Utility Power Loss but ATS Does Not Transfer Decision Tree

Why this matters

A customer reports the lights went out, the genset never started, and the lights came back on when utility returned. The ATS never transferred. The unit ran fine on its weekly exercise the day before. The fault sits at the ATS voltage-sensing circuit, not the genset itself. Misreading this as a generator fault burns hours on the wrong subsystem; the genset cannot start until the ATS tells it to start, and the ATS does not tell it to start until the ATS sees utility voltage drop below the dropout threshold.

Symptom presentation

Confirm the utility loss actually happened. Check the utility's outage reporting system, the neighbor's report, or the digital clock reset times on appliances. A momentary brownout below the dropout threshold may not register on the utility outage map but is enough to trigger or fail to trigger the ATS depending on the dropout settings.

Pull the ATS event log if the ATS has logging. Many modern units do; Generac RTS, Kohler RXT, ASCO 300, and Eaton ATC each log voltage sense events and transfer attempts with timestamps. The log will show whether the ATS saw the utility drop and what action it took. No event in the log during a confirmed utility loss is the diagnostic anchor.

Quick checks at the ATS

Open the ATS cover with the utility live and the genset off. Read voltage on the utility line side at the terminals. Confirm the voltage matches the panel voltage; a single-leg loss inside the residence past the ATS is not a utility loss to the ATS and the ATS will not transfer. Read the voltage at the ATS voltage-sense input terminals; a wire break between the line side and the sense input means the ATS reads a permanent loss and may already be transferred or stuck in a fault state.

Check the ATS dropout voltage setting. Most residential ATS units have a dropout threshold around 70 to 80 percent of nominal voltage; a service drop that browns to 200 volts may still be above the dropout and the ATS interprets the brownout as healthy power. Verify the threshold matches the OEM default and the local utility's expected brownout behavior.

Isolation tree

Start with the voltage-sense circuit.

Voltage at line side: present and matches panel. Move to sense input.

Voltage at sense input: present but ATS does not act. Read the ATS controller's reported voltage versus actual; a divergence indicates the sense board is reading wrong. Replace the sense board or the controller per the OEM service procedure.

Voltage at sense input: not present despite line side good. The wire break between line side and sense input is the fault; repair the wire run, confirm voltage at the sense input under live utility, and retest.

Voltage at sense input matches utility but ATS interprets as healthy: check dropout threshold. A unit set at 60 percent dropout will not transfer on a 70 percent brownout. Adjust dropout threshold per the OEM service procedure and the local utility's expected brownout signature.

Voltage at line side and sense input both 0 during the reported outage: the ATS saw the utility drop. If the ATS did not signal the genset, the fault is in the ATS transfer logic or the time delay setting. Most residential ATS units have a delay-to-transfer setting around 10 to 20 seconds to avoid nuisance transfers on momentary utility blinks; a setting beyond the actual outage duration will cancel the transfer when utility returns before the timer expires. Confirm the delay-to-transfer matches the desired sensitivity for the property.

Confirming the call

Document the ATS event log, the voltage at line side and sense input under live utility, the dropout threshold setting, the delay-to-transfer setting, and the result of a simulated utility loss. The simulation is the operational sign-off; isolate the utility feed with the main breaker, confirm the ATS sees the loss, confirm the transfer happens within the OEM-rated time after the delay expires, confirm the genset starts and carries the load, restore utility, and confirm the retransfer happens within the OEM-rated time after the cooldown.

Remediation by branch

Wire break between line side and sense input: repair the wire run, torque to OEM spec, retest under live utility.

Sense board or controller fault: replace the board or controller per the OEM service procedure. Verify firmware against the OEM bulletin list before swapping; firmware updates have addressed nuisance transfer and missed transfer events on multiple residential ATS units.

Dropout threshold misconfigured: adjust to OEM default or to a setting that matches the local utility's expected brownout signature. Document the setting in the customer record.

Delay-to-transfer too long: shorten the delay if the outages in the territory are typically momentary blinks; lengthen if the territory has frequent momentary blinks the customer does not want the genset to chase. The setting is a customer judgment supported by the utility's outage statistics.

ATS contactor failed mechanically: a contactor that cannot move under its operator is a separate fault from the sense circuit. Replace the contactor or the full ATS per the OEM service procedure.

References

1. UL 1008, Standard for Transfer Switch Equipment, current revision.
2. NEC NFPA 70 Article 702, Optional Standby Systems, 2023 edition.
3. NFPA 110, Standard for Emergency and Standby Power Systems, 2022 edition.
4. Generac Power Systems, RTS Transfer Switch Service Manual, current revision.
5. ASCO Power Technologies, Series 300 Transfer Switch Installation and Service, current revision.