Automatic Transfer Switch Maintenance Guide: NFPA 110 Requirements and Preventive Maintenance Schedules

The Critical Role of the Automatic Transfer Switch

The automatic transfer switch is arguably the most critical single component in an emergency power supply system (EPSS). It is the device that detects a loss of normal utility power, signals the emergency generator to start, and transfers the building's emergency loads from the utility source to the generator. When utility power is restored, the ATS monitors the returning source for stability and transfers the loads back.

A generator can be perfectly maintained, fully fueled, and tested monthly -- but if the ATS fails to detect the outage, fails to send the start signal, or fails to transfer the load, the entire emergency power system is rendered useless. Many of the conditions that stop an ATS from working, such as loose connections, drifting settings, and stiff mechanisms, can be found and corrected through routine maintenance and testing.

NFPA 110 includes transfer switches in its weekly inspection and monthly testing requirements. This guide covers those requirements, explains where manufacturer instructions and NFPA 70B fill in the details, and provides a practical maintenance framework for facility managers and maintenance teams. Because section numbers have shifted between editions, confirm them against the edition your AHJ enforces.

How an Automatic Transfer Switch Works

Understanding ATS operation is essential for effective maintenance. The transfer sequence follows a specific logic:

Normal-to-Emergency Transfer

1. Voltage sensing -- The ATS monitors utility voltage, ideally on all three phases. When voltage falls below the "dropout" setting, the ATS treats the source as failed. ASCO describes 85% of nominal as a typical dropout setting for most loads.
2. Time delay on engine start -- NFPA 110 requires a time delay on starting the generator so that harmless momentary dips do not start it. The NFPA 110 annex says a nominal 1 second is adequate for most applications, and the delay must be short enough for the system to still meet its Type (for example, 10 seconds for Type 10).
3. Engine start signal -- After the time delay expires, the ATS sends a start signal to the generator control panel, initiating the engine cranking sequence.
4. Generator voltage sensing -- The ATS monitors the generator output, waiting for voltage and frequency to reach their "pickup" settings (ASCO's Group 5 controller defaults to 90% of nominal voltage and 95% of nominal frequency for the emergency source).
5. Time delay on transfer -- An optional brief delay after generator voltage is confirmed allows the generator to stabilize before accepting load.
6. Transfer operation -- The ATS mechanically transfers the load contacts from the utility source to the generator source. In a standard open-transition transfer, there is a brief interruption as the contacts break from one source and make to the other.

Emergency-to-Normal Retransfer

1. Utility restoration sensing -- When utility power returns, the ATS monitors voltage and frequency for stability.
2. Time delay on retransfer -- A programmable delay ensures the utility source is stable before retransferring. This prevents cycling between sources during intermittent utility restoration. Settings vary; ASCO's Group 5 controller ships with a 30-minute default, adjustable to suit the site.
3. Retransfer operation -- The ATS transfers loads back to the utility source.
4. Generator cooldown -- The ATS maintains the engine start signal for a programmable cooldown period (the same ASCO controller defaults to 5 minutes), allowing the generator to run unloaded before shutdown.
5. Engine shutdown -- After the cooldown period, the ATS removes the start signal and the generator shuts down.

NFPA 110 classifies systems by Type, the maximum time the transfer switch load terminals may be without acceptable power. For a Type 10 system -- required for NEC Article 700 emergency loads and healthcare essential electrical systems -- the entire sequence from utility failure to power at the load must complete within 10 seconds. Every time delay, every sensing threshold, and every mechanical operation must be set and maintained to fit within that window.

NFPA 110 Chapter 8 ATS Testing Requirements

NFPA 110 Chapter 8 establishes the testing and maintenance requirements for all EPSS components, including the ATS. The section numbers below are used in the 2010 and 2016 editions; confirm them in the edition your AHJ enforces. In healthcare, CMS surveyors check that generator and transfer switch maintenance and testing follow NFPA 110 (Life Safety Code tag K918).

