Modern circuit breakers are much fancier. They break not only when the current reaches a threshold but also when the currents passing in both directions (i.e. also back through the ground wire) are unequal. This prevents things like grounding through someone's body.
> Modern circuit breakers are much fancier. They break... when the currents passing in both directions [is] unequal
What you're describing is properly called GFCI[0] — which in some countries is referred to as an RCD[1] — not just a standard circuit breaker.
You can get devices which fit into a standard circuit breaker slot which perform both functions. However a conventional circuit breaker (which are still widely available) doesn't do any of that.
In fact in the US, especially residential, GFCI circuit breakers are not terribly common for 120V circuits, people much prefer and are used to having the GFCI in the outlet, which can of course protect multiple conventional outlets downstream.
But, what is quite common in the last 30 years are AFCIs, arc fault interruptors which are code for pretty much all living area branch circuits (just not bathrooms). These are built in to the circuit breaker. You can get dual GFCI/AFCI breakers, but these tend to be expensive and not commonly used. Nobody wants to have go down to the basement just because of a ground fault in the kitchen. Also the propensity for GFCIs to nuisance trip increases with the length of the branch circuit. There’s also cases where they are specifically disallowed.
GFCI outlets also seem to wear out eventually, whether they trip a bunch or not. Lately everyone I know that's sold a house has had to replace one or more of them due to them failing to work during inspections.
Part of the issue with "not terribly common" is that no one ever retrofits these things into older houses and new houses only get them where code requires it since it's an increased cost.
When I mentioned not terribly common I was speaking specifically of GFCI breakers in the breaker panel box as opposed to GFCI in the receptacle. Even in a retrofit or remodel it isn't common to put GFCI in the panel box for the reasons noted. There aren't 240V receptacles widely available with GFCI so these tend to use the breakers.
They should of course be added to any kitchens or bathrooms. It's cheap insurance.
Yep I thought my house has that until I tested cheap LED dome light from Aliexpress. It had an exposed screw and I was bare feet in garage (concrete floor). Lesson learned.
> (i.e. also back through the ground wire) are unequal.
You mean neutral or return wire. No current should be normally passing through the ground wire. In fact in the US at least, GFCIs can be used on grandfathered ungrounded outlets. When you buy GFCI outlets they come with little stickers: “No Equipment Ground” to affix on the outlet in that case.
