Why to multiple bulbs in series ALL die?

Sometimes, when bulbs fail, they fail shorted. That's why they make that big flash. The filament burns open and the loose ends drop down and make contact where they shouldn't.

In a series string, the momentary higher current stresses the rest of the bulbs (all nearing the end of their life anyway) and they fail as well.

Same thing with the fused dimmer. Bulb goes 'pow' and takes out the fuse. Better that than the expensive bits.

It's called a cascading failure mode, when the failure of one section inreases the stress on on others, whether they are shorts occuring in voltage-source-fed series loads or open circuits appearing in current-fed parallel loads. The same failure mode can occur in series and parallel-connected sources, under the influence of constant-power loading.

If the single lamp failure produced an open circuit in the voltage-fed series string, others would normally be protected and there would be no cascading effect, but the single point of failure would be difficult to locate immediately.

There is a mechanism involving low-breakdown-voltage oxide contacts which, when located in parallel with low voltage lamps, will tend to 'fuse' as a ~short, intentionally, if a large voltage occurs across a failed-open lamp filament. In a series string, with the short in effect, only the failed lamp would fail to illuminate, but the remaining lamp stresses increase proportionally while the condition persists. If the increased stress causes other failures, a cascade occurs until one mechanism fails to short circuit as intended or more points finally fuse open circuit due to abnormal current levels.

The resulting dark string is not necessarily composed entirely of dead lamps, due to the imprecise and untested function and limitations of the bypass mechanism, though a high percentage of failures (including

When the mechanism fails to break down as intended, one fast method of restoring continuity for trouble-shooting is to stress the string with a higher voltage source; one that is internally limited to low power or current (something like a hipot tester, if you've got one lying around). This may produce the intended conduction in the open circuit location, allowing it's identification when diagnostic (low) voltages are slowly re-applied to the string, to illuminate surviving lamps.

A common repair procedure, dictated by economic reality, presently involves the scrapping and replacement of the entire product. The replacement may use entirely different materials and methods for illuminaton.

RL

If feasible, try the more expensive dimmable fluorescent variant if you were using an incandescent bulb. Also, avoid dimmers that allow on/ off at any brightness level.

I'm not asking how to fix it. I've already done that in the clock by rewiring all bulbs in parallel to a 12V xformer. I only made the post to try and understand what causes multiple series-connected bulbs to fail at once. These were normal low watt (maybe 2W) 12V tungsten (not halogen) bulbs wired in series and connected to the AC line. Thanks for the replies.

What did you do in the case of the dimmer? Stock up on fuses?

Yes. I haven't messed with that. Though so far the fuse has blown twice when the bulb blew....or the bulb died when the fuse blew.

I have seen failure modes on incandescent lamps where the filament opens, and an arc forms, the arc then migrates back down to the support leads, and sustains there until the support wire vaporises. If that happens in a series string, I could guess that the increased voltage across the rest of the string causes a similar failure on each globe in succession. 12 volt globes certainly have short spacings across the support leads. They were not designed to withstand mains voltage.