How to Test the Ballasts in Fluorescent Light Fixtures

Fluorescent light ballasts control the voltage delivered to the fluorescent tubes, allowing them to start and to operate at rated output. When the fixture is switched on, the ballasts heat the electrodes or generates a high voltage to start the electric arc through the tubes. As the electrodes heat up, the ballasts control the voltage to maintain the operating point that delivers the longest possible fluorescent tube life. If the ballasts are not operating properly, the fluorescent lights may not start, may flicker or may burn out prematurely. Replacing defective ballasts lets you prolong the life of the fluorescent lights.


Visual Inspection

Make sure the ballast cases and the light fixtures are grounded. Verify that there are grounding wires going from the ballast cases to the fixtures or to grounding points in the junction boxes where the power for the lights is connected.

Check that the ballasts are connected in accordance with the instructions printed on the ballast case. Verify the connection points visually to make sure that the wires are held in their terminals and that there are no obviously damaged components.

Take good bulbs from a functioning light and insert them in the fixture you are testing to make sure the problem is not a defective bulb. If the lights use starters, place a good starter in the light to be tested to make sure the problem is not with the starter.

Testing the Ballasts

Remove any lights in the light fixture to be tested. Use the voltmeter to check for voltage at the output of the ballast. If there is no voltage present, check for voltage at the incoming leads to the light. If they are supplying 120 volts AC to the ballast, replace the ballast.

Measure the voltages at the output of the ballast. For rapid-start lamps, use the voltmeter to find the heater filament leads with a voltage between 3 and 10 volts and the high voltage leads with a voltage of 85 to 400 volts. Check the voltages against the rated open circuit voltages that are printed on the ballast label. For electronic ballasts, the high-voltage circuit readings may be higher. For instant-start lights, there is no heater filament circuit.

Check or replace the light fixture if the ballast output voltages are normal but the lights don't work. Replace the ballast if the voltages don't correspond to the values printed on the label.