Automatic Transfer Switch (ATS) Basics: Why Transition Type Matters in Critical Power
An Automatic Transfer Switch (ATS) detects a mains failure and transfers the load to a standby generator, then returns it when utility power stabilises. In a typical chain, the mains feeds the main switchboard via the ATS; the diesel generator feeds the ATS; any UPS or static transfer switches sit downstream on critical circuits.
Open transition is break-before-make. Delayed transition adds a brief centre-off. Closed transition is make-before-break with a short, controlled parallel. Your choice dictates outage time, inrush risk and compliance with BS EN 60947-6-1, BS 7671, HTM 06-01 and EN 50600. For engineered ATS panels, commercial diesel generators and answers to common queries, see our FAQs. PowerTech Generators provides ATS, diesel generators, UPS integration and 24/7 support across the UK.
Open Transition ATS (Break-Before-Make): Simple and Robust
Open transition momentarily disconnects the load from all sources before making to the generator or mains. Transfer and retransfer typically complete in 100–500 ms. Most IT power supplies and lighting tolerate this short dip, but retransfer can cause inrush and transformer remagnetisation.
- Pros: simple, economical, widely available.
- Cons: step change in voltage/frequency and possible flicker.
Good practice settings and checks:
- Undervoltage pickup around 85–90% with a short time delay; add underfrequency checks.
- Retransfer delay to ensure stable mains and avoid nuisance transfers.
- Engine warm-up before load; cool-down after.
- Mechanical and electrical interlocks to prevent backfeed.
- Prove performance with generator load testing; for voltage control principles see our AVR guide.
Delayed Transition ATS (Centre-Off): Protecting Motors and Transformers
Delayed transition introduces a brief centre-off so residual voltages decay and transformer cores demagnetise before re-energisation. This reduces inrush and back-EMF when returning to mains, helping prevent nuisance trips and mechanical stress.
Typical centre-off delays are 300–1000 ms. Longer delays reduce inrush but may upset processes with limited ride-through. Choose delayed transition for motor-heavy sites, chillers, lifts and large distribution transformers. Avoid it on no-break clinical circuits that cannot tolerate any interruption. PowerTech Generators configures delays to match the load profile and site standards for stable, repeatable transfer.
Closed Transition ATS (Make-Before-Break): Near Zero-Break Transfer
Closed transition briefly parallels the generator and mains (often <100 ms) while synchronised. The ATS checks voltage, frequency and phase angle, and uses interlocks, reverse power protection and non-export controls to prevent backfeed.
Benefits include minimal transient, no visible flicker and reduced inrush—ideal for theatres, imaging suites, labs, data halls and control rooms. Constraints include DNO permission (typically under ENA G99), protection coordination and added cost/maintenance complexity. Where permitted, PowerTech Generators engineers synchronism checks, anti-parallel logic and settings to BS EN 60947-6-1 and BS 7671 for assured operation.
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Inrush and Step Load: Engineering Your Generator–ATS Pair
Across-the-line motors can draw 5–7x FLC; transformer magnetising inrush can reach 6–12x rated current; some UPS rectifiers and chargers add short spikes. These shape the transfer method and generator size.
Specify sets with strong step-load acceptance and tuned AVR/governor response to keep voltage and frequency dips within equipment limits. Many Cummins, Perkins, FG Wilson and Baudouin sets can accept 30–50% steps with quick recovery when configured correctly. Reduce retransfer stress with soft starters/VSDs, staged restarts or load sequencing. Size with realistic diversity and headroom for retransfer inrush. Use our kVA calculator to estimate demand before a detailed study.
Neutral Switching, Earthing and RCD Selectivity
Choose 3-pole (solid neutral) or 4-pole (switched neutral) ATS based on earthing and fault paths. On TN-C-S/PME systems, a 4-pole ATS often creates a separately derived generator source, preventing circulating neutral currents and reducing shock risk.
Apply the correct N–E bond at the active source only, and maintain clear earth fault return paths. Poorly managed neutrals can trip RCDs/RCBOs or defeat discrimination. Commissioning should verify insulation, continuity, earth loop values and the absence of parallel neutrals. For sensitive healthcare and data environments, confirm leakage currents and RCD selectivity are documented and tested.
