My little maintenance project did not all go well. One light ran fine with the replacement bulbs, the other ran fine for about 4 minutes and then went bzzzZZZAAATT!! - and went out. The bulb is ok (retested in 1st fixture.)
The fixtures are both rapid start types and should be compatible, but, old ballasts often can’t hack it, according to my web search. In this case, Brave Search failed, but Google AI gave a great reply:
(Sorry for the slightly run on text below, I only partially corrected that.)
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Me:
Why did a led replacement bulb put in a 48” t8/12 fixture kill the ballast? the fixture is rapid start, the bulb only says not to use in a pre-heat fixture, and it worked for about 4 minutes before the ballast (i presume) failed. the bulb still works, in another 48” rapid start fixture.
Reply per Google:
The replacement LED bulb killed the ballast because of an impedance mismatch and overheating, which commonly happens when plugging “ballast-compatible” (Type A) LED tubes into older magnetic or aging fluorescent ballasts. Because the bulb still works in a different fixture, it proves the bulb’s internal driver survived, but your first ballast could not handle the unique electrical load of the LED.
Why the Ballast Failed in 4 Minutes.
Age and Existing Degradation: Fluorescent ballasts degrade silently over time. The sudden change in electrical demand from the LED bulb often acts as the final “shock” that pushes an old, compromised ballast into total failure.
Impedance Mismatch: Type A LED tubes are designed to mimic the resistance of a standard fluorescent gas tube. However, if the ballast’s output frequency or internal circuitry doesn’t align perfectly with the LED’s driver, the ballast will experience an over-current condition, causing it to burn out almost instantly.
Thermal Tripping or Meltdown: Many older rapid start ballasts utilize magnetic coils. When paired with an incompatible electronic LED driver, these coils generate immense heat. Within 4 minutes, the ballast either melted its internal insulation or permanently tripped its internal thermal safety fuse.
How to Proceed
Do not buy another “plug-and-play” bulb or replace the ballast. The most reliable and energy-efficient solution is to completely eliminate the ballast from the equation.
Keep the current bulb for your other working fixture. Do not risk swapping it back and forth.
Convert the failed fixture to Ballast Bypass (Type B). Buy a Type B LED tube (also called a “direct-wire” tube). Cut the ballast out: Turn off the breaker, cut the wires leading to the ballast, and connect the building’s main AC power wires (Hot and Neutral) directly to the socket holders (tombstones) according to the Type B bulb’s instructions.
Bypassing the ballast removes the failure point permanently, saves more electricity, and ensures your lights turn on instantly without humming or flickering.
So... I ended up replacing 3 of the 4’ fluorescent light fixtures I was dealing with, with new LED fixtures — not just the replacement LED bulbs. The 3rd shop light has a weak ballast in it, so I didn’t even risk blowing it up. :-) What I’m going to do with all my extra 4’ LED bulbs, I don’t know. It appears that any rapid start ballast that is a little weak will fry itself with the “Rapid Start” compatible LED bulbs installed. At least the bulbs survive.
One of the new shop lights has a motion sensor in it. That’s very useful for the spot I put it in, and will save some energy too (compared to me remembering - not - to go back and turn off the light if my hands were full as I exited the shop. That “not” business happens a lot!) I have another of these, and there’s a second somewhat similar spot that I may swap it in to. It’s a bit harder to access, tho’...
Whew! Long, long day. I’m beat!
To follow up - about 40% of the “rapid” (or “instant”) start 4’ fluorescent shop lights I’ve attempted to put LED “rapid” (or “instant”) start compatible replacement bulbs in are still running fine after a day “on”. However, some of these LED replacement bulbs don’t work (or even come on) in rapid start fixtures that other brand rapid start compatible LED replacement bulbs DO seem to work fine in. The situation may be reversed in other fixtures. What the variance is, I don’t know...
I guess anything that goes in a really difficult spot to get to will have to be a LED light fixture that I’ve modified for better heat dissipation of the LED’s, and then make sure I have surge suppression on that circuit.