Two bulbs, one fixture
November 6, 2019 9:38 PM   Subscribe

How could two CFL light bulbs burn out at the same time?

Tonight, I turned on my overhead kitchen light, as I do, except it didn't. I had put in two CFL bulbs years ago, to replace the incandescents I'd had in there – long enough so that I've forgotten how many years ago it was exactly. What's weird is that they both burned out at the same time – I've checked both sockets on the overhead fixture with a known good CFL bulb from the same batch, so I know they work and that it wasn't a circuit breaker issue. It's an old house (probably over a hundred years old) in less than great condition, but the wiring is not that old – I believe my landlord redid the electrical about twenty years ago. The other light in the kitchen, in a good above the stove, has the same bulb from the same batch, but is still working just fine.

What could have caused the bulbs in the overhead light fixture to die at the same time? And are they in any way fixable? I have my suspicions, but I'm curious as to what someone who actually knows about electricity and CFL technology thinks is possible/likely.
posted by skoosh to Home & Garden (12 answers total) 3 users marked this as a favorite
Check electric current CFL bulbs are vulnerable to current breaks...
But better to use LED lights
posted by ElliotChang at 10:10 PM on November 6, 2019 [1 favorite]

Did they burn out when they started up?

As I understand it, CFLs generate a high start up voltage using coils/transformers, and if they happen to short out or develop an open circuit at that time, those coils can then generate big back voltages which might burn out another bulb connected in parallel with them, such as another bulb in the same fixture would be.
posted by jamjam at 11:34 PM on November 6, 2019

I didn't notice any sound or flicker indicating that they were burning out when I flipped the light switch, only the lack of light. They had worked fine the last time they were on, so this seems consistent with a silent, instantaneous failure as soon as I flipped the switch.
posted by skoosh at 11:42 PM on November 6, 2019

Is it a confined enclosure? A hot thermal environment can lead to early electrical failure.

Is there excess dark coloration near the electrodes or a corner of the bulb? That is erosion of the cathodes, which can lead to failure as well.

Were the bulbs surprisingly loose in the sockets? Bulbs can work their way loose due to thermal cycling, and then get intermittent current at startup which degrades the electronics.

Are you sure they both worked last time, or could one have preceded it without noticing behind thick opalescent glass?

CFL's are all crap now that LED's exist. The factories are just being run into the ground. Look forward to superior LED light.
posted by nickggully at 6:02 AM on November 7, 2019 [1 favorite]

They were put in at the same time, if I'm reading that right, so it's pretty likely they would burn out at around the same time. That they went at the exact same time may just be coincidence.
posted by fiercecupcake at 6:56 AM on November 7, 2019 [1 favorite]

It's not a confined enclosure; there's a translucent glass plate that sits under the bulbs, but it's open space to the sides and above. I saw no discoloration on the bulbs. The bulbs were not loose in their sockets. It is possible that one of them had already burned out and I hadn't noticed, although in times past when that occurred, I have eventually noticed a diminution in brightness.

I already planned to replace my CFL bulbs with LEDs as they die. All of my current CFL bulbs were purchased several years ago, before LED bulbs were a viable/attractive option.
posted by skoosh at 7:16 AM on November 7, 2019

It could have been a small power surge — not enough to bother other bulbs and fixtures in your house, but these two bulbs, both being near the end of their life, could have been more vulnerable than newer bulbs and as a result both succumbed to the surge. The bulb in the hood above your stove, from the same batch, was probably not used as much, therefore not as close to end of life, not as vulnerable, and didn't blow. Yet. And no, they are not fixable.
posted by beagle at 7:38 AM on November 7, 2019

I recently bought a big box of bulbs because I was out. Aside from the one I noticed, around my house there were two other fixtures with one bulb out that I hadn't bothered to notice, but now that I had a bunch of bulbs I replaced them all and now it's bright again everywhere.

So I think there are two likely explanations: it was a coincidence and they both stopped at once, or more likely, if you're like me, one had been out for a while and you didn't notice because it wasn't dark.
posted by fritley at 7:41 AM on November 7, 2019

Electronic components like the ballast in your bulbs follow a bathtub curve of failure. Which means that if they make it out of an initial beginning of life high failure zone they tend not to fail until they reach end of life at which time the failure rate starts zooming up. So if your bulbs had a five year rated life they would tend to fail sometime between five and six years. Also many electronic components fail on start up because it stresses transistors and capacitors more than just running. So if your bulbs were destined to fail over the course of a year and you turn on your light five times a day there is a (365x5)2* chance they fail on the same start up event. 1 in 3.3 million aren't great odds but they are better than many lotteries which someone wins regularly. Today the winner was you.

* This probability is probably understating the possibility because the failure of these bulbs isn't truly independent. They were made at the same time from the same components and then exposed to exactly the same usage pattern. I wouldn't be surprised if the actual probability of burning out at the same time was an order of magnitude less.
posted by Mitheral at 7:42 AM on November 7, 2019 [2 favorites]

How bright does it look with just one bulb? Many modern light fittings are much brighter than they need to be so it's possible that one blew a while back and you just didnt notice until the second one went out.
posted by Lanark at 10:52 AM on November 7, 2019

Can't add a diagnosis, but a pro-tip: write the month/year on the base of the bulb when you put it in. With long life bulbs it is hard to remember when a blown bulb was put in, and therefore assess whether it gave good service.
posted by GeeEmm at 2:22 PM on November 7, 2019

"Once is happenstance. Twice is coincidence. Three times is enemy action."
posted by porpoise at 8:21 PM on November 7, 2019 [1 favorite]

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