Power infrastructure
Both move a load from one power source to another. An automatic transfer switch (ATS) uses mechanical contacts and usually sits upstream of the UPS, choosing between the utility and the generator. A static transfer switch (STS) uses solid-state switches, transfers in a few milliseconds, and usually sits downstream of the UPS, choosing between two live UPS outputs.
It moves the building load from the utility to the generator when the utility fails, and back when it returns. The ATS senses the outage, signals the generator to start, waits for the generator to reach the proper voltage and frequency, then transfers. Cummins notes that without a UPS, power to the loads is lost for approximately 10 seconds while that happens. In a data centre the UPS carries the IT load through the gap, which is why the ATS sits upstream of the UPS and the UPS battery only needs to last until the generator is on line. How the UPS should behave on generator power is covered in UPS and generator compatibility.
Two ways of making the transfer, with and without a break. Cummins lists three open-transition types: open-delayed, open-fast-transition with sync and open-fast-transition without sync. All of them break before they make, so the load sees a short interruption, and the delayed version pauses in a centre position to let motor loads spin down. Closed transition makes before it breaks: both sources are synchronized in phase, voltage and frequency, then paralleled momentarily, under 100 ms, so downstream loads see no gap. Cummins notes that some utilities require closed transition for their interconnect rules and that local service requirements must be checked first, because paralleling with the utility is regulated.
It switches between two live sources faster than IT equipment can notice. LayerZero describes the static transfer switch as a solid-state system that transfers critical loads between independent live sources in less than 4 milliseconds, approximately one-quarter cycle, using silicon-controlled rectifiers. LayerZero notes that servers and sensitive controls often tolerate only a few milliseconds of disturbance before they trip or reboot, which is the point of the speed. An ATS is generally intended for utility-to-generator transfer; an STS is used between two sources that are both already running, such as two UPS systems.
| Automatic transfer switch (ATS) | Static transfer switch (STS) | |
|---|---|---|
| Switching device | Mechanical contacts | Solid-state (SCR) |
| Typical position | Upstream of the UPS | Downstream of two UPS systems |
| Sources | Utility and generator | Two live sources, usually two UPS outputs |
| Transfer | Open (with a break) or closed (paralleled briefly) | Less than 4 milliseconds |
| Protects | The building from a long outage | Single-corded IT loads from a source failure |
The UPS sits between them. On a utility failure, the UPS battery carries the load while the ATS brings the generator on; on a UPS failure, the STS moves single-corded loads to the second UPS.
Mostly because some equipment has only one cord. The Uptime Institute notes that single-corded devices are found in a larger share of installations than should be expected, and that some owners use large static transfer switches, or STS power distribution units, to give them dual power paths. It also points out the trade-off: a large STS concentrates risk, while a rack-mounted or point-of-use transfer switch spreads it as low as possible in the distribution. Vertiv adds an engineering detail: switching between sources can saturate the downstream transformer and cause an inrush current that overloads the sources and trips upstream breakers, which is why STS transfer timing is designed around the transformer.
With an annual inspection that measures, not just looks. ASCO's maintenance paper cites NFPA 110 for annual ATS inspection and maintenance including the measurement of specific electrical values, and recommends more frequent visits where the environment is severe. NFPA 110, as reprinted by Cummins, also requires transfer switches to be operated monthly. A thorough visit inspects the main and arcing contacts for wear, pitting and discoloration, checks the transfer mechanism, and measures the contact resistance across the main and arcing contacts. Where the switch has a bypass-isolation design, Cummins notes that the ATS can be drawn out for service while the load is fed through the bypass switch, so maintenance does not need an outage.
As power electronics, close to how a UPS is maintained. The switching devices have no contacts to wear, but the fans, the control boards and the connections age, and the transfer has to be proven. A visit typically reviews the event log, checks the fans and filters, scans the connections, and runs a controlled transfer between sources with both available. Because the STS sits right before the PDUs, it is usually serviced in the same window as the UPS maintenance for data centres that feeds it.
Yes. Transfer switch maintenance is delivered through specialist partners and quoted with the UPS work, so the transfer test, the generator run and the UPS check can be planned as one sequence. Send the switch make, model and rating through the quote request form.
LayerZero puts it at less than 4 milliseconds, approximately one-quarter cycle, using solid-state switching.
An open-transition ATS breaks before it makes, so there is a short interruption; a closed-transition ATS parallels both sources for under 100 ms and does not.
Because the generator must start before the ATS can transfer; Cummins puts the gap at approximately 10 seconds without a UPS.
Annually, with measured electrical values, per NFPA 110 as cited by ASCO, and operated monthly; severe environments call for more frequent visits.
Sources, checked 2026-10-04:
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