ATS Fundamentals: What It Does And Who Needs It
An Automatic Transfer Switch (ATS) – also called a generator changeover switch – monitors the mains and, on failure, starts the generator, warms it up, transfers the load, then retransfers and cools down when the utility returns. It prevents dangerous back‑feeds, protects uptime and improves electrical safety across critical facilities.
Typical users include data centres, healthcare, manufacturing, utilities and construction. An ATS is often paired with new or used industrial generators for sale and delivered as part of a professional generator installation. Explore our engineered ATS panels range built for resilience and compliance.
UK Compliance Framework: BS 7671, Device Standards And DNO Approvals
Design in line with UK requirements:
- BS 7671 (notably Section 551 for alternative supplies) for earthing, switching and disconnection.
- ATS devices certified to BS EN 60947‑6‑1 with clearly stated Icw (short‑time withstand), Icm (peak making) and the correct utilisation category for the duty.
- Assemblies and panels built to BS EN 61439 where applicable.
Closed transition (momentary paralleling) usually requires DNO G99 permission (and G100 export limitation where relevant). Keep single‑line diagrams, protection settings, labels and signage current, and commission to a written test plan. Our turnkey installation maintenance services and proven case studies ensure safe, compliant outcomes.
Open Vs Closed Transition: Choosing The Right Topology
Open transition (break‑before‑make)
- Creates a brief interruption (typically 100–500 ms) during transfer.
- Simpler, lower cost, no paralleling with the mains.
- Suits most lighting and socket‑outlet loads; motors/transformers can see inrush on re‑energisation; some UPS systems ride through better than others.
Closed transition (make‑before‑break)
- Synchronises sources for a near‑bumpless transfer (overlap typically <100 ms).
- Reduces transients for motor, transformer and UPS loads; requires sync controls and DNO approval.
- In‑phase and delayed transfer features help limit torque shocks and nuisance trips.
For temporary resilience or trials, consider short‑term generator hire with the correct transfer mode.
This image was generated with AI and may not always represent the product or service exactly.
Neutral Switching: 3‑Pole Vs 4‑Pole And Where To Bond Neutral–Earth
Three‑pole ATS (solid neutral)
- Can be acceptable on some TN‑S systems with no risk of parallel neutral paths.
- Verify with BS 7671, considering RCD operation, SPDs and any shared neutral conductors.
Four‑pole ATS (switched neutral)
- Preferred in PME/TN‑C‑S buildings to prevent circulating currents and PEN hazards.
- Provide a single, defined neutral–earth bond point for islanded operation (often at the generator) to maintain disconnection times.
- Confirm settings, labels and interlocks during commissioning; see our concise FAQs.
PME Earthing: Safe Standby Operation And PEN Conductor Considerations
- On loss of mains, avoid reliance on a PEN by switching the neutral and creating a local TN‑S from the generator via a 4‑pole ATS.
- Install appropriately sized earth electrodes and verify impedance to achieve protective device operation and touch‑voltage control.
- Document the neutral–earth bond location, provide test points and ensure clear labelling at handover.
For preliminary sizing support, use our kVA calculator and compare options across our new generators portfolio.
Inrush And Transients: Protecting Motors, Transformers And UPS
- Transformers can draw 8–14× rated current on energisation; across‑the‑line motors 6–8× FLC. Residual flux alignment can worsen peaks.
- Mitigate with in‑phase transfer, controlled delays, soft‑starters/VSDs, and pre‑transfer contacts to signal UPS “on generator” modes.
- Generator performance matters: alternator/AVR stability, governor recovery and adequate kVA limit voltage/frequency dip.
Brands such as Cummins, Perkins, FG Wilson and Baudouin offer robust platforms. For AVR behaviour and settings, see a simple guide to automatic voltage regulators AVRs.
This image was generated with AI and may not always represent the product or service exactly.
Integrating With Existing Switchboards And Protection
- Confirm busbar ratings, ACB/MCCB/contactor interfaces, thermal clearances, cable entries, gland plates and required form of separation (Form 3/4).
- Match enclosure IP rating to the plantroom or outdoor location; plan earthing bars and segregation for safe termination.
