What Is an ATS? Automatic Transfer Switch

When the mains fails, the ATS (Automatic Transfer Switch) panel starts the generator automatically and transfers the load to it. The generator starts a few seconds after the outage; the load is typically transferred to the generator within 10–15 seconds. When the mains returns, the load is transferred back to the mains and the generator stops automatically after a cool-down run of a few minutes.

Most problems seen on site with ATS panels come from the wrong number of poles, delay times set to zero, or panels that were not tested during commissioning.

What is inside an ATS panel?

• Switching device: A contactor, motorised circuit breaker or motorised changeover switch. It connects the load to either the mains or the generator.
• Control unit: Monitors mains voltage, frequency and phase sequence. Manages the delays. Sends the “start” and “stop” commands to the generator.
• Mechanical and electrical interlocking: The mains and the generator can never be connected to the load at the same time. The safety of the system depends on this.
• Metering and alarms: Voltage, current, status information. Many models also offer remote monitoring.

The international reference for transfer switching equipment is IEC 60947-6-1. The utilisation category of the switching device must suit the type of load. In a facility with heavy motor loads, this detail matters.

How does the transfer proceed? Timings

Step	What happens?	Typical setting (approximate)
1. Outage detection	The mains fails or goes out of limits	1–5 s delay
2. Engine start	Start command goes to the generator	A few seconds
3. Steady state	Voltage and frequency settle	A few seconds
4. Transfer	Load moves to the generator	Total usually ~10–15 s
5. Mains return	Mains stability is monitored	Usually a few minutes
6. Retransfer	Load returns to the mains	—
7. Cool-down	Generator runs off-load and stops	Usually 3–5 min

The values are examples. Actual settings are made according to the facility, the generator control panel and the relevant regulations.

Remember two things. First: during those 10–15 seconds the load has no power. Servers, medical equipment or PLCs cannot tolerate that gap; for them, UPS and generator together is the answer. Second: the waiting time on mains return is there for a reason. Handing the load straight back while the mains is dropping in and out a few times hits motors and equipment with shock after shock.

Types of transfer

• Open transition (break-before-make): One source opens first, then the other closes. This is the most common solution. There is a short interruption during the transfer.
• Delayed transition (centre-off): The switch waits briefly in the “0” position. On large pump and fan motors this gives the residual voltage time to decay and reduces the inrush current.
• Closed transition (make-before-break): On mains return, the two sources run in parallel for a very short time and the load is transferred without interruption. It needs synchronization and additional protection. Paralleling with the grid may require permission from the distribution company. Details are in our article on synchronized (parallel) operation.

How do you choose the right ATS?

1. Current. In a 400 V three-phase system:

I (A) = kVA × 1,000 ÷ (√3 × 400) ≈ kVA × 1.44

Generator	Rated current (approximate)	Typical ATS rating
50 kVA	72 A	80–100 A
100 kVA	144 A	160 A
250 kVA	361 A	400 A
500 kVA	722 A	800 A
1,000 kVA	1,443 A	1,600 A

Select the next standard rating above the calculated current. Cables and protective devices must also be matched to the same current.

2. Number of poles. 3-pole or 4-pole? The answer lies in the facility’s earthing system. Systems in which the generator neutral is earthed separately usually need a 4-pole ATS. We cover this in our article on generator earthing.

3. Short-circuit withstand. The ATS must suit the short-circuit current at its connection point and the upstream protective device.

4. Control compatibility. The control unit must be able to talk to the generator panel. Delays, test mode and alarm outputs must be adjustable.

5. Scope. The whole building, or only critical loads? Grouping critical loads on a separate panel often solves the job with a smaller generator and ATS. You can estimate the requirement with the power calculator.

Common mistakes on site

• The number of poles is chosen without regard to earthing. Result: residual current devices trip for no apparent reason when the load moves to the generator.
• The outage detection delay is set to zero. The generator starts on even a dip of a few hundred milliseconds in the mains. Dozens of unnecessary starts a day.
• The cool-down time is shortened. The turbo and engine stop while hot. A habit that eats into service life.
• Phase sequence is not checked. On transfer to the generator, a pump runs backwards. Usually noticed at the first real outage.
• The ATS is never tested. The generator is run every month, but off-load. The transfer is tried for the first time in a real outage, and sometimes it fails.

Checklist

• Has the rated current been calculated and the next standard ATS rating up selected?
• Are the earthing system and the 3/4-pole decision defined in the design?
• Are short-circuit withstand and the upstream protective device compatible?
• Is there mechanical and electrical interlocking?
• Have the delays (detection, return, cool-down) been set?
• Has the phase sequence been checked?
• Has communication with the generator control panel been tested?
• Is a periodic transfer test included in the maintenance plan?

Periodic testing

Only a real transfer shows that the ATS works. During scheduled maintenance the generator should be run on load, transfer and retransfer should be tried, and delays and alarms checked. If possible, a real outage should be simulated by opening the mains breaker. For the scope of maintenance, see our scheduled maintenance page.

Next step

The Terex Diesel team helps you choose the ATS panel to suit the generator rating and your facility. For product information, see our ATS transfer panel page; for your project, contact us.