Automatic Transfer Switch (ATS) for emergency power supplies (IEC 60947-6-1)
Automatic Transfer Switch (ATS) for emergency power supplies
The §551 emergency-generators guide and the generator-load-test guide cover the generator itself and testing it under full load. This article covers the switch that determines which source — mains or generator — actually supplies the installation: the Automatic Transfer Switch (ATS), whose switching requirements are laid down in IEC 60947-6-1.
Open transition: break-before-make
The most commonly used ATS design works on the break-before-make principle: the load is fully disconnected from one source before the other source is connected, with a short gap during which both switches are open. This prevents the mains supply and the standby generator from ever becoming unintentionally paralleled — a situation that can lead to a dangerous short-circuit current or backfeed towards the grid (see also the portable-generator-earthing guide for the earthing risks of a generator not correctly disconnected from the mains).
Closed transition: briefly paralleled
In a closed-transition ATS, both sources remain synchronized and briefly paralleled for a very short overlap period — for example to switch over sensitive electronics or a continuous process without any interruption at all. This requires an additional synchronization check (frequency, phase, voltage) confirming that both sources match closely enough before the overlap is permitted — a closed-transition ATS is therefore considerably more complex and expensive than an open- transition design, and is only used where the brief interruption of an open-transition switch is itself already unacceptable.
Transfer time
The total transfer time of an ATS (from the loss of mains voltage to voltage being available again via the generator) is a sum of: the mains-failure detection time, the disconnection time of the mains-side switch, the generator's start-up/warm-up time to nominal voltage/frequency, and the closing time of the generator-side switch. For critical applications (for example emergency lighting, see the emergency-luminaires guide) a separate UPS is often deployed to bridge the time until the generator is up to voltage, because an ATS transfer to a cold-starting generator is typically too slow for those applications (see also the UPS-continuity guide).
Note: this article gives the switching principle. The exact transfer time, the required synchronization tolerances for closed transition, and the precise test requirements vary per product and application and follow from IEC 60947-6-1 and the manufacturer's product documentation.
Practical relevance
When designing a standby power supply, it must be established up front whether the connected load can tolerate a brief interruption (open transition), or whether an uninterrupted transfer (closed transition, or a separate UPS bridge) is required — this choice determines both the required ATS type and whether an additional UPS is necessary.
Common mistakes
- Using a closed-transition ATS without correctly configuring the required synchronization check — an insufficiently controlled overlap between mains and generator can lead to a short-circuit current instead of a seamless transfer.
- Assuming any ATS transfer is fast enough for emergency lighting or other time-critical applications — a cold-starting generator typically has a start-up time far longer than the permitted interruption time for that application.
- Running the generator directly in parallel with the mains "for testing" without a correct ATS or synchronization arrangement — this is one of the most common causes of backfeed into the grid and a hazard to grid-operator personnel.