How to Test a Light Ballast

An isolated electrical ballast against a white background.
  • 1 hours
  • Intermediate
  • 125
What You'll Need
Protective eye wear
Philips screwdriver

If you're having problems with a fluorescent light turning on, and you have tried the simple solutions, the problem could originate with a bad ballast. After all, without a working ballast, you can't have a working light. However, an electrical ballast can be checked quickly and easily with the right tools. Here are the steps you need to take to find out whether this part should be replaced.

Step 1 - Safety

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Safety is always paramount when working with electricity, so put on protective eyewear and switch the power off to the lights before you do anything else. Allow the fixture to cool before you remove the bulb. Put it in a safe place until you're ready to use it again.

Step 2 - Check the Ballast's Appearance

Remove the screws holding the protective cover in place and set it aside. Inspect the ballast closely to see if there is any leakage. If you see any indication of such, there's no need to continue with testing; the ballast will have to be replaced.

Tip: You can continue this testing process after you have a new ballast installed if you feel the need.

Step 3 - Locate the Tombstones and Wiring

Test the high voltage side of the ballast to be sure the electricity flowing between the wires and the tombstones is continuous. The tombstones are the plastic fittings at either end of where the bulbs sit. These contain small metal contactors, and you should see either one or two wires going to each one.

Step 4 - Test the Ballast

Turn the multimeter on the ohms scale. Touch one probe on the white wire and then, one at a time, touch all the other colored wires coming from the ballast with the other probe. If the multimeter gives a resistance reading when the second probe touches any one of the colored wires, the ballast is bad and needs replacement. Test the resistance of the other colored wires on the high voltage side as well, still holding one probe on the white wire and then test the wires from the transformer. Finally, test the wires from the power side of the ballast.

Step 5 - Check Wire Connections

If all of your readings indicate no electrical problems with the ballast, yet you're still unable to get the light working, be sure that the nut connectors and all of the wire connections to each of the tombstones are secure and that the metal connectors in the tombstones are not corroded.

Step 6 - Reinstall the Bulb

Screw the protective cover back on and refit the bulb in the fixture. Then, you can switch the power to the light back on and test whether your problems have been solved.

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