- Coordinate protection so ATS withstand (Icw/Icm/kAIC) exceeds site fault levels, with discrimination upstream/downstream.
- Add metering and Modbus/BACnet for BMS/SCADA, with clear status and alarm points for remote monitoring.
Control Logic And Communications: Future‑Proofing Your ATS
- Modern controllers (e.g., Deep Sea, ComAp) provide remote start, breaker status, alarms, test modes and adjustable timings.
- Integrate demand signals, pre‑transfer contacts for UPS coordination, and scheduled exercise routines (with/without load).
- Use staged load shedding for limited generator capacity and black‑start sequences for dead‑bus recovery.
- Secure remote access and logging support 24/7 monitoring, maintenance planning and incident investigation.
Specifying The Right ATS: Ratings, Form And Environment
- Select a continuous current rating that meets or exceeds the load; choose the correct utilisation category (e.g., AC‑33iB for mixed motor duty).
- Verify short‑time withstand (Icw) and making capacity (Icm) against site fault level.
- Choose between mechanically interlocked breakers and purpose‑built contactor ATS, considering serviceability and fault ratings.
- Match form of separation and IP class to the installation environment; allow space and I/O for future expansion (tie‑breakers, multi‑source, multi‑set).
- For export generators projects or phased upgrades, design spare capacity and clear pathways to avoid rework.
Commissioning, Testing And Maintenance For Reliability
- Perform FAT/SAT with documented tests: source sensing, interlocks, open/closed transition, both transfer directions.
- Prove protection, earthing integrity, neutral switching, phase rotation, and labelling. Record all controller settings and as‑built drawings.
- Adopt planned generator maintenance and generator servicing with periodic live transfers, thermal imaging and exercise runs.
- Check contact wear, firmware, comms, battery health and charger settings to ensure readiness.
Procurement Checklist, Budget Drivers And Delivery
- Provide load data (kW/kVA, motor/UPS content, step loads), site fault level, earthing arrangement, and switchboard details (busbar, device types, space).
- Budget drivers: open vs closed transition, 4‑pole neutral switching, controller I/O and comms, form/IP rating, metering and integration scope.
- Clarify spares, SLAs, training and documentation requirements.
- Plan delivery, cranage, access, export packing and CDM/H&S documentation.
- Options include buy new or used, short‑term hire, trade‑in and decommissioning. For early pricing and scope alignment, talk to PowerTech Generators.
Why PowerTech Generators For ATS And Critical Power
PowerTech Generators is independent and multi‑brand, supporting Cummins, Perkins, FG Wilson, Baudouin and more. We design, supply, install, commission and maintain ATS and generator systems across healthcare, data centres, manufacturing, utilities and construction.
We deliver UK‑wide with export capability, 24/7 support and proven project delivery. Start your specification with our engineered ATS panels and end‑to‑end installation maintenance.
FAQs
Do I Need Closed Transition For My Site?
Not always. Closed transition benefits motor, transformer and UPS‑heavy loads but needs sync controls and DNO G99 approval. Many sites run reliably with open transition.
Should I Specify 3‑Pole Or 4‑Pole ATS?
In PME/TN‑C‑S systems, 4‑pole is usually preferred to avoid parallel neutrals and circulating currents. Confirm with BS 7671 earthing design and site testing.
What Size Generator Works With My ATS?
Size for starting currents, UPS charging and step loads to limit voltage/frequency dip. Use our kVA calculator and share load profiles for accurate selection.
Will A UPS Eliminate The Need For Closed Transition?
Not necessarily. A well‑sized UPS can bridge open transition breaks for IT loads, but motor and transformer loads may still benefit from in‑phase or closed transfer.
How Often Should I Test The ATS?
Monthly exercise with periodic live transfer testing is good practice. Include annual protection checks, thermal imaging and controller firmware reviews.
Can I Retrofit An ATS To An Existing Switchboard?
Yes, provided mechanical space, busbar access and protection coordination are achievable. A site survey confirms ratings, earthing and communication options before design.
What Documentation Is Required At Handover?
As‑built drawings, settings, test results, manuals, spares list and safe operating procedures. Include clear labelling and signage for sources, earthing and isolation.