Weekly Inspection -- Section 8.4.1

The EPSS, including its transfer switches and other appurtenant components, must be inspected weekly. For the ATS, a weekly look typically covers the controller display, alarm and fault indications, the switch being in automatic mode, and the general condition of the enclosure.

Monthly Operation -- Section 8.4.6

Transfer switches must be operated monthly. The monthly test consists of electrically operating the transfer switch from the primary (normal) position to the alternate (emergency) position and then back to the primary position. This is usually done during the monthly generator exercise required by Section 8.4.2. During each monthly test, verify at least the following ATS functions:

• The ATS detects the simulated or actual loss of normal power
• The ATS sends the engine start signal to the generator
• The ATS transfers loads to the generator after the generator achieves acceptable voltage and frequency
• The ATS retransfers loads to normal power when the utility source is restored (or simulated as restored)
• All time delays operate within their programmed parameters
• Transfer and retransfer operations complete without abnormal noise, arcing, or hesitation

Monthly testing should be performed by simulating a utility outage at the ATS (using the test switch or by opening the normal source breaker) rather than waiting for an actual outage. This ensures the test is controlled and documented.

Periodic Inspection and Maintenance

Beyond weekly inspection and monthly operation, NFPA 110 relies on a routine maintenance program based on the manufacturer's recommendations. It also requires EPSS circuit breakers to be exercised: annually (with the generator off) for Level 1 systems at 600 volts or less, and every six months for breakers over 600 volts, with a simulated overload test every two years. Industry guidance on ATS programs commonly calls for one major maintenance service per year plus periodic (for example, quarterly) inspections covering:

• Physical condition of the enclosure, wiring, and mechanical components
• Tightness of all electrical connections (torque verification)
• Condition of main transfer contacts (look for pitting, erosion, or discoloration)
• Operation of the mechanical transfer mechanism
• Verification of all sensing and control circuits
• Review of the ATS event log (if equipped with electronic controls)

NFPA 70B and Annual Maintenance

NFPA 70B, which became a standard (rather than a recommended practice) with its 2023 edition, sets maintenance intervals for electrical equipment based on an equipment condition assessment and calls for infrared thermography at intervals not exceeding 12 months. If your AHJ, insurer, or owner has adopted NFPA 70B, it applies to transfer switches alongside NFPA 110. Most ATS manufacturers also recommend a comprehensive annual maintenance service that goes beyond the monthly operational check.

2025 Edition: Start Signal Monitoring

The 2025 edition of NFPA 110 adds a requirement for Level 1 systems: the integrity of the start signal circuit from a remote device such as an ATS must be monitored for broken, disconnected, or shorted wires, and a fault in that circuit must start the generator. Where your AHJ has adopted the 2025 edition, include this monitoring in your ATS and generator checks.

Monthly ATS Exercising Procedures

The monthly ATS exercise should follow a consistent, documented procedure. The following protocol covers the essential steps:

Pre-Test Checks

1. Verify the generator is in automatic standby mode and ready to start
2. Confirm fuel level is adequate for the test duration
3. Check the ATS controller display for any active alarms or fault indications
4. Notify building occupants and relevant personnel that a power transfer test is about to begin
5. Ensure any connected loads that are sensitive to momentary power interruption are protected or bypassed

Test Execution

1. Initiate the test by activating the ATS test function. Most modern ATS units have a test position on the controller that simulates a utility failure without requiring the normal source breaker to be opened.
2. Observe the transfer sequence -- Verify that the generator starts within the expected cranking time, achieves rated voltage and frequency, and that the ATS transfers smoothly.
3. Monitor operating parameters during the loaded run period -- voltage, frequency, and current on the emergency bus.
4. Time the transfer -- Using a stopwatch or the ATS controller's built-in timer, record the elapsed time from simulated utility loss to load transfer. For Type 10 systems, this must be within 10 seconds. In healthcare, if the 10-second criterion is not met during the monthly test, CMS expects a process to confirm the capability annually for the life safety and critical branches.
5. Initiate retransfer by returning the ATS test switch to the normal position. Verify the ATS retransfers to the utility source after the programmed time delay.
6. Verify generator cooldown -- Confirm the generator continues to run for the programmed cooldown period before shutting down.
7. Check for alarms -- After the test is complete, verify the ATS controller shows no active alarms and has returned to normal standby mode.

