Circuit breaker versus GFCI outlets (which also have breakers). I find the former far more convenient—I know they don't serve the same purpose, but the way one interacts with them is similar, except that the GFCIs are scattered all over the place and may require moving things to find.

> If it's even possible, such a wiring configuration seems like an obvious building code safety violation.

Norm in every house I've lived in in the US (except one so old it had knob-and-tube wiring... hahaha) is one GFCI for a set of outlets. Like, if you have a long vanity in a largish bathroom, you might end up with two or three outlets, one GFCI, the others following after it so they trip it, too. The only times I've seen a single one used with nothing hanging off it is when it's the only outlet in the room, or for under-sink outlets for dishwashers and disposals to plug into. This can turn into a real mess if anyone got "creative" with wiring at any point. We've had a garage GFCI kill a couple outlets in the house proper (in a nearby laundry room, not super distant), plus an outdoor outlet probably 30 feet away, all of which were evidently attached to it.

Every GFCI outlet has a line side and a load side designed specifically for protecting additional downstream outlets. it's perfectly safe and legal (and a great way to protect multiple outlets, especially in older houses with ungrounded outlets).

What OP is saying is that one of those downstream outlets can trip the GFCI and if you don't know which GFCI it tripped, then you have to go looking.

Right, that. And it's not like they're intolerable or anything, but they do represent a pretty high percentage of the time I spend messing with my house's electrical system. It's probably light bulbs (I swear, all but the crazy-expensive LED bulbs are blatantly lying about their lifespan, like by a literal order of magnitude) then GFCI stuff after that.

In the UK, yes, in lots of other parts of the world they are integrated into the circuit breakers.

Would this be a regular circuit breaker fuse? Otherwise what's different?

If single phase you'll notice sometimes it has a little extra ground wire connected (now mostly obsolete), for 3 phase it doesn't, in both cases it will say so on the outside (typically: the test current, and usually there is a test button which makes them easy to identify, this test button forces a small leak causing the circuit to become unbalanced resulting in a trip (if it doesn't trip when the test button is pressed the breaker is considered faulty and should be replaced).

For instance, here is an 'Eaton' 3 phase breaker with integrated ground fault protection circuitry:

https://www.omnical.co/products/eaton/1742431/2259595

Note the little yellow 'test' button. That's a 30 mA fault current device, you can have higher permissible fault currents for certain gear that tends to be a bit more leaky which would otherwise cause nuisance trips. These little things are quite the work of art inside, if you ever have a faulty one I would encourage you to pick it apart to see what makes it tick.

Circuit breakers trip when there's too much current going through the circuit (enough to melt the wiring for example), and so protect from short-circuits: hot and neutral touching each other, or a short in equipment.

GFCIs protect against current leaking to ground, by detecting if the current flowing on a hot and neutral leg are different. If they're different, the current must be going somewhere else: to ground, through you, etc. The GFCI breakers do this in the breaker, the outlets do it at the outlet. As someone else mentioned, you can also have other non-GFCI outlets chained to a GFCI outlet, so that upstream outlet is the one that pops if there's a fault.

TL;DR they detect and protect against different fault conditions.

Good GFCI receptacles are almost 10x the cost. Adding them everywhere would add a few thousand extra to building costs.

In any case, new houses must use arc fault breakers which do the same thing and more for most things.

I would add that big motors will trip these every time, so they aren’t usable for every circuit.