A ballast controls the current through a fluorescent bulb to keep the light even and to protect the bulb from damage. When the ballast fails, the bulb will flicker or fail to light. Although changing a ballast is safer with the power off, sometimes it is impractical, such as in a crowded store or office space, and you must replace the ballast without cutting the power. This can be done safely with some preparation and the right tools.
Set up your ladder. Place your tools and replacement ballast within reach. Put on your safety glasses.
Remove the bulbs and covers, and locate the ballast inside the lighting fixture. It is a thin metal box about 10 inches long, with wires coming out of it.
Cut the power wires first, using the insulated wire cutters. Cut about an inch from the ballast to give yourself enough wire to work with. The power wires are usually black and white for 120 volts, or red, blue or brown and white for 277 volts. Cut each wire individually, and then screw a wire cap on the end of each wire to prevent accidental contact.
Cut the rest of the wires from your faulty ballast about an inch from the ballast, then strip the ends of the wires with the wire strippers about 1/4 to 1/2 inch. These are the wires that go to the bulb ends and are normally a combination of red, yellow and blue. Do not strip the power wires yet.
Remove the old ballast from the fixture with a screwdriver or nut driver, depending on how the ballast is mounted.
Install the new ballast in the same orientation as the old one, paying attention to on which ends the different colored wires were positioned. Usually ballasts have one color for one side of the bulb (such as yellow) and one or two colors for the other side (red, blue or both).
Strip the ends of the ballast wires, if they are not already stripped. Use the wire strippers to remove about 1/4 to 1/2 inch of the wire insulation.
Connect the wires from the bulb sockets to their respective colors on the ballast. Put two wires together at their stripped ends, insert the ends into the wire nut, and screw the wire nuts onto the ends, connecting them together.
Remove the wire nut from one of the power wires and carefully strip about 1/2 inch of insulation from the end, making sure not to touch the end. Hold the ballast wire next to this wire with the stripped ends parallel but not touching. Quickly slip on the wire nut and screw onto the ends. (If the wire sparks, it will be inside of the wire nut.) Repeat this step with the other power wire.
Take up the slack in the wires, and group them together. Twist them into a bundle or tie them together using a zip tie. This will make it easier to put the wires back under the cover.
Replace all covers and bulbs. The bulbs should light as soon as they are installed. If the lights fail to operate, redo Steps 1 and 2, and check the wiring to make sure all the wires are in the correct places and are making good contact inside the wire nuts.