Post-Test Documentation

Record the following information for each monthly test:

• Date and time of test
• Name of person conducting the test
• Transfer time (seconds from utility loss to emergency power at the load)
• Retransfer time delay (minutes)
• Generator cooldown duration
• Any abnormalities observed (unusual noise, hesitation, arcing, alarm conditions)
• Corrective actions taken or required

Annual ATS Maintenance Checklist

The annual ATS maintenance service is the most thorough maintenance event in the ATS lifecycle. It should be performed by a qualified technician familiar with the specific ATS make and model.

Mechanical Inspection

• Inspect main transfer contacts for pitting, erosion, discoloration, or excessive wear
• Measure contact gap and wipe per manufacturer specifications
• Check contact spring tension and mechanical linkage alignment
• Lubricate mechanical pivot points per manufacturer instructions (use only approved lubricants)
• Inspect the operating mechanism (motor operator or solenoid) for wear or damage
• Verify mechanical interlocks operate correctly -- contacts must not bridge between sources
• Check the enclosure for moisture intrusion, corrosion, or pest entry

Electrical Inspection

• Perform infrared thermography scan of all connections under load (if accessible without de-energization)
• Torque all power connections to manufacturer specifications
• Torque all control wiring terminations
• Inspect all wiring for damaged insulation, loose terminations, or signs of overheating
• Check grounding connections for tightness and continuity
• Verify proper phase rotation on both normal and emergency sources
• Test overcurrent protective devices (if integral to the ATS)

Controls and Sensing

• Calibrate voltage sensing -- Verify the ATS picks up (transfers) and drops out (retransfers) at the correct voltage thresholds
• Calibrate frequency sensing -- Verify frequency sensing thresholds are set correctly
• Test and verify all time delays: time delay on failure, time delay on transfer, time delay on retransfer, engine cooldown timer
• Test all auxiliary contacts and control relays
• Verify the engine start signal circuit operates correctly
• Test the ATS exerciser timer (if equipped) for correct day and time programming
• Download and review the ATS event log for anomalies
• Replace the controller battery (if applicable) per manufacturer schedule

Comprehensive Annual Maintenance Schedule

Voltage and Frequency Sensing Calibration

The ATS voltage and frequency sensing circuits are the decision-making elements that determine when to transfer and retransfer. Incorrect calibration can cause either nuisance transfers (sensing thresholds too tight) or failure to transfer (thresholds too loose). The values below are manufacturer defaults and typical settings, not NFPA 110 requirements; use the settings in your switch's documentation and design.

Voltage Sensing Parameters

Dropout voltage (failure threshold): The voltage level below which the ATS considers the source to have failed and begins the transfer sequence. ASCO gives 85% of nominal as a typical dropout setting for most loads. For a 480V system, that means the sequence starts when voltage drops below about 408V.

Pickup voltage (restoration threshold): The voltage level above which the ATS considers the source acceptable again, a prerequisite for retransfer. ASCO gives 90% of nominal as typical. Pickup is set higher than dropout so the switch does not cycle while voltage hovers near the threshold. Sensitive loads such as healthcare and data equipment may call for closer settings.

Frequency Sensing Parameters

Frequency dropout and pickup: Frequency works the same way. ASCO's Group 5 controller, for example, defaults to a normal-source frequency dropout of 90% of nominal (54 Hz on a 60 Hz system) and a pickup of 95% (57 Hz). Many controllers also offer over-frequency and over-voltage trip settings.

Frequency sensing is particularly important for the generator source, as governor instability or overloading can cause frequency deviations that should prevent load transfer until the generator stabilizes.

