An automatic transfer switch (ATS) watches the utility supply and, when it fails, starts the generator and moves the load to it. When utility power returns and stays stable, it moves the load back. It is the link between the generator and the building, and it needs the same attention as the generator itself.
What the ATS does
A typical transfer sequence works like this:
- The ATS controller senses that utility voltage or frequency has dropped outside set limits.
- After a short time delay, which avoids starting the generator for momentary dips, it signals the generator to start.
- Once the generator reaches correct voltage and frequency, the switch transfers the load to the generator.
- When utility power returns, the controller waits through a retransfer delay to make sure the supply is stable.
- The switch transfers the load back to utility, and the generator runs through a cool-down period before stopping.
The timers and pickup settings in the controller shape this sequence. They should be set to suit the building and documented, so nobody changes them without understanding the effect.
Types of transfer switches
| Type | How it transfers | Where it is used |
|---|---|---|
| Open transition | Breaks the connection to one source before making the connection to the other, with a brief interruption | Most standby and emergency systems |
| Delayed transition | Open transition with a deliberate pause in the neutral position | Loads with large motors or transformers, where the pause lets residual voltage decay |
| Closed transition | Briefly parallels both sources during transfer so there is no interruption when both are available | Sites that test often and want to avoid a blip on retransfer; needs utility approval |
| Bypass-isolation | Adds a bypass so the ATS can be isolated and serviced while the load stays powered | Hospitals, data centres and other critical loads |
Transfer switches also differ in whether they switch the neutral, which matters for grounding and for ground fault protection. This is a design question for the electrical engineer, covered in part in our guide on grounding and bonding.
Emergency, legally required and optional loads
Larger buildings often have more than one ATS. Life safety loads such as emergency lighting, fire alarm and fire pumps may be on separate transfer switches from loads like elevators, heating or business equipment, and the electrical code requires certain separation between emergency and other circuits. CSA C282 covers emergency power supply systems for buildings, and the Canadian Electrical Code sets wiring rules. Check the current editions and your province's adoption.
Common problems
- Controls in the wrong mode. An ATS left in manual or test after maintenance will not transfer automatically.
- Dirty or worn contacts. Arcing during transfers wears contacts and can cause overheating.
- Loose connections. Terminations can loosen over time and heat up under load. Infrared thermography can find these while the switch is energized.
- Failed controller components. Batteries in the controller, sensing circuits and timers can fail without obvious signs.
- Mechanism problems. Switches that rarely operate can stick or move slowly.
- Undocumented setting changes. Timers adjusted during troubleshooting and never reset.
Testing and maintenance
The ATS should be exercised as part of every generator test, so that each test proves the full chain from utility failure to load on generator and back. Good practice includes:
- Testing by simulating a utility failure, not only by starting the generator manually
- Recording transfer and retransfer times and comparing them with the settings
- Checking indicator lights, alarms and remote annunciation
- Periodic de-energized maintenance by a qualified electrical technician: cleaning, inspecting contacts and arc chutes, checking mechanism operation and torque on terminations
- Infrared scans of the switch under load between de-energized services
- Keeping controller settings documented and checking them against that record
- Returning the switch to automatic and confirming no alarms after any test or service
Qualified workers only. Transfer switches are fed from two sources, so isolating one does not make the equipment safe. Maintenance must be done by qualified electrical workers following the site's lockout procedures and arc flash precautions. See arc flash basics.
De-energized maintenance usually needs a planned outage or a bypass-isolation switch. Plan it with building operations, and arrange temporary power for critical loads where necessary. For the generator side of the system, see standby generator maintenance.
How ehZee can help
Our diesel generator inspections look at the transfer switch as part of the standby power system: condition, settings, test records and how it performs during a test. We report what needs attention and can coordinate the follow-up with your electrical contractor.
Common questions
Why does the generator keep running after utility power returns?
The ATS waits through a retransfer delay to confirm the utility is stable, then the generator runs through a cool-down period with no load. Both are normal and protect the equipment.
Can we test the ATS without disrupting the building?
An open transition test causes a short interruption to the loads on that switch. Schedule tests with occupants in mind, or discuss closed transition or bypass options with your electrical engineer if interruptions are a problem.
How often should an ATS be serviced?
It should be exercised with every generator test and serviced on the schedule in the manufacturer's instructions and the applicable standard. Critical facilities often service more frequently.