How to Check If the Ballast Is Bad in a Fluorescent Light

How to check if your ballast is failing with your fluorescent lights?


Unlike incandescent and LED lights, fluorescent lighting requires a ballast that sends the correct amount of power into the bulb or tube.

The ballast delivers enough voltage to start the lighting process. Once the illumination begins, it backs off the electricity to produce a steady light.


Without a ballast, fluorescent bulbs would use the 120V current and quickly burn out and ruin the product. If it goes bad, you'll see or hear symptoms that tell you it's time for a potential replacement.


What Are the Symptoms of a Failing Ballast?


When a ballast starts failing, it will typically buzz or causes the bulbs to flicker. These issues can sometimes happen while it is operating as expected, so it's important to check the equipment before assuming a replacement is necessary.


Here are the steps to follow to check if the ballast is bad in fluorescent light.


1. Turn Off the Power

You'll need to safely turn off the fixture at the circuit breaker to check the ballast. Check to see if the voltage is still there any way for an extra layer of safety? If you flip the switch and get no response when one was there before, you're typically okay – but electricity is not something to mess with at all.


Remove the fixture's lens cover. Some wrap around the fixture or are held in place with slides or screws. Be careful when doing this work to avoid breaking it.


2. Take out the blubs

Once the cover is gone, you can remove the fluorescent bulbs from your light fixture. If they are circular, you'll see a plug that connects the product to the ballast. Straight and U-shaped bulbs install in sockets that connect it instead.


If you have a U-shaped bulb, it will be held in place with a tension spring. Gently pull the bulb from it to release it from the socket.


Tubes turn clockwise or counterclockwise to be removed from their sockets.


3. Remove the Ballast Cover

Once you have the bulbs out of the fixture, you can remove the ballast cover if this component isn't already exposed. Most covers attach with slot inserts or screws.


Some ballasts have visual damage you can see immediately. If it's leaking oil, swollen, or burned, it'll need to be removed.


If you're unsure, proceed to the next step.


4. Check the Ballast with a Multimeter

Turn the multimeter to its ohm setting. If you have multiple options available, you'll want it on the X1K option.


Insert one probe into the wire connector while holding the white wires together. Touch the other probe to the ends of yellow, red, and blue wires coming from the ballast. Some might exclude the yellow.


If the ballast doesn't cause the needle to move, you know it needs to be replaced. A good one will sweep the needle or read across the face of the tool.


5. Install the New Ballast

When you need to replace the ballast, you'll just swap in the new one for the hold one. Install the cover again, replace the fluorescent bulbs, and then test the fixture to see if it works by turning the power on at the breaker box.


If it works as intended, you'll see the lights turn on without buzzing or flickering. It might take a couple of minutes for the equipment to warm up before the symptoms disappear.


Should the fixture not work, you'll need to go back to check the bulb connections, the ballast, and other connections. Continue to recheck until you receive power to the lights.


Once the fluorescent lights turn on, you can replace the fixture cover.


How to Work on a Ballast Safely


When you need to switch out a bad ballast for a good one, you'll need to have access to the fixture. Since most are installed on ceilings, you'll want a stepladder that can reach that space.


Although it might be tempting to stand on a chair or another piece of furniture, the ladder is the safest way to work on the unit. Have someone there to spot you to prevent any slippage while you're checking the lights and ballast.


If you have a ballast that keeps shorting or burning out, there could be a deeper wiring issue to fix. This problem could also be evidence of a short in the line.


Once you've replaced the ballast and still have unwanted flickers or noise, it might be time to call in an experienced electrician to look at the issue. When dealing with electricity, it's always better to take extra precautions. ​


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