Common ATS Failure Modes and Prevention

Understanding the most common ATS failure modes allows you to focus maintenance efforts where they will have the greatest impact.

Contact Welding

Cause: High-current arcing during transfer or retransfer welds the contacts in the closed position. This is most common in older ATS units that have not been exercised regularly.

Prevention: Monthly exercising keeps contacts clean and free-moving. Annual contact inspection identifies early signs of pitting that could lead to welding. Properly sized ATS rated for the connected load prevents contacts from operating near their thermal limits.

Controller Failure

Cause: Electronic control board failure due to power surges, moisture intrusion, component aging, or firmware corruption.

Prevention: Surge protection on the ATS control circuit, regular inspection for moisture intrusion, and maintaining controller firmware updates per manufacturer recommendations. Keep a spare controller board on-site for critical installations.

Mechanical Binding

Cause: Corrosion, debris, or lack of lubrication in the mechanical transfer mechanism prevents the contacts from moving freely.

Prevention: Annual lubrication per manufacturer specifications, enclosure integrity maintenance to prevent moisture and debris entry, and monthly exercising that keeps mechanical components moving.

Sensing Circuit Drift

Cause: Voltage and frequency sensing thresholds drift over time due to component aging, resulting in either nuisance transfers or failure to transfer.

Prevention: Annual sensing calibration verification and adjustment as part of the maintenance program.

Loose Connections

Cause: Thermal cycling of power connections causes gradual loosening over time. Loose connections create resistance, which generates heat, which accelerates further loosening in a destructive cycle.

Prevention: Annual torque verification of all power and control connections. Infrared thermography at least every 12 months (more often for critical or poor-condition equipment) to identify hot spots before they cause failure.

Open-Transition vs. Closed-Transition Transfer

The transfer method used by the ATS has implications for both the connected loads and the maintenance program.

Open-Transition Transfer

In an open-transition transfer, the ATS breaks contact with the current source before making contact with the new source. There is a brief power interruption during which the load is disconnected from both sources. When transferring between two live sources (as during a test), in-phase monitoring or a delayed-transition switch helps prevent damage to motor loads from residual voltage.

Characteristics:

• Simplest and most common transfer method
• Momentary interruption acceptable for most emergency loads (lighting, fire alarm, smoke control)
• No risk of paralleling the generator with the utility
• Lower equipment cost

Closed-Transition Transfer

In a closed-transition (make-before-break) transfer, the ATS briefly connects both sources simultaneously before disconnecting the original source. The overlap is kept very short, and the controller checks that voltage, frequency, and phase angle are closely matched before it occurs. Controllers also include an extended-parallel timer that acts if the overlap runs too long (ASCO's Group 5 default is 0.5 seconds).

Characteristics:

• No power interruption to the load during transfer
• Requires source synchronization capability
• Brief period of generator-utility parallel operation (the utility may need to approve this under its interconnection rules)
• Higher equipment cost and more complex controls
• Used for loads, such as digital equipment, that cannot tolerate a power interruption during transfers between two live sources

Maintenance implications: Closed-transition ATS units require additional maintenance of the synchronization controls, phase-matching circuits, and the paralleling logic. Annual maintenance should verify synchronization accuracy and confirm the overlap duration does not exceed the programmed limit.

Bypass Isolation Switch Considerations

Many critical ATS installations include a bypass isolation switch that allows the ATS to be electrically isolated for maintenance while maintaining power to the emergency loads through a manual bypass path.

Why a Bypass Isolation Switch Matters

Without a bypass isolation switch, performing ATS maintenance requires either de-energizing the emergency loads or working on energized equipment. Neither option is acceptable for facilities where continuous emergency power availability is required (such as hospitals or data centers).

