Automatic Transfer Switch (ATS) Testing Procedure

Automatic Transfer Switch (ATS) Testing Procedure

An Automatic Transfer Switch (ATS) is a device in electrical systems that automatically switches power supply between two sources, usually the utility power grid & a backup power source like a generator or battery system. 

During normal operation, the ATS distributes power from the primary source, which is often the utility grid. 

If the primary power supply fails, the ATS recognizes it right away and automatically transfers the load to the backup power source. 

This ensures that essential loads receive uninterrupted power supply, making it important in applications where continuous power is required, such as hospitals, data centers, & industrial facilities.

ATSs are available in a variety of designs, including 

  • Open-transition (break-before-make) &
  • Closed-transition (make-before-break), and 

can be constructed with accurate voltage & current ratings for many different applications.

It is essential to evaluate whether the ATS is within manufacturer specifications during a planned evaluation to avoid the danger of finding this when an unexpected loss happens and you are unable to convert your system to secondary power.

Occasional testing of your ATS ensures that the emergency system is reliable.

  • The check should be done both outside and inside on the transfer switch.
  • The transfer switch must be kept in good condition by completing a weekly comprehensive inspection. 
  • This check must include looking for indications of high heat, vibration damage, any amount of deterioration, leakage, or contamination.
  • Dirt and dust must be eliminated. Dirt, dust, and other contaminants must always be cleared from both the outside and interior using a vacuum cleaner, dry cloth, or brush. 
  • Blowing away pollutants and debris using compressed air is not recommended. This might result in debris becoming trapped in components, causing damage to the switch.
  • If the check reveals broken or loose components, contact a certified specialist to do the repairs.
  • Any damaged, broken, or missing external elements must be replaced using the manufacturer’s specified components.
  • Contact your local authorized distributor (or) dealer to order the specific part number you need.
  • All electrical sources must be shut off prior to any inside inspection.
  • When opening the switch door, check to see whether any external malfunctions have disturbed inside components.
  • All service work must be performed by a trained service technician. If one of the following conditions are identified:
    • Dirt, dust, moisture, and other pollutants accumulate on the surfaces of the unit & its components.
    • Any indications of corrosion
    • Loose, missing, or broken components.
  • Signs of overheating include melted plastic, discolored metal, or a burning odor, as well as damage to wiring or insulation from cuts, abrasions, or wear.
  • Any additional signs of damage, wear, (or) failure of the transfer switch & its components
  • Internal service work & inspection must be performed by a professional technician on a standby system that does not allow for power interruption during the inspection process.
  • Check both mechanically and physically.
  • Check alignment, anchorage, needed clearances, and grounding.
  • Check if the unit is clean.
  • Check for proper lubrication on the moving current-carrying parts & sliding surfaces.
  • Ensure that manual transfer notifications are attached & visible.
  • Perform a manual transfer procedure.
  • Check for positive mechanical interlocking between alternate and normal sources.

When interior inspections need more than a visual check by the operator, they must be carried out by an authorized distributor (or) distributor as part of a planned preventative maintenance agreement.

In terms of grounding, conduct insulation resistance testing on all control wiring.

  • Perform a contact/pole resistance test.
  • Check the settings and functionality of control devices.
  • Calibrate all relays & timers.
  • Check phasing, phase rotation, and the synchronized function as required.

Perform automated transfer tests:

  • Simulate the loss of normal electricity.
  • Return to usual power.
  • Simulate the lack of emergency power.
  • Simulate all types of single-phase conditions.

The transfer switch functioning testing includes both electrical & manual tests. A manual operator handle is included with the transfer switch for the maintenance purposes only. The switch should be tested manually before it may be activated electrically.

A typical technique of an automatic transfer switch (ATS) functional test for a standby generator is described below:

  • To start the test, turn off the regular source circuit breaker. When the appropriate voltage is detected, the switch controller will switch on the available LED. 
  • If source 1 activates the automatic transfer switch (ATS) mechanism, the LED at source 1 will light up. Verify the phase-to-phase voltages at the power line terminals.
  • Close the alternative source breaker and start the engine generator. When the appropriate voltage & frequency levels are detected, the S2 (Alternate) Available LED illuminates. After validating both sources, switch off the engine generator and set the start control to automatic.
  • Open Source 1, also known as the normal side breaker, to simulate a utility failure. The delay to the engine start timer starts its timing cycle. After the timer has finished its timing cycle, the engine start connections are closed to start the generator.
  • When the generator frequency & voltage reach the fixed reinstate points, the Source 2 available LED illuminates. Simultaneously, the delay generator timer starts its timing cycle. 
  • When the time delay is complete, the ATS will switch to Generator, the S1 location LED will turn off, and the S2 location LED will illuminate. Systems must transfer in no more than ten seconds where device failure could result in human death or serious injury.
  • Reclose the Source 1 breaker to return to the usual source. The utility timer’s timing cycle begins when the delay occurs. When the timer finishes its timing cycle, the automatic transfer switch (ATS) will transfer. The S2 position LED turns off, while the S1 position LED glows.
  • The delay engine stop timer will start its timing cycle. The generator is emptied for the duration of the timing cycle. The generator gets switched off once the timer has completed its timed cycle, and the S2 accessible LED will turn off. 
  • A minimum of 5 minutes should be allowed for unloaded EPS operation before shutdown to allow the engine to cool down (NFPA 110). Small air-cooled prime movers of 15 kW or less do not require a minimum 5-minute delay.

The way automatic transfers work should be in line with the manufacturer’s specifications. The following describes a general procedure for the functional testing of a standby generator and automatic transfer switch.

Automatic transfers should follow manufacturer design. A general automatic transfer switch and standby generator functional testing approach is below:

  1. Start the test by closing the standard source circuit breaker. If voltage is correct, the switch controller shall illuminate the utility available LED. 
  2. When ATS is on Source 1, the Source 1 LED lights. Check utility line terminal phase-to-phase voltages.
  3. Continue by closing the Alternative source breaker and starting the engine generator. The S2 (Alternate) Capable LED will light up when voltage and frequency are correct. 
  4. After verifying both sources, turn off the engine generator and set the start control to automatic.
  5. Open Source 1 (normal side) breaker to simulate utility outage. The engine start delay timer starts. The generator starts when the engine starts connections close after the timer cycles.
  6. Source 2 Available LED illuminates when generator voltage & frequency meet restore points. Generator timer delay starts simultaneously. 
  7. When the time delay is over, the ATS transfers to Generator, turning off S1 and lighting S2. Systems must transfer in 10 seconds if device failure could kill or injure people.
  8. To return to the normal source, close Source 1 breaker. Utility timer delay starts. ATS transfers after the timed cycle. The S2 LED turns off and the S1 LED shows.
  9. The delay engine shutdown timer starts. The generator runs unloaded throughout this timing cycle. After the timer cycle, the generator stops. S2 Available LED turns off. NFPA 110 requires a 5-minute pause for unloaded EPS running before shutdown to cool the engine. 
  10. Small air-cooled prime movers under 15 kW don’t wait the 5-minute.

After testing, apply an NFPA-compliant field testing label certifying that the transfer switch is electrically & mechanically sound and ready for duty.

  • Inspection and Cleaning Before working on the transfer switch, turn off all power sources.
  • The switch should be checked for moisture, debris, or dust and vacuumed or wiped off with a soft brush (or) dry cloth.
  • Do not use a blower because material can get trapped in electrical and mechanical components, causing harm.
  • Any surface residues must be cleaned with a clean cloth.
  • Provides uninterrupted electrical supply.
  • Enables efficient switching to generator power.
  • Ensures safety
  • Faster connection