How to Test a Contactor

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Things You'll Need

  • Screwdriver

  • Clear tape

  • Pen

  • Volt ohmmeter


Buzzing or vibrating contactors can be the result of dirt and debris inside the plunger. Clean the contactor.

Arc flashing of the contacts indicate carbon scorching or pitting on the faces of the contacts. Remove the contacts and clean with an emery cloth or fine mill file.

Contacts that stick in the closed position indicate a broken return spring or debris in the mechanism. Clean or replace the spring.

Screw terminal arcing is a sign of overamperage draw by the device being controlled. Internal damage may have occurred to the contactor. Replace the contactor unit. Terminal arcing is observed by heavy black carbon buildup on the face of the contactor. Carbon will conduct electricity.

Contactors are used to remotely switch electrical power. The coil-actuated switch operates exactly like a relay, but a contactor typically handles higher amperages in the switch mechanism itself. The coil consists of many winds to a wire and is energized by a smaller voltage. A movable solenoid, called a plunger, is mechanically connected to a set of spring-loaded copper discs. These discs are called the contacts. When the coil is energized, the solenoid pushes against the spring and closes the contacts. When the coil voltage is removed, the force of the spring returns the contacts to their normally open position. Typically the contacts, two each to a switch, are identified as the line and terminal. The line, or L, is the voltage feed, and the terminal, or T, is connected to the electrical device being controlled. Contactors can have many sets of contacts or switches.


Step 1

Remove all electrical power to the L or line side of the contactor. Use the screwdriver and pull the wires from the line-side screws. Identify the wires, if needed, with the clear tape and pen. Place identifier marks such as L1, L2 and L3 with the pen over the tape. Adhere the tape to the corresponding wires.

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Step 2

Loosen and remove the wires from the T or terminal side of the contactor. Identify each wire as in Step 1, but with a T1, T2 and T3. Wires left in place on the contactor can lead to a false reading in the following procedure.

Step 3

Energize the contactor's coil by turning the control switch to the "on" position. You should hear an audible "click," and a humming sound will emit from the contactor. Some contactors may have some form of visual aid or manual actuator that will physically move when the coil is energized.


Step 4

Switch the volt ohmmeter on and to the ohms position on the front selector switch. Place the red lead into the "ohm" connector and the black lead into the "common." Touch the two leads together. The meter short read 0 ohms or a direct short.

Step 5

Test each set of L1-to-T1 contacts. Place the red lead to the L1 and the black lead to T1. The meter should read 0 ohms. Perform the test on each separate set of contacts. Keep each set test together--L1 to T1, L2 to T2 and L3 to T3. Each set should read 0 ohms. If any set does not, then that contact is bad. The contacts may have to be replaced.


Step 6

Observe all readings on the meter. If all the readings are not 0 ohms or a direct short, the coil may not be properly connecting the contacts. Cycle the control switch for the coil. Can you hear an audible click? If not, check the voltage to the coil.

Step 7

Switch the meter to volts and place the red lead in the volts connector on the meter. Apply power to the coil. Touch each lead of the meter to each connector on the coil. The meter should read the supplied voltage. Read the specification for the circuit to find the voltage supplied. If the voltage is correct and the coil still does not close, shut all power off to the coil.


Step 8

Remove the wires to the coil with the screwdriver. Reset the meter to read ohms again as in Step 4 above. Touch each lead to the coils connectors like reading the voltage, but with the meter in the ohms position. The meter should read from 10 to 100 ohms. If the meter does not, the coil is bad and must be replaced.