Automatic Transfer Switch (ATS) and Generator Changeover Panels
Automatic transfer switch: The operating logic and design criteria of the ATS panel that moves the load to the generator during an outage.
ContentsAn automatic transfer switch is the control and power arrangement that moves critical loads to the generator when the utility supply fails, and back again once it returns. In hospitals, data centres and continuous process plants the length of the interruption depends directly on how this panel is designed.
How an ATS panel works
The controller monitors the incoming supply continuously. When voltage or frequency leaves the configured window, a confirmation delay starts; this prevents needless transfers during brief disturbances. Once the delay expires the generator is started, and the load is transferred as soon as voltage and frequency are stable.
Types of transfer
Three approaches are used depending on the load mix. Where motor loads dominate, the way the transition is made matters as much as the starting current itself.
| Transfer type | Interruption | Application |
|---|---|---|
| Open transition | 40–200 ms | General purpose sites |
| Delayed open transition | 1–5 s dwell | Sites with many motor loads |
| Closed transition | Uninterrupted | Sites permitted to parallel with the utility |
Mechanical and electrical interlocking
If the utility and generator devices ever close at the same time, the generator is connected to the network out of synchronism, and the damage is severe. For that reason the panel carries both electrical interlocking through auxiliary contacts and a mechanical interlock. The mechanical interlock keeps protecting even if the control supply is lost.
Neutral switching and earthing
Where the generator is a separately derived source, the earthing point of the neutral must be defined clearly. With a four-pole transfer the neutral is switched as well and each source is earthed at its own point. Otherwise parallel neutral paths appear and residual current protection misoperates. See neutral and earth bar sizing for the busbar side.
Points to watch in panel design
An ATS panel has requirements a normal distribution board does not:
- Backed-up control supply (battery or UPS)
- Generator cable selected for the prospective short-circuit level
- Manual bypass for testing and maintenance
- Volt-free contacts or a communication port for remote monitoring
- A periodic exercise schedule configured in the controller
Key takeaways: automatic transfer switch
- How an ATS panel works — The controller monitors the incoming supply continuously.
- Types of transfer — Three approaches are used depending on the load mix.
- Mechanical and electrical interlocking — If the utility and generator devices ever close at the same time, the generator is connected to the network out of synchronism, and the damage is severe.
- Neutral switching and earthing — Where the generator is a separately derived source, the earthing point of the neutral must be defined clearly.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is an ATS an alternative to a UPS?
No, they complement each other. A UPS gives a seamless transition but supplies for a limited time; a generator provides long-duration energy but takes seconds to come online. Critical sites use both.
What transfer time is acceptable?
For loads that tolerate a short break, ten to fifteen seconds is acceptable. For IT and medical equipment the gap is bridged by a UPS, while the ATS is responsible for bringing the generator on stably.
Can closed transition be used everywhere?
No. Because the generator runs briefly in parallel with the utility, it requires the distribution operator’s permission and synchronising equipment.
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Devpan designs ATS panels around generator rating and load profile, treating interlocking, the earthing arrangement and maintenance access as one problem. Share your standby power scenario and we will review it with you.