Protection and Discrimination During Transfer
Selective coordination ensures downstream devices trip before upstream ones under both mains and generator fault levels. Short-circuit levels on generator are lower, so time–current curves must be checked for both sources.
ATS-related protections include under/overvoltage and frequency, sync-check and reverse power for closed transition, and robust backfeed prevention. Where applicable, vector shift or ROCOF can support protection schemes during brief paralleling. Complete a fault level study, set and label protection devices, and hand over a settings schedule at commissioning.
Healthcare and Data Centre Use Cases
Hospitals seek compliance with HTM 06-01: closed transition suits theatres, imaging and life-safety circuits where a visible dip is unacceptable; delayed or open transition can support non-critical areas. Routine testing, logging and user training are essential.
Data centres align to EN 50600 and Tier objectives. The ATS typically sits ahead of UPS input; UPS/STS manage ride-through while the ATS shifts source. Closed transition reduces transfer anomalies and inrush into UPS rectifiers, while non-export controls protect the grid. Match transition mode to RTO/RPO and the sensitivity of the downstream IT plant.
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Selection Framework: Choosing Open, Delayed or Closed Transition
Decide using four factors: load criticality, outage tolerance, grid permissions and budget/complexity.
- If a brief dip is acceptable and loads are mostly resistive/lighting: choose open transition.
- If motors/transformers dominate and inrush trips occur: choose delayed transition.
- If no visible interruption is required and DNO permits paralleling: choose closed transition.
- Where mixed loads exist: consider open for non-critical boards and delayed/closed for critical boards.
Examples: a plant with large compressors often benefits from delayed transition; construction welfare and site cabins typically use open transition; a water utility control centre may justify closed transition for zero-break returns. PowerTech Generators can package ATS and generator solutions to suit each scenario.
Installation, Commissioning and Ongoing Maintenance
Good installation starts with correct cabling, glanding and segregation, verified earthing, and proven interlocks. Label source and load sides clearly.
Commissioning checklist:
- Witnessed mains-fail and retransfer tests (record transfer times and stability).
- Step-load checks and harmonic observations under realistic load.
- Protection verification: device settings, function tests, and labelling.
- Documentation: as-left drawings, settings schedule and O&M packs.
For resilience, exercise the ATS and generator under load, log events, and use remote monitoring for alerts. Planned generator maintenance and periodic testing reduce failure risk and help ensure compliance. PowerTech Generators provides 24/7 callout and service plans for critical sites.
Procurement and Project Delivery with PowerTech Generators
Achieve resilience fast: buy new generators and ATS for a turnkey upgrade, source used generators UK for value, or arrange generator hire for temporary cover. We handle design, RAMS, permits, FAT/SAT, documentation and training.
We support export generators, financing and full compliance packs. End-of-life? We manage safe decommissioning and asset resale. Brands include Cummins, Perkins, FG Wilson and Baudouin. Speak to PowerTech Generators to scope an ATS upgrade or a complete critical power solution.
FAQs
Do I need DNO approval for closed transition?
Yes. Closed transition creates a brief parallel with the grid, which requires DNO permission (typically under ENA G99) and non-export controls. We manage the application and settings.
Will open transition harm sensitive IT loads?
Most IT PSUs ride through a 100–500 ms dip, but some legacy or poorly sized UPS may alarm. If zero-break is essential, consider closed transition.
When should I specify a 4-pole (switched neutral) ATS?
Use 4-pole on many TN-C-S/PME systems to avoid parallel neutrals and circulating currents. It also supports a separately derived generator source.
How do I size for motor and transformer inrush?
Account for 5–7x motor FLC and up to 6–12x transformer inrush. Add headroom and check step-load acceptance with appropriate AVR/governor performance.
How often should the ATS be tested?
Exercise monthly where practical, with annual witnessed on-load tests. Record transfer times, voltages, frequencies and any alarms.
Can a UPS remove the need for closed transition?
A UPS can ride through open or delayed transfers, but closed transition reduces flicker and retransfer inrush. Choose based on RTO/RPO and DNO constraints.