How to Check a Fluorescent Light Ballast

Written by tzvi raphael
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How to Check a Fluorescent Light Ballast
Learn how to check a fluorescent tube light ballast. (Fluorescent overhead lamp in office image by Alexey Kuznetsov from

Fluorescent lights have in fitting inside known as the ballast, which creates the necessary current to create light. Sometimes you may hear a buzzing or humming sound coming from one of the fluorescent lights in your home. Or your light may be flickering on and off in a strobe light type of action. These are all indications that the ballast might be damaged. You can check your fluorescent light ballast and determine if you need to replace it.

Skill level:
Moderately Challenging

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Things you need

  • Screwdriver
  • Volt ohmmeter

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  1. 1

    Disconnect the light fixture from the electricity and wait a few minutes. This lets any stored current remaining in the ballast to disperse in the lighting circuit.

  2. 2

    Take the fluorescent light bulbs out of the end pieces of the light fixture, which are called the tombstones because of their shape. Find a safe place to set the bulbs so they do not break.

  3. 3

    Using a screwdriver, take off the protective cover that covers the ballast. Examine the ballast before going any further. If you see oil leaking from the ballast, you must remove the ballast. When oil leaks from the ballast, the internal seal has been ruptured and it is damaged beyond repair.

  4. 4

    Run a test on the high voltage side to check for continuity between the wires connected to the light fixture's tombstones. You will see one or two wires connected to each tombstone end fixture. With either one or two wires, the colours will be the same as follows: the power wires are yellow or blue and the neutral or common wire is white.

  5. 5

    Choose the setting "ohms" on the Volt Ohmmeter and take the probes' ends and touch them together. The meter will show either a direct short or continuity.

  6. 6

    Take the probes of the meter and touch them to one of the white wires and one of the coloured wires. You can use either the yellow or blue wires that come out from the ballast. This test should reveal either a short or a continuous circuit. If you don't get these results, the ballast is damaged and must be replaced.

  7. 7

    Run a check on all the wire pairs that run from the ballast on the high voltage side. If you see that all the wire pairs test correctly, check the transformer's low voltage side.

  8. 8

    Take the wire nuts off the black and white wire that runs from the ballast's power feed side. Touch the probes of the meter against these wires. The circuit should show a continuous or short circuit. If you don't get these results, then the ballast is damaged on the transformer's low voltage side.

  9. 9

    Run a test on the wire connections to the tombstones if your ballast checks out. If you have the "clip on" type connection to the light, one wire might be loose.

Tips and warnings

  • Fluorescent bulbs can be brittle, so handle with care.
  • Always take all necessary precautions when working with electrical fixtures, as it can be dangerous.

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