I might have a project coming up in which I would wire up about 10 Christmas mini lights in series on 120V. They would of course be 12V lamps. My question is about the cascade failure that can occur, as if this goes forward, it could very well be a reality. It would not be a Christmas-only use and the long hours would mean that burnt-out lamps would be an eventuality.

I plan on using a fuse on this circuit, most likely quite close (but slightly above) the amperage draw of the lamps. Now, when a lamp burns out, the shunt usually shorts it out. When too many lamps have burnt out, the system acts like there is a smaller quantity of lamps on the same 120V feed, which increases the current in each. When too many have failed (or too few remain), they all burn out in short order (cascade failure). When this happens, in theory, you would have a complete short circuit on the 120V. I have read online that the shunts are supposed to act like a fuse, and one will blow and interrupt the current. I am wondering if this is the case, that a cascade failure usually results in a blown plug fuse on Christmas lights, or if usually a shunt burns out and cuts the power like a fuse would.

Alternatively, since this is not a typical Christmas light set, I could substitute one of the lamps with a standard 12V incandescent which would burn out as well and cut the power (like the "fuse bulb" in UK mini Christmas light sets).

@sol:
I've had it happen, and usually a shunt in one or more bulbs will go even before all the bulbs burn out. Never had it blow the little fuses in the plug.

If using 12v bulbs, I reall suggest using atleast 12 of them, it will greatly increase the lifespan of the bulbs, plus gives you some allowance for dead bulbs!

All sets i seen (classic series strings with bimetal flash lamp) had no fuse in the plug or anywhere. The packaging stated that the lamp shunts are the fuse

I assume the lamps dont have the interruption capability of even a 5x20 glass fuse (let alone a sand fill fuse), so wonder if any lamp might explode when acting as a fuse. Maybe they count on fusing several lamps in series for the interruption capability, but i think that the lamp that burns last would also take most of the fusing action, since it already got arc going on from the breakdown of the oxide isolating the shunt

@Ash:
Interesting. I'm surprised, especially being in a 240v country that your sets wouldn't have fuses!
All even-somewhat-modern sets here in the US have a fused plug (I ofcourse mean store-bought sets, the ultra-cheap stuff off eBay doesn't count .. that may or may not have fuses .. plus might have such thin wire that if shorted you might even burn through it before the breaker trips .lol. )

I know a shunt can 'burn out' if there's too many dead bulbs (too much current)

Probably if you put 120v straight through a single one of those little bulbs it would explode .LOL. (that'd be fun to try)

Thank you for your comments. When the time comes, I will probably put an inline fuse, just in case.

I have been searching everywhere on the internet for this, and I have not arrived at any conclusive information other than to make sure to unplug the set before relamping because if not, you will blow lamps as you install them.

I have also considered about 12 lamps to increase the lifespan. Not knowing yet what form this will take, I haven't decided yet.

to make sure to unplug the set before relamping because if not, you will blow lamps as you install them.

In all the many years of replacing dead bulbs in Christmas lights with the set plugged in (I always do), I have never blown a bulb from it. .LOL.
The reason I saw you should unplug is to prevent from getting shocked.

Yes, but let's say all the lamps are burnt out and all the shunts are active. A new lamp will burn out immediately, so you're getting no where.

As for the occasional burnt out lamp, like you, I always relamp with the set plugged in. You get immediate feedback if the new lamp operates correctly.

Not being able to handle maintenance of incandescent strings to stop cascade failure is one of the reasons I switched to all LEDs...

Interesting. I'm surprised, especially being in a 240v country that your sets wouldn't have fuses!
I know a shunt can 'burn out' if there's too many dead bulbs (too much current)
...

Actually a single fuse is technically way less reliable in higher voltage environment, so a fuse may be viable option for 120V strings, for 230V it wont help that muc, unless it would be some high grade one (so more expensive).

Plus such fuses "have the tendency to stop protecting" after it blows the first time. The reason is, the user will very likely not have the proper fuse so use some wire of Al foil "fix".
The fuses are really there just to make an excuse for lawers, they are in no way expected to be the main protection against the cascade failure.

What is used and actually works is the shunts being actually a kind of resistors in series with a brake-over device (all in the form of a ceramic like material). When the filament is OK, the voltage does not reach the breakover voltage, so the resistor is disconnected. Once the filament breaks, the voltage has to exceed the trippoing voltage for the breakover device to turn ON. But it turns off at every zero cross again, so next halfwave needs the voltage again.
Now when many bulbs burn out, the breakover voltage of the shunts adds up, so they turn ON at higher voltage. If too many lamps burn out, the mains voltage becomes insufficient to trigger all the shunts in series, so they remain off.
At home burned down from a Christmass tree the lawers blamed there was no fuse so it "must have been the failing lights to cause the fire". And not the sparkler that used to be hung on the tree regularly...

The Christmass fire statistics here are quite "interresting":
Iconic Christmass picture: A family with burning sparkles hanging on a tree behind them. That is, what I remember from my childhood too.
Usually a dozen trees burnt down so nothing is left from them every year (sometimes taking the complete home with it, fortunatelly often with just limited damage).
But all the reports list "failed electrical lighting" as the "most probable cause". Not a single "careless use of open fire" when tree burns down (dont get me wrong, open fire is sometimes blamed, but only with the "advent wreath" fires where it catches fire from the candles).
Now you want me to believe the lights are really the culprit to blame...