Transfer Switch
The Power Failover
The listed automatic transfer switch (ATS) that decides in milliseconds whether the fire pump runs on utility power or generator power.
What an ATS Does
An automatic transfer switch is a power-switching device with two sources and one load. In fire pump service, the two sources are Normal (utility feed) and Emergency (standby generator or second utility feed), and the one load is the fire pump controller. The ATS monitors Normal power continuously. When it detects a loss of phase, a sustained voltage sag, or an over-frequency event, it initiates a transfer to Emergency — typically within 10 seconds for a healthy generator.
NFPA 20 §9.7 requires the ATS used with a fire pump to be listed for fire pump service. That listing adds features a normal ATS doesn't have: locked-in operation against pump inrush, dedicated controller communication contacts, annunciated transfer and return status, and inhibit logic to prevent transfer during a pump start.
Transfer Sequence
- ATS senses loss of Normal power.
- After a short time delay (typically 1–3 seconds) to ride through momentary events, the ATS sends a start signal to the generator.
- Generator starts, warms up, and reports healthy voltage and frequency to the ATS.
- ATS transfers load from Normal to Emergency in a break-before-make or closed-transition sequence.
- Fire pump controller sees power again. If the pump was already commanded on, it re-starts immediately.
NFPA 25 Annual Test
§8.3.2.7 requires the ATS to be exercised in conjunction with the annual fire pump flow test. That means, with the pump running at rated flow, simulate a loss of Normal by opening the utility breaker or disconnect. The ATS should transfer, the generator should carry the load, and the pump should never stop pumping.
Field tip: Observe the ATS return-to-Normal sequence after the test. A healthy unit retransfers smoothly. If the pump drops out for more than a second or two, you've found something worth investigating before the real emergency.
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Watch on YouTube →References
1. NFPA 20 (2022), §9.7 — Transfer of power for fire pumps.
2. NFPA 110 (2022) — Standard for emergency and standby power systems.
3. NFPA 25 (2023), §8.3.2.7 — Transfer-switch test in conjunction with annual pump flow test.
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Discussion (2)
Great breakdown of the technical details. The NFPA 25 maintenance table is exactly what I needed for my ITM schedule.
Really clear explanation. Would love to see a companion video walkthrough of the inspection process.