How to Test a Light Ballast
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
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 and the stored electrical current to dissipate 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 conductors, and you should see either one or two wires going to each one.
Step 4 - Test the Voltage
Turn the voltmeter to the the ohms reading. Touch the probes a white wire and a colored wire coming from the ballast. The voltage meter should present a reading of a continuous circuit or a short. Any other reading indicates that the ballast needs to be replaced. Test the voltage of the other colored wires on the high voltage side as well, 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 all of the wire connections to each of the tombstones is 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.