In short. Choose by the interruption the load can tolerate. If seconds are acceptable, an ATS with a generator is enough. If milliseconds matter between two clean sources, use an STS. If no break is acceptable, use a UPS, often with an STS and a generator behind it.
The decision table
| Load tolerance | Sources available | Suggested architecture |
|---|---|---|
| Seconds to minutes | Utility and generator | ATS |
| Fraction of a second, single-corded | Two clean feeds | STS |
| None, with generator backup | Utility, generator | Online UPS plus ATS and generator |
| None, highest availability | Two independent systems | Dual UPS with dual-cord loads, or UPS with STS |
| Poor-quality supply, brief outages accepted | Utility only | Stabiliser, or line-interactive UPS |
| Both quality and continuity | Utility and generator | Online UPS plus ATS |
Role of each device
- ATS: moves the building load between utility and generator, with a break of tens of milliseconds to seconds. See ATS service.
- STS: moves a load between two clean sources in milliseconds. See static transfer switches.
- UPS: carries the load on battery through the gap and conditions the supply. See UPS architecture.
Typical combined arrangement
Utility and generator feed an ATS. The ATS feeds the UPS input and non-UPS essential loads. The UPS feeds critical loads. If a second UPS or path exists, an STS or dual-cord arrangement joins them. Coordination of timing and settings matters. See ATS, STS and UPS coordination.
Questions to ask
- What is the load’s ride-through time?
- Is the load dual-corded or single-corded?
- What are the sources, and are they synchronised?
- What fault levels and protection exist?
- How will the system be maintained while live?
Other selection points
- Transition type for ATS: open, delayed or closed. See closed-transition control.
- Pole count for ATS: three or four, depending on neutral treatment. See earthing and neutral.
- Bypass for maintenance on each device.
- Ratings: current, voltage, fault withstand.
Worked illustration
A manufacturing plant has process loads that ride through two seconds, and a 40 kW control and IT room that cannot. The design: one ATS for the plant between utility and generator, and a 40 kW online UPS behind the ATS for the control room. No STS is needed because the loads are not single-corded across two sources. Adding STS or dual UPS would raise cost without benefit.
Which switch for which job
| Need | Device |
|---|---|
| Move a board between utility and generator | ATS |
| Switch a load between two clean synchronised sources in milliseconds | STS |
| Hold the load with no break through an outage | UPS |
| Allow service of a UPS without dropping load | Maintenance bypass |
| Give single-cord equipment two feeds | Rack STS |
Typical arrangements
- ATS upstream, UPS downstream: common and simple; the UPS rides through the ATS transfer.
- Two UPS systems, STS at the load: removes UPS as a single point of failure for single-cord loads.
- Dual-bus with dual-cord loads: no STS needed for those loads.
Selection checks
- Fault level and withstand rating of each switch.
- Transfer time versus load tolerance.
- Neutral switching: three-pole or four-pole.
- Control and sync arrangement.
- Maintenance bypass for every critical switch.
Example (illustrative)
A data room with dual-cord servers and some single-cord network devices: two UPS systems on A and B buses fed from separate ATS-backed generator sections; rack STS for the single-cord devices.
See transfer coordination and static transfer switch systems.
Common mistakes
- Choosing UPS for loads that tolerate an ATS gap.
- Using an STS between unsynchronised sources.
- No maintenance bypass.
What to send us
Send load ride-through data and supply details. Send your application details.
Questions engineers ask
Do I need both ATS and STS?
Not always. ATS serves the utility to generator change; STS serves two clean sources for single-corded loads.
Can a UPS replace an ATS?
No. It bridges short gaps; the ATS connects the generator for longer outages.
When is a four-pole ATS needed?
When the neutral must be switched to keep source earthing separate.
What transfer time can an ATS achieve?
Open transition is typically tens to hundreds of milliseconds, plus the generator start time before it.
This guide gives general principles. Selection needs a design review.