How to test the ballasts in fluorescent light fixtures

Updated February 21, 2017

Testing the ballast of a fluorescent light fixture is a basic process that can be accomplished quickly once you understand the operation of the high voltage transformer. The high voltage transformer is also encased in an oil bath that reduces the heat created by the action of the transformer. A low voltage, generally 120 VAC, is transformed into a high voltage, generally in thousands of volts. There must be a continuity between the wires of the high voltage side in order for the circuit to operate correctly. Heat can damaged the internal connections of the ballast. By using a volt ohmmeter the internal circuit can be quickly checked.

Remove the electrical power from the light fixture and allow a few minutes to pass. This will allow any stored current in the ballast to be dissipated in the lighting circuit.

Remove the fluorescent light bulbs from the end light holding fixtures also called the tombstones. Set the bulbs in a safe place so they do not break.

Use the screwdriver and remove the protective cover that conceals the ballast.

Observe the ballast before proceeding. If oil is leaking from the ballast, stop here and remove the ballast. Leaking oil indicates that the ballast is damaged beyond any doubt as the internal seal has been ruptured by excessive heat.

Test the high voltage side for continuity between the wires going to the light fixture's tombstones. There will be either one or two wires going to each tombstone fixture. Regardless of the number of wires, the colours will be the following: Blue or yellow are the power wires and white is the common or neutral wire.

Turn the Volt Ohmmeter to "ohms" and touch the ends of the two probes together. The meter should show continuity or a direct short.

Touch the probes of the meter to one of the white wires and one of the coloured, blue or yellow, wires that comes from the ballast. This should show a short or continuous circuit. If it does not, the ballast is bad and should be replaced.

Check all the wire pairs from the ballast on the high voltage side. If they all test correctly, then check the low voltage side of the transformer.

Remove the wire nuts from the black and white wire that come from the power feed side of the ballast. Touch the probes of the meter to these wires. The circuit should show a short or continuous circuit. If it does not, then the ballast is bad on the low voltage side of the transformer.

Check the wire connections to the tombstones if the ballast checks out OK. One wire can be loose in the "clip on" type connection of the light fixture.


If oil is leaking from a ballast, caution should be exercised as some older transformers may contain harmful chemicals. Check with local codes for proper disposal of any types of ballasts.

Things You'll Need

  • Screw driver Philips or slotted
  • Volt ohmmeter
bibliography-icon icon for annotation tool Cite this Article

About the Author