How can I tell whether a fluorescent tube has failed, or an electronic ballast?

Question

The light fitting in my utility room is a 5' fluorescent tube, and it's stopped working. It's about 7 years old, with a quick starting electronic ballast. The failure has been a little odd - a completely dark tube for several days whenever I instinctively tried to turn it on, then it worked for a bit, then yesterday it came on for less than a second and went off again.

I don't want to buy a tube just for testing - if the ballast is gone I'm stuck with a large fragile thing to store, and will probably end up replacing the whole fitting with something that takes LED bulbs. My only other fluorescent tube fitting is 6', so borrowing the tube out of that isn't an option.

I do have a few multimeters, and I'm used to working with mains, but don't know what to look for.

Answer 1

I wouldn't guarantee this to be a foolproof method, but as you have three potential fail-points [assuming you checked the power] & as it's a bit scary to test these things with a multimeter, then I'd start with…

^{This all assumes you have standard tubes with a slot-in starter, not something 'smart' like a T4 or T5 where it's built into the fitting itself}

How dark is the tube at the ends? Tired old tubes can get almost black at the ends. They will also start to stress your starter, needing several attempts to fire up, before they eventually fail.
They also sometimes have a period where they will not glow so well as when they were new, or they can flicker without needing restarting - this would be sign they were leaking, so not always a symptom.

Try a new starter first - they're cheap & easy to store. if the starter is banging away at the tube with no joy, then we're back to it being the tube; but bad tubes wear out starters fast, so it's always wise the change a starter when changing a tube, & always wise to try a new starter before blaming the tube.

If no joy with either of the above, it still could be the tube… but it could be the ballast. You now have to weigh up whether you're going to risk a new tube to find out it's the ballast.

Answer 2

I am the biggest supporter of Real Fluorescent that you will ever meet. Fluorescent technology has dramatically improved in the last 20 years, and I strongly feel that the standard 4 foot tubes are the best light available with 90 CRI readily available and 98 CRI available... far more reliable since you can get Tier 1 vendors like GE and Sylvania very affordably (the field isn't glutted with cheap Cheese junk like LEDs are). and cost actually less overall. They are not as efficient, though, so will burn more energy.

That said, anytime I am dealing with an oddball size of fluorescent, I will either go to the standard 4’ or switch to LED. The lamps are too costly when they are odd sizes.

That said, if you pay careful attention to your ballast specifications, you may be able to borrow a ballast from another fixture to try in this one. While tubes are particular to one fixture, ballasts typically work with a range of tube lengths. The ballast labeling will state the most common sizes they work with, and their spec sheet (on the Web) will list every single size they work with. Follow the wiring instructions on the label, as these differ "instant-start" vs "rapid or programmed start".