A bypass isolation switch provides three operational positions:

1. Normal operation -- Load is fed through the ATS, which operates automatically
2. Bypass to normal -- Load is fed directly from the utility source, bypassing the ATS, allowing ATS maintenance while maintaining power
3. Bypass to emergency -- Load is fed directly from the generator source, bypassing the ATS

Bypass Switch Maintenance

The bypass isolation switch itself requires maintenance, though less frequently than the ATS:

• Annual inspection of bypass contacts and mechanical operation
• Torque verification of all bypass switch connections
• Functional test to verify the bypass path provides power to the load
• Verification that the ATS is properly isolated when the bypass switch is in the service position
• Interlock verification to confirm the ATS cannot transfer while in bypass mode

Building a Sustainable ATS Maintenance Program

A reliable ATS maintenance program integrates the following elements:

1. Weekly inspection and monthly exercise testing with documented transfer times and operational verification
2. Thermographic inspections at least every 12 months to catch connection issues before they cause failure
3. Annual comprehensive maintenance by qualified technicians familiar with your specific ATS make and model
4. Spare parts inventory for critical components -- at minimum, keep a spare controller and a set of main contacts on-site for critical installations
5. Condition-based replacement planning -- Track contact wear over annual inspections and plan replacement before contacts reach their wear limits
6. Training for facility staff -- Ensure on-site personnel can perform monthly exercise tests, recognize alarm conditions, and operate the bypass isolation switch in an emergency
7. Service contract with qualified provider -- Maintain an ongoing relationship with a service provider who knows your equipment and can respond quickly to emergency repairs

The ATS is the bridge between your generator and your emergency loads. Maintain it with the same priority you give to the generator itself, and it will deliver reliable automatic power transfer when your facility needs it most.

Sources and References

• NFPA 110: Standard for Emergency and Standby Power Systems -- National Fire Protection Association. https://www.nfpa.org/codes-and-standards/nfpa-110-standard-development/110
• NFPA 110 Maintenance and Testing -- Curtis Power Solutions. https://www.curtispowersolutions.com/nfpa-110-maintenance-testing
• NFPA 110 for Emergency Power Systems: 2025 Updates (PowerHour, August 21, 2025) -- Cummins Inc. https://www.cummins.com/sites/default/files/2026-04/2025-aug-nfpa-110-for-emergency-power-systems.pdf
• NFPA 110 Levels 1 and 2 Standards for Generator Systems (Information Sheet 25) -- Clifford Power Systems. https://cliffordpower.com/wp-content/uploads/2024/02/IS_25_NFPA110_Level1_2-REV-11-1-19.pdf
• Form CMS-2786R, Fire Safety Survey Report, 2012 Life Safety Code, Health Care (K918) -- Centers for Medicare and Medicaid Services. https://www.cms.gov/Medicare/CMS-Forms/CMS-Forms/downloads/cms2786R.pdf
• Basic Control of Automatic Transfer Switches (white paper) -- ASCO Power Technologies, hosted by Curtis Power Solutions. https://www.curtispowersolutions.com/hubfs/Files/Technical%20Information/ASCO/White%20Papers/ATS/ASCO%20Basic_Control_of_Automatic_Transfer_Switches_168394_0.pdf
• User's Guide, Group 5 Controller for 4000 and 7000 Series Transfer Switches -- ASCO Power Technologies, hosted by CI Group. https://cigroup-us.com/wp-content/uploads/2023/01/Group-5-Control-Panel-Users-Guide-PN-381333-126.pdf
• NFPA 70B: Standard for Electrical Equipment Maintenance -- National Fire Protection Association. https://www.nfpa.org/codes-and-standards/nfpa-70b-standard-development/70b
• Don't Panic: A Practical Approach to NFPA 70B and IR Thermography -- Rozel (70E Consultants). https://www.70econsultants.com/dont-panic-a-practical-approach-to-nfpa-70b-and-ir-thermography/
• NFPA 70E: Standard for Electrical Safety in the Workplace -- National Fire Protection Association. https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e
• NFPA 70: National Electrical Code (NEC) -- National Fire Protection Association. https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70