection for wear, discoloration, loose connections, and signs of overheating
• Contact cleaning to keep connections free of dust, grime, and corrosion
• Lubrication of moving parts, including the manual transfer handle
• Electrical testing—voltage checks, simulated transfer tests, and time-delay checks
• Battery check on the control system, including terminals for corrosion
• Mechanical testing under load to surface-worn relays, coils, or micro switches before they fail
• Calibration back to the manufacturer’s specifications

Having a certified technician inspect your ATS at least once a year ensures potential problems are caught early. These experts run in-depth tests that go beyond a basic inspection, such as checking internal components and conducting detailed load tests, and they verify the switch is calibrated properly and every component is functioning as intended.

Documentation matters just as much as the work itself. Keeping thorough records of every inspection, test, and repair lets you track the health of your ATS over time. Those records serve as a reference for future maintenance and provide the paper trail you need to support a warranty claim or troubleshoot a recurring issue, which is especially important for facilities that depend on the ATS to keep critical systems powered.

Where Automatic Transfer Switches Are Used

Any facility that cannot afford an unplanned power outage relies on an ATS to bridge the gap between utility power and its generator. The switch is standard equipment anywhere critical loads and critical applications need power without interruption, including:

• Data centers: Servers and cooling cannot pause, so the transfer has to be fast and dependable.
• Hospitals and medical facilities: Life-safety and patient-care systems require power the instant the grid drops.
• Manufacturing and industrial applications: A stalled line means lost output, scrapped material, and missed deadlines.
• Government and infrastructure sites: Water, communications, and emergency services depend on continuous power.

In each case, the ATS is the piece of power management that decides, in real time, whether the utility or the backup source carries the building.

 

Keep Your Backup Power Ready

An automatic transfer switch is the component that turns a generator into a working backup power system. It watches the utility, starts the generator, and moves the load in seconds, then hands power back when the grid returns. Routine inspection, testing, and calibration are what keep the switch ready to perform the one time your facility actually needs it.

That dependability only holds if the switch is maintained, and that is where an experienced service partner matters. Duthie Power has kept Southern California facilities powered since 1965, with certified technicians who service transfer switches and generators of every make and model. To have your ATS inspected or to talk through your backup power needs, request a quote or call us at 1-800-899-3931.