Transfer Switch
The Power Failover

Voltage (All 3 Phases)

The ATS monitors voltage on all three phases of the normal source. If any phase drops below the adjustable undervoltage setpoint (typically 80-85% of nominal), the ATS begins the transfer countdown. The setpoint must be low enough to ride through momentary sags but high enough to detect a real outage.

Frequency

Monitors the power supply frequency (60 Hz in North America). Abnormal frequency indicates generator instability or utility grid problems. An overfrequency or underfrequency condition beyond the adjustable window (typically ±3-5 Hz) triggers a transfer or prevents retransfer to a source that is not stable.

Phase Sequence

Verifies all three phases are in the correct A-B-C rotation. If two phases are swapped (phase reversal), the motor would spin backward. The ATS blocks transfer to any source with incorrect phase sequence and annunciates a trouble signal.

Time Delay to Retransfer (TDRE)

After normal power returns, the ATS starts a timer — typically 30 minutes. This ensures the utility is truly stable and not flickering on and off (which is common during storm recovery). If normal power drops again during this window, the timer resets. Only after the full TDRE elapses with stable normal power does the ATS initiate retransfer.

Engine Cooldown Timer

After retransfer to normal, the ATS keeps the generator running for an adjustable cooldown period — typically 5-10 minutes. This allows the engine to dissipate heat under no-load conditions before shutting down. Abrupt shutdown of a loaded diesel engine can cause turbocharger bearing damage and thermal shock to the engine block.

Transfer Switch (Broad Category)

Any device that switches an electrical load between two power sources. Can be:

• Manual — operator physically moves a handle or lever to switch sources
• Automatic (ATS) — senses power loss and transfers without human intervention
• Non-automatic — requires a signal from a separate control to initiate transfer

ATS — Automatic Transfer Switch (Specific Type)

A transfer switch that operates automatically — senses normal power failure and transfers to the alternate source without any human action. For fire pump service:

• Must be listed for fire pump service per NFPA 20 §9.7
• Must handle locked rotor inrush current without tripping
• Must transfer in-phase to prevent motor damage
• Must not have load-management or load-shedding features

A Standard Building ATS Will Damage Your Fire Pump

General-purpose ATSes for HVAC and lighting have time delays (30-60 seconds), load shedding, and programmed transfer sequences. A fire pump ATS must transfer immediately — within seconds — and cannot shed the fire pump load for any reason. Using a standard building ATS on a fire pump circuit violates NFPA 20 and can cause the pump to drop offline during a fire when utility power is lost.

The ATS Must Handle In-Phase Transfer

When transferring a running motor between two power sources, the voltage phase angle must align. An out-of-phase transfer creates a momentary short circuit through the motor windings that can destroy the pump coupling, damage bearings, and trip breakers. Fire pump ATSes use phase monitoring to ensure the transfer occurs at the correct moment. This is one reason fire pump ATSes cost 3-5x more than standard units.

Retransfer Is More Dangerous Than Transfer

The transfer from normal to emergency is a planned event — the ATS knows normal power is gone. The retransfer back to normal is trickier: the ATS must verify that utility power has been stable for a preset time (typically 30 minutes) before switching back. A premature retransfer during unstable utility restoration can cause the pump to cycle off and on repeatedly, creating voltage transients and water hammer.

A Seized ATS Contactor Is the Most Common Failure Mode

ATS contactors are heavy-duty mechanical relays that carry hundreds of amps. If the ATS is never exercised (common in buildings that skip monthly testing), the contactor can seize in one position from corrosion or contact welding. When utility power finally fails for real, the ATS tries to transfer and the contactor does not move. Monthly exercise is not optional — it is the only thing that keeps the mechanical parts from seizing.

Some Fire Pump ATSes Have a "Bypass" Mode

Higher-end fire pump ATSes include a bypass-isolation switch that allows the ATS to be removed from the circuit for maintenance while the fire pump remains connected to one power source (usually normal). Without bypass, any ATS maintenance requires shutting down the fire pump — creating an impairment. Bypass-isolation ATSes are strongly recommended for healthcare and high-rise installations.

The Generator Must Be Sized for Locked Rotor Current

When the ATS transfers to the generator and the fire pump starts, the motor draws locked rotor current (5-7x FLA) for several seconds. The generator must be sized to handle this massive inrush without voltage collapse. A 100 HP fire pump can draw 800+ amps on startup — if the generator is sized only for running load, the voltage sags so badly the pump cannot start. Generator sizing for fire pump service follows NFPA 20 §9.6.