Many, many big power-smoothing capacitors inside those jumping from 0 to 120V in a microsecond, that’s why. The better-smoothed the power supply, the more capacitors and the bigger the sparks tend to be, although some really high quality ones put most of them behind inrush-current limiters to reduce the sparking, but that can also marginally reduce efficiency. High power electronics are always a bit of a tradeoff. The problem is that capacitors charge and discharge almost instantly in most cases, and when empty they act like a short circuit until they’re filled, so they can create some pretty big sparks, even though the actual energy going in is minuscule by any reasonable measurement. It’s almost like a static shock, huge spark, tiny energy.

Some motors will also spark badly when disconnected, but the reason is slightly different. They have a huge electromagnetic field which suddenly fills or collapses and that inductance in the coils can draw a lot of amps on startup and generate some pretty high voltages, more than enough to spark across the gap. Like the capacitors, they are very nearly a short circuit until they start moving.

It’s also kind of misleading. This map labels North America as 115-120V like everyone always does, when in fact it’s ALSO a 240V system, it’s just that the “common” plug and the “typical” circuit don’t use it, they only use half of a center-tapped 240V line. So that’s the “standard” they choose to use to label the whole system.

But it’s kind of unfair. It’s 240V coming into the house just like everywhere else in the world, except you also get the choice for it to be 120V. Being split-phase makes it easy to run multiple 120V circuits with a minimum of wire and still allows 240V for high-wattage appliances on their own dedicated circuits. It’s actually a very clever system and basically every house is effectively supplied with both voltages. It’s often poorly utilized, yes, with a few practical limitations and a lot of limitations due to historical conventions, but as a technical design it’s really kind of the best of both worlds, and it could be utilized a lot more effectively than it is.

If I was allowed to have an outlet with two 120V sockets, and one 240V European-style socket, there’s no technical reason I could not safely do that in a single outlet box. I could choose to plug in whatever I want at either voltage as long as it wasn’t more than 15 amps. Of course code would never allow that, because we consider the higher voltage “more dangerous” but it’s always right there, across two opposite phase 120V lines. We’re just not allowed to use it, except for large electrical appliances like air conditioners and clothes dryers. It’s frustrating.

The fuse is actually in the UK plug (the big brick-like thing with the wire on it), not the socket. But yes, it’s a thing, and most of the rest of the world considers it overkill. Also a lot of cheap junky equipment (ironically the stuff where you’d most want the fuse) omits the fuse in the plug, go figure.

No you can’t. You can lock yourself out, but a typical residential house built to code in North America has a latch handle that always turns from the inside, even when locked, and usually unlocks by doing so to prevent accidental lock-outs. And likewise if the door has a deadbolt, it must have a deadbolt with a handle on the inside. Most other kinds of locks are also easily accessible and removed by hand from the inside. The point is that they can’t require a key from the inside, because if you can’t find the key then you are locked inside and in thick smoke and fire that the key may be impossible to reach. If any egress door requires a key to unlock from the inside it is considered a serious fire hazard and will never pass a code inspection. (Of course, foolish people can still add them later but you can’t prevent stupid and it’s still a fire hazard not to mention impractical)

These types of building code and fire code rules are typically written in blood. People have died because of this.