how to test ac contactor


How to Test AC Contactor


AC contactors are essential components in electrical systems, responsible for controlling the power flow to motors, compressors, and various other high-power devices. Regularly testing the functionality of an AC contactor ensures that it is operating efficiently and preventing any potential failures. In this article, we will explore the step-by-step process of testing an AC contactor, ensuring the safety and reliability of your electrical system.

Understanding AC Contactors

AC contactors are electromechanical devices designed to handle high-voltage AC currents in electrical systems. They consist of a coil, contacts, and an iron core, with the coil being energized to close the contacts and establish a connection between the power source and the load. When an issue arises with an AC contactor, it can lead to equipment malfunctions, excessive energy consumption, and even electrical hazards. That's why periodic testing is crucial to identify any potential problems and address them promptly.

Gathering the Necessary Tools

Before proceeding with testing, it's important to gather all the necessary tools and equipment. The following tools are typically required to test an AC contactor:

1. Multimeter: A digital multimeter is an essential tool for measuring voltage, current, and resistance in electrical circuits.

2. Screwdriver: Depending on the contactor model, a screwdriver may be needed to remove access panels and secure connections.

3. Insulation Discharge Device: This device is essential for discharging the residual electrical energy stored in the contactor to ensure safety during testing.

Checking for Power Supply

The first step in testing an AC contactor is to ensure the power supply is disconnected. This can be achieved by switching off the circuit breaker or removing the fuse that supplies electricity to the contactor. Once this is done, use a multimeter to verify the absence of voltage at the contactor's terminals. Carefully check the electrical panel or wiring diagram to locate the correct breaker or fuse.

It is crucial to wear appropriate personal protective equipment (PPE) before proceeding, including safety goggles and insulated gloves. Remember, safety should always be the top priority when working with electrical systems.

Testing Coil Continuity

The coil of an AC contactor is responsible for opening and closing the contacts. Testing the coil ensures its proper functionality. To test the continuity of the coil, perform the following steps:

1. Disconnect all power sources: Double-check that all power sources to the contactor have been switched off and confirmed to be de-energized using a multimeter.

2. Remove access panel: Using a suitable screwdriver, remove any access panels necessary to access the contactor's coil.

3. Locate the coil terminals: Identify the coil terminals on the contactor. These are typically labeled as "A1" and "A2."

4. Set the multimeter: Set the multimeter to the resistance or continuity mode.

5. Test for continuity: Place the multimeter probes across the coil terminals (A1 and A2). If the meter displays a low resistance value or beeps, this indicates the coil has continuity.

6. Repeat the test: Reverse the multimeter probes and perform the test again. The multimeter should display the same low resistance or continuity.

If the multimeter does not display continuity or shows an open circuit, it indicates a faulty coil that needs to be replaced.

Inspecting Contact Integrity

The contacts of an AC contactor are responsible for making or breaking the electrical connection. Over time, due to wear and arcing, the contacts may deteriorate, resulting in poor performance and potential failures. Testing the contact integrity ensures a reliable connection between the power supply and the load. Follow these steps to inspect the contact integrity:

1. Disconnect the power supply: Ensure that all power sources to the contactor have been disconnected.

2. Remove access panel: Use a screwdriver to remove the access panel to gain access to the contacts.

3. Visually inspect the contacts: Look for signs of burning, pitting, or any other visible damage on the contacts. If any damage is found, the contacts likely need to be replaced.

4. Measure contact resistance: Set the multimeter to the continuity or resistance mode. Place one probe on each of the main power input terminals and note the resistance reading. It should be as close to zero ohms as possible. Any significantly higher resistance indicates poor contact integrity.

5. Wiggle test: Gently wiggle the contacts using an insulated tool or screwdriver. If the multimeter reading fluctuates or shows an open circuit during the movement, it indicates loose or worn-out contacts that require immediate attention.

Testing the Switching Action

The switching action of an AC contactor refers to its ability to open and close the contacts effectively. This action should be smooth and reliable, without any hesitation or arcing. To test the switching action, follow these steps:

1. Disconnect the power supply: Ensure that all power sources to the contactor have been disconnected.

2. Remove access panel: Use a screwdriver to remove the access panel to gain access to the contacts and coil.

3. Energize the coil: Reconnect the power supply to the contactor while keeping a safe distance from any exposed wires.

4. Observe the action: Watch the contactor closely as it energizes. The contacts should close firmly, indicating a proper switching action. Similarly, when the coil is de-energized, the contacts should open promptly without any hesitation or sticking.

5. Listen for unusual sounds: Pay attention to any buzzing or humming noises during the switching action. Unusual sounds may indicate a faulty coil or worn-out contacts that need to be addressed.


Regularly testing your AC contactor is vital for maintaining the safety and efficiency of your electrical system. By understanding the components and following a systematic testing process, you can identify any potential issues and address them promptly. Remember to always prioritize safety by disconnecting the power supply, using appropriate PPE, and following the manufacturer's guidelines. By investing time in testing your AC contactor, you can prevent costly equipment failures, reduce energy consumption, and ensure the longevity of your electrical system.


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