Generator ATS Selection: Confirm the Electrical and Transfer Sequence Before the RFQ

Start with the approved electrical one-line

Identify both sources, generator(s), transformers, service and distribution arrangement, grounding and bonding points, protective devices, ATS location, downstream panels, emergency/legally required/optional branches as applicable, and interfaces to fire alarm, building management, UPS or load controls.

Do not ask a supplier to infer neutral switching, fault duty or sequence from a photo of the existing switchboard. The one-line must have an owner, revision and approval status. Record temporary or future sources separately.

Match electrical ratings and fault duty

Provide system voltage, frequency, phase, number of conductors, continuous current, load type, expected motor or transformer inrush, generator voltage and frequency tolerances, ambient, altitude, enclosure/environment and installation location. Select from the manufacturer's applicable ratings and listings.

Available short-circuit current and upstream protective devices must be coordinated with the ATS withstand and closing rating. A current rating alone does not prove fault suitability. Record the protective-device type, settings or fuse class required by the equipment listing and the study. A specific-breaker rating applies only with the breaker models on the equipment label; a time-based rating applies with other devices; and selective-coordination schemes that remove upstream instantaneous trips require the switch to carry fault current for a longer defined interval.

Decide poles and neutral from the grounding design

ASCO's ATS Selection Basics white paper explains that transfer switches are available with a solid or a switched neutral, and that a switched neutral pole is required where the alternate source is a separately derived system with its own ground point. Bonding both neutrals in that case can create more than one grounding path and leave a ground fault undetected by ground-fault sensing equipment. The decision affects ground-fault paths, neutral current and protection; it is not a preference or a default “more poles is safer” rule.

Provide the grounding and bonding diagram, source neutral-ground relationships, ground-fault protection, residual-current devices, four-wire loads and applicable code analysis. The responsible engineer should state whether the ATS switches the neutral and what other bonding or protection conditions follow.

Choose the transition method from the load and source relationship

Open transition breaks one source before making the other and briefly interrupts power — about 40 milliseconds for a solenoid mechanism, up to about a quarter second for a motor-operated switch. Delayed transition holds a timed disconnect position so inertial loads slow before transfer. Closed transition makes before breaking, momentarily paralleling the two sources under monitored source parameters and completing in under 100 milliseconds to limit impact on the utility. Paralleling is subject to the utility's and the design's requirements, and other transfer schemes have their own constraints.

Record the load's interruption tolerance, motor residual voltage, transformer behavior, UPS ride-through, process restart, phase relationship and any prohibition on paralleling. Do not select closed transition only to avoid a visible blink; verify synchronization, protection and failure behavior, including that the controller falls back to an open-transition transfer when a source fails.

Write the complete normal, failure and return sequence

Define normal-source monitoring; outage or abnormal-source pickup and time delay; generator start; warm-up and source-acceptable criteria; transfer; load shedding or sequencing; generator supervision; normal-source return; retransfer delay; cool-down; stop; exercise; test and manual modes.

Add failure branches: generator fails to start, source becomes unacceptable during transfer, switch does not reach position, both sources unavailable, controller power loss, communication failure, emergency inhibit and manual operation. State alarm, safe state, operator action and reset ownership.

Timers and thresholds must come from the approved design, load and equipment data. Do not copy another site's settings.

Plan commissioning, maintenance and safe bypass

Factory documentation should confirm model, ratings, poles, neutral, transition, controller, accessories, wiring and test results. Site commissioning should verify conductor identification and phasing, torque/installation records, sensing, start interface, source acceptability, transfer/retransfer, timers, alarms, interlocks, load response and documented manual mode under the approved safety plan.

If bypass-isolation is needed for maintenance continuity, include it in the one-line, space, switching, interlock and procedure review. Maintenance bypass is not permission to work energized or bypass safety controls.

Prepare a decision-ready ATS RFQ

Send the one-line and load schedule; source voltage/frequency/phase and generator controls; continuous current; available fault current and upstream protection; poles/neutral decision; transition; bypass; enclosure/environment; controller I/O and communications; sequence; standards/listing; documentation; factory tests; site commissioning; spares and support boundaries.

Use the generator sizing resource to define the generating set separately. Review available diesel generator packages, then request a quotation with the approved ATS inputs. GENKVA can configure a package against the brief; final electrical design and code acceptance remain project-specific.

Generator ATS FAQs