How An ATS Works And Where Changeover Schemes Fit
Imagine a small hospital ward losing mains power during a storm — lights, ventilators and monitoring all need to come back quickly and predictably. An Automatic Transfer Switch (ATS) does that: it monitors the mains and a generator, starts the generator on mains failure, then transfers the load once the set is stable. If mains returns, it controls a safe retransfer.
This guide is for facilities managers, consulting engineers and site engineers in the UK who must choose or specify an ATS. In our experience, you’ll get clear criteria to decide between open, closed and soft‑loading schemes, what ratings to check, and the practical tests to insist on before handover. See our ATS panel options and compatible commercial diesel generators for context: ATS panels, commercial diesel generators.
What Most People Get Wrong
A common issue we see is selecting an ATS by amp rating alone. People overlook utilisation category, neutral switching and short‑circuit capability. That causes nuisance trips or unsafe backfeed risks when the site is commissioned.
Open Transition (Break‑Before‑Make): The Safe Default
Open transition opens one source before closing the other, so there is a brief interruption. It is simple, low‑cost and avoids any parallel with the public network. Typical sequence: detect mains fail, start engine, warm up, open mains contactor, then close generator contactor; reverse on return with a controlled cool‑down.
Downside: sensitive PLCs, VFDs and motor control can see a bump. If you need temporary resilience or short deployments, open transition with reliable servicing is often the right choice — consider short‑term generator hire while you assess longer‑term options: generator hire.
Closed Transition (Make‑Before‑Break): Near No‑Break Transfer
Closed transition synchronises the generator to mains so both breakers overlap briefly (typically milliseconds). Sync‑check relays and protection prevent uncontrolled backfeed. In our experience this reduces transients for sensitive process lines and drives.
Be aware: any momentary parallel with the public network can trigger ENA G99 and DNO consent. Plan protection settings and permissions early and involve the installation and maintenance team during design: installation and maintenance.
This image was generated with AI and may not always represent the product or service exactly.
Soft Loading And Paralleling: Ramped Import/Export
Soft loading uses synchronising controls, governor/AVR management and controlled ramp rates to parallel and move load with minimal disruption. You can configure kW and kVAr ramp rates to limit torque and voltage dips while hitting power‑factor targets.
If you run a data centre, hospital or a high‑value production line and cannot accept interruptions, soft loading or extended paralleling is appropriate. For any export to the public network you will need ENA G99 engagement, anti‑islanding protection and commercial agreements.
Specifying The Right ATS (63A–3200A): Ratings That Matter
Start with load current. At 400 V three‑phase use the kVA→A rule of thumb: ~1.44 A per kVA, then apply diversity and motor starting allowances. Check the utilisation category (AC‑33B for mixed motor/resistive loads), Icw/Icu fault ratings and whether neutral switching is required.
Quick Checklist
- Calculate worst‑case starting current and continuous kW
- Confirm AC utilisation category and Icu/Icw ratings
- Decide 3‑pole or 4‑pole switching based on neutral policy
- Specify bypass‑isolation for maintainability where needed
- Plan for metering, IP rating, cable entry and footprint
Use a calculator to sanity‑check sizing: kVA calculator.
Controls, In‑Phase Monitoring And Safety Interlocks
Controllers set start delays, transfer/retransfer delays and cool‑down. In‑phase monitoring enforces phase‑angle and slip thresholds to reduce disturbance during open transition. We recommend clear HMI diagnostics and remote alarm outputs for fault analysis.
Layered safety includes mechanical interlocks, electrical shrouding and key exchange. Standard I/O should include remote start/stop, genset ready, breaker status and Modbus or equivalent for BMS/SCADA integration. In our experience good commissioning saves repeated site visits.
Sector‑Based Recommendations: Matching Scheme To Risk
Match the transfer method to the consequence of interruption. Healthcare and utilities usually need closed transition or soft loading with documented testing, bypass‑isolation and strict governance. Advanced manufacturing and data centres favour soft loading with redundancy and load‑shed strategies.
When This Doesn’t Apply
If you run a small commercial shop or a short‑term hire site, the complexity and cost of closed or soft‑loading schemes often outweigh the benefit — open transition with robust maintenance may be the most pragmatic choice.
This image was generated with AI and may not always represent the product or service exactly.
Commissioning And Testing: Engineer Best Practice
Commissioning starts on paper: wiring checks, protection settings and controller configuration. Witness FAT/SAT, record transfer timings, in‑phase thresholds and alarm logic. Prove closed/soft synchronisation and backfeed protection under controlled conditions.
Load‑bank tests across kW steps and power factors validate performance. Capture O&M manuals, as‑builts, risk assessments and training records. For a practical test programme, follow industry guidance and our testing methodology: understanding generator load testing and why it matters.
Safety, Standards And Compliance
Apply BS EN/IEC 60947‑6‑1 for transfer switching, BS 7671 for wiring and, where paralleling is involved, ENA G99 with DNO engagement. In healthcare align with HTM 06‑01. Confirm earthing system (TN‑S, TN‑C‑S or TT) before finalising neutral switching and RCD strategy.
Enforce lockout/tagout, permits‑to‑work and arc‑flash risk controls. Supply test certificates, timing reports and operator training as part of handover to demonstrate compliance and due diligence.
Procurement, Deployment And Lifecycle Support
PowerTech Generators sources ATS panels and gensets to balance specification, lead time and cost. We deliver turnkey installation, planned servicing, 24/7 support and rapid hire for outages. Our UK engineering teams manage survey, design, installation and commissioning to reduce single‑point risk.
Consider lifecycle costs: planned generator servicing, spares strategy and periodic exercising keep an ATS dependable. In our experience, a modest annual budget for maintenance avoids expensive emergency call‑outs.
FAQs
How Do I Decide Between Open, Closed And Soft Loading?
Decide on allowable interruption and equipment sensitivity. Open transition suits low‑consequence sites; closed reduces transients at lower cost than continuous paralleling; soft loading is for no‑break requirements and export capability but needs DNO engagement.
When Should I Involve The DNO?
Engage the DNO early if any paralleling or export to the public network is planned. ENA G99 compliance and consent processes can affect design and commissioning timelines.
Is A 4‑Pole ATS Necessary For Portable Generators?
Use a 4‑pole ATS where TN‑C‑S, RCD selectivity or circulating neutral currents could cause nuisance trips or safety issues. For many portable installations it is the safer choice.
What Tests Should I Require At Handover?
Insist on wiring verification, recorded transfer timings, synchronisation checks, load‑bank results and full O&M documentation with training for onsite staff.
How Can I Keep My ATS Reliable Year‑Round?
Schedule planned generator servicing, battery maintenance, periodic exercising under load and annual protection setting reviews. Clear maintenance records and seasoning runs prevent failures when you need them most.