Not sure if I understand - can you clarify? Did you mean that a normally safe 1.5kW appliance runs the risk of cooking itself if it's connected to a 40amp circuit? If yes, then sure, that makes sense.

> Lots of smaller (kw) circuits is safer than a few very large circuits.

Not fully convinced about the safety aspect but it definitely feels more convenient (can selectively turn rooms off if you need to work on something) - if I'm ever opening the walls, I'm going to switch to that approach. At the very least, I want to have a dedicated 20amp circuit in the home office...

Largely yes a 1.5 kW device (a 12.5 amp space heater for example) is less safe on a 40 amp circuit. Since if it fail pulling more load than designed but not truly shorted the breaker may not trip. But if you put that space heater on a 15 amp circuit it would most likely trip the breaker. As other commenters have said the UK addresses that risk with fuses in the plug.

Also I recently redid my kitchen and I added an additional 20 amp circuit just for the kitchen island outlets and it has been super nice.

Are safety concerns alleviated here by GFCI breakers? In what cases could this appliance pull more amps than it was designed for? I know it's not required for circuits outside kitchen and bathroom, but I was thinking about just replacing all the breakers with combined GFCI/AFCI ones (especially given that my circuits are so old that there's no ground on them, in which case code I believe mandates that the outlets are GFCI-protected and labeled as "no ground").

So the risk would be reduced. Since if the failure causes a electricity to travel anywhere other than the neutral it will cause the gfci to trip. A scenario where a device would pull more power would be a short developing in a heating element reducing its resistance. So it would just pull more power but all of the power is still going to the neutral and wouldn't trip the GFCI.

Also with replacing all your breakers it can increase safety. But first the breakers are not cheap. Second it can lead to a lot of nuisance trips, albeit that is better than it used to be with modern breakers.

Gotcha, so I guess it'll only protect from electrical hazard but fire hazard is still going to be there...

Breakers are probably going to be a bit cheaper and easier than opening all the walls to redo the circuits though...

Yeah your AFCI in theory is supposed to help reduce fire risk. Since failures like that cause arcing. But yeah it will be around $60-$80 per breaker, but there is definitely cheaper than ripping open walls to re-run wires.

GFCI is not a magic “make everything perfectly safe” - you could electrocute yourself to very dead without tripping a GFCI if all the power went down the main wires and none went to ground.

Britain solves this with fuses inside appliance plugs.

The continental European system doesn't really solve it, as far as I know, so you have thicker wires on low power appliances than you would in the USA.

The US also has fused plugs but their are not widely used for some reason. I see them mostly on Christmas lights, but I have also seen it on some small home electronics like a window fan and air purifier.

That’s usually because the lights draw such little power that a short at line amperage could turn them very hot without tripping a breaker.

The fuse prevents that.