A standby generator is useless if nothing connects it to the load when the utility fails - and no one wants to run out to a site to throw a switch during an outage. The automatic transfer switch does that job in seconds, sensing the failure, starting the generator, and moving the load across, then reversing when normal power returns. This guide explains what an ATS is, how it works, the difference between open and closed transition, and where it fits in oil and gas backup power.
ATS in one line: An automatic transfer switch (ATS) is a device that automatically switches an electrical load between two power sources - typically the utility and a standby generator. When it detects that the primary source has failed or gone out of tolerance, it signals the generator to start and transfers the load to it, then transfers back and shuts the generator down once normal power is restored and stable.
An ATS continuously monitors the primary (normal) source for voltage and frequency. When it detects a failure or sustained out-of-tolerance condition, it starts a timing sequence: after a brief delay to ignore momentary dips, it signals the standby generator to start, waits for the generator to reach stable voltage and frequency, then operates its transfer mechanism to connect the load to the generator.
When the utility returns and stays stable for a set retransfer delay, the ATS transfers the load back to the normal source, then keeps the generator running unloaded for a cooldown period before stopping it. The whole cycle is automatic and unattended. The transfer mechanism itself is mechanically interlocked so the load can never be connected to both sources at once during a standard open-transition switch, preventing a dangerous cross-connection and generator back-feed.
Most transfer switches are open transition, or break-before-make: they disconnect from one source before connecting to the other, so there is a brief interruption - typically a fraction of a second to a couple of seconds - as the load transfers. This is simple and safe because the two sources are never tied together, but the momentary outage can drop unprotected loads (which is why critical controls sit behind a UPS).
A closed-transition (make-before-break) ATS briefly parallels the two sources - synchronizing the generator to the utility - so the load transfers with no interruption, then opens the source being left. This requires synchronizing controls and utility coordination and is used where even a momentary break is unacceptable. A third variant, soft-loading or bypass-isolation, adds the ability to service the switch without dropping the load. The choice balances cost, complexity, and how much interruption the load can tolerate.
Remote and unmanned oil and gas sites rely on standby generators to keep production, safety systems, and telemetry alive through utility outages, and the ATS is what makes that backup automatic - no operator has to travel to the site to switch sources. It protects critical loads such as SCADA, ESD systems, cathodic protection, and essential pumps, often working alongside a UPS that covers the momentary open-transition gap and a paralleling scheme on multi-generator sites.
An ATS reports its state - on normal, on emergency, transferring, generator running, and fault - through contacts or a communications port. A cloud SCADA such as Merobix can read those ATS status contacts or Modbus registers, so operators get an immediate alarm when a site transfers to generator, know that backup power engaged correctly, and can watch for a failure to transfer before it becomes a lost site.
A manual transfer switch requires a person to be on site to sense the outage and physically move the load to the generator and back. An automatic transfer switch does this by itself - detecting the failure, starting the generator, transferring the load, and reversing when utility power returns - which is essential for unmanned and remote sites.
Open transition (break-before-make) disconnects from one source before connecting to the other, so there is a brief power interruption but the sources are never tied together. Closed transition (make-before-break) briefly parallels the two synchronized sources for a seamless, no-break transfer, but needs synchronizing controls and utility coordination. Open transition is the common, simpler choice.
Yes. When it detects the primary source has failed, the ATS sends a start signal to the standby generator, waits for it to reach stable voltage and frequency, then transfers the load to it. When utility power returns and stabilizes, it transfers back and runs the generator through a cooldown period before shutting it down.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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