Automatic Transfer Switch (ATS) Programming

Understanding Automatic Transfer Switches

An Automatic Transfer Switch (ATS) is a critical component in any standby power system. It continuously monitors the utility power source for voltage and frequency deviations and, upon detecting an outage or power quality event, automatically signals the standby generator to start and transfers the electrical load from the utility to the generator. When utility power is restored and stable, the ATS retransfers the load back and signals the generator to cool down and shut off.

ATS equipment ranges from small residential units rated at 100-200 amperes to large industrial and mission-critical switches rated at 4000 amperes and above. Manufacturers include ASCO (Schneider Electric), Eaton, Russelectric, GE Zenith, and Cummins. Each manufacturer offers microprocessor-based controllers with extensive programming parameters that must be configured correctly for the specific application.

ATS Operating Modes

Open Transition (Break-Before-Make)

Open transition is the most common ATS operating mode. The switch disconnects from the current source before connecting to the alternate source, resulting in a brief power interruption (typically 60-100 milliseconds for contactor-type and 6-10 cycles for breaker-type). This mode is simple, reliable, and does not risk paralleling two power sources. It is suitable for most commercial and industrial loads including HVAC, lighting, pumps, and general-purpose equipment.

Closed Transition (Make-Before-Break)

Closed transition ATS units momentarily parallel the two power sources (for less than 100 milliseconds) during transfer, providing a seamless transition with zero interruption. This requires the generator and utility to be synchronized in voltage, frequency, and phase angle before the transfer occurs. Closed transition is specified for loads that cannot tolerate any interruption, such as data centers with synchronous UPS systems or continuous manufacturing processes.

Soft Loading Transfer

Some advanced ATS controllers support programmed load ramping during retransfer from generator back to utility. This prevents the sudden load impact on the utility transformer and reduces voltage transients on sensitive downstream equipment.

Key Programming Parameters

Voltage and Frequency Sensing

The ATS controller continuously monitors all three phases of the utility source. Critical threshold settings include:

• Undervoltage pickup: Typically set at 80-85% of nominal voltage. Below this threshold, the ATS initiates transfer to generator.
• Overvoltage pickup: Typically set at 110% of nominal voltage to protect loads from sustained overvoltage conditions.
• Underfrequency pickup: Typically set at 95% of nominal frequency (57 Hz on a 60 Hz system) to detect utility instability.
• Single-phase sensing: Detects loss of one phase, which can damage three-phase motors running on single-phase power.

Time Delay Settings

Proper time delay programming prevents nuisance transfers and ensures equipment safety:

• Time delay to engine start (TDES): 1-6 seconds. Allows momentary utility dips to recover before starting the generator unnecessarily.
• Time delay to transfer (TDTD): 0-30 seconds after generator reaches rated voltage and frequency. Ensures generator is stable before accepting load.
• Time delay to retransfer (TDRE): 5-30 minutes after utility restoration. Ensures utility power is stable and not experiencing intermittent outages.
• Engine cool-down time: 5-15 minutes of unloaded running after retransfer to allow gradual engine cooling before shutdown.

Retransfer Logic

Retransfer programming determines when and how the ATS returns the load to the utility source after an outage. Options include immediate retransfer upon utility restoration, time-delayed retransfer for stability verification, and programmed retransfer windows that avoid transferring during peak demand periods. For closed-transition ATS units, the retransfer sequence includes automatic synchronization verification before the momentary parallel operation.

Generator Exerciser Programming

ATS controllers include built-in exerciser functions that automatically start and run the standby generator on a programmed schedule to ensure readiness. Common exerciser configurations include:

• No-load exercise: Generator starts and runs for 15-30 minutes without transferring the load. Maintains engine lubrication and battery charge.
• Loaded exercise: Generator starts and the ATS transfers the building load for 30-60 minutes. Tests the complete transfer sequence under real load conditions.
• Schedule: Typically weekly or bi-weekly, programmed during low-occupancy hours to minimize disruption.
• Cooldown period: Post-exercise unloaded running time before shutdown.

Multi-ATS Sequencing

Facilities with multiple ATS units connected to a single generator require load sequencing to prevent overloading the generator during a transfer event. The ATS controllers are programmed with staggered transfer delays so loads are added to the generator incrementally. Priority loads (life safety, IT, medical) transfer first, followed by secondary loads (HVAC, lighting, convenience outlets). Load shedding logic may disconnect non-essential loads if generator capacity is insufficient for the total connected load.

Testing and Commissioning

ATS commissioning must verify every programmed parameter under actual operating conditions. NFM Consulting performs comprehensive ATS testing including simulated utility failure, generator start and transfer verification, retransfer timing, exerciser function testing, and load bank testing to verify generator capacity. All test results are documented in commissioning reports with oscillograph recordings of transfer events showing voltage and current waveforms during the transition.

Maintenance and Monitoring

Modern ATS controllers support remote monitoring via Modbus TCP, BACnet, or SNMP protocols. Integration with building management systems or SCADA platforms provides real-time status indication, transfer event logging, exercise schedule compliance, and alarm notification for equipment faults. NFPA 110 requires monthly no-load tests and annual load transfer tests for Level 1 emergency power systems.