Automatic Transfer Switch Testing Procedures: When and How to Do Them?

What happens when the primary power source goes off? Alternate source starts operating and powers the electrical equipment.  How does this happen? There is a transfer switch that switches the electrical loads between two power sources. Sometimes, these transfer switches are manual. In this, an operator throws the switch whenever any of the two power sources lose or gain power. However, now you can find an automatic transfer switch. This switch has a specific control panel system that makes the switch automatic. The control panel detects the primary source failure and leads to the start of the engine generator. To ensure that the ATS works properly, you must conduct ATS testing. Here are the helpful automatic transfer switch testing procedures.

How do Automatic Transfer Switches Work?

The automatic transfer switch is a self-acting device. The control logic of the switch is automatic and microprocessor-based. It regularly monitors the frequency and voltage of power sources. In general, automatic transfer switches connect to the primary power source or utility by default. However, when the primary power source fails, it connects to the alternate power source i.e., generator or backup utility.  The switch transfers the load to the primary power supply on normal power restoration.  You have to install these switches at the location of the backup generator.

NFPA guidelines for automatic transfer switch testing:

  • You must do inspection and operational testing.
  • Test all the EPSSs along with all internal and mechanical components once a month.
  • Operate the automatic transfer switch monthly.
  • You must perform an electrical function test periodically that includes transferring of switch to the alternate source of power from the standard source and returning.
ATS installed under a building water

Image: ATS installed under a building water

Automatic Transfer Switch Testing Procedures– Monthly Automatic Transfer Switch Functional Testing

Most facilities such as hospitals, data centers, factories, etc. require continuous power to deal with power failure circumstances. They use an emergency system for alternate power sources. This can be an engine generator or any other backup utility.  If your automatic transfer switches (ATS) do not work, it may lead to serious issues. Thus, you must do operational testing of automatic transfer once a month to ensure the smooth working of the automatic transfer switches. 

Precautions to take:

  • Firstly, make sure that you close the enclosure door of the generator set to prevent injury.
  • Secondly, ensure that the normal and emergency sources of power should be there.
  • Finally, make sure that the emergency sources of power should be in working condition.

Follow these steps for electrical operation transfer testing of an ATS and standby generator

  • First of all, turn the isolation handle anti-clockwise to the test position. This will turn on the transfer switch connected to normal and normal source accepted lights.
  • Now, turn and hold the transfer control switch in a clockwise direction to the transfer test. The delay to the engine start timer begins its cycle. This cycle will take 15 seconds until the engine starts and runs. When the cycle completes itself, the “engine start contacts” close and it starts the generator. With this, the emergency power source lights will turn on.
  • This triggers a 2B time delay, and after this, the transfer switch will work in an emergency position. As a result, the transfer switch connected to the emergency light or the S2 position LED turns on. Along with this, the transfer switch connected to normal will turn off.
  • After the feature 3A time delay, the transfer switch will go back to the normal position.
  • To re-transfer at once, turn the transfer control switch in an anti-clockwise direction to retransfer delay bypass. As a result, the transfer switch connected to normal will turn on and the transfer switch connected to emergency or the S2 position LED will go off. Now, turn the isolation handle in a clockwise direction to the connected position.
  • After feature 2E time delay, the generator will stop. The cycle of the delay engine stop timer starts. During this time, the generator set runs without load.
  • It is important to give a minimum 5-minute delay for the unloaded running of the generator. Let the engine cool down during this time. 

With these steps, you can verify whether:

  • All the necessary functions are running correctly when load shifts to an emergency power source in case of primary source failure.
  • S1 (source1) LED and S2 position LED are working properly.
  • There is any vibration or noise during operation.
  • All the time delays during the load transfer are working correctly. 

Automatic Transfer Switch Testing Procedures– Annual preventive transfer switch maintenance process

To ensure the reliability and life of the automatic transfer switches, proper maintenance is important.  The switch uses dangerous voltage that can lead to shock, burn, and even death. Thus, make sure that you disconnect ATS with the normal power source and the emergency power supply system before any automatic transfer switch maintenance activity.

  • First of all, conduct a visual inspection and look for any dirt, dust or moisture. With the help of a soft brush or a vacuum cleaner, remove all the dust. Clean the moisture with a cloth.
  • You will find a manual operator handle in the switch for maintenance purposes. You must check for any issues in manual operation. 
  • Next, remove transfer switch barriers. Check all the internal components and contacts of the switches. Contacts must last till the life of the switch. Reinstall all the barriers carefully.
  • Due to harsh operating conditions, the lubrication on the switch may get lost. Thus, apply lubricants on all movements and linkages.
  • Then, check the connections of all cables and wires. Re-tighten them if required.
  • Finally, make sure that the switch has manual transfer warnings to avoid accidents.

Automatic Transfer Switch Testing Procedures–Who performs this test?

If it is a normal automatic transfer switch testing, any local person can do it. However, for advanced testing procedures, you have to contract out this task to experts. When doing so, make sure the contracting company meets all the safety requirements of the OSHA (Occupational Safety and Health Administration). Also, the company must also have industry-trained technicians to do this job. Having qualified personnel means they are very well familiar with the procedures, related risks, and safety procedures of the ATS.


Testing an automatic transfer switch ensures that you get a continuous power supply. Additionally, it will also ensure that the load transfer process, from primary power supply to emergency power supply, is efficient, safe, and fast. During the installation of ATS, if you need any help regarding wires and cable assemblies, contact Cloom. We deal in high-quality ones for your electrical system needs.

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