[dvh]

I was thinking about this and came to conclusion that the only correct type is UK style socket because it has fuse.

In our houses, there are circuit breakers. They don't protect you or devices, they can only protect wires in the wall, those who installed the wires knew how much current they can take and installed appropriate circuit breakers.

When you plug the plug in the wall socket, the circuit breaker has no idea what you plugged in so it cannot protect it, so there has to be a fuse in the plug, like in the UK plug. Whoever chose the wires for this device choose appropriate fuse.

There is one more case possible, the wire is not permanently attached to the device but via another socket, for example C14 socket like in PC. In that case manufacturer of the PC should know what kind of currents it is capable of handling and should put fuse inside it.

Now everything is protected (at least for over-current, if you touch live and neutral with two hands, 30mA through heart is enough to kill you but that's something that cannot be avoided, not even GFCI can do it).

[UebVar]

I disagree. The plug is usually part of an appliance connector cable, that has no idea what happens to be on the other side aswell. If you size that cable for the same current as the socket, the cable itself is protected by the circuit breaker.

The correct spot for the fuse is the appliance itself. Fuses used to be easily replaceable, often with fuse holders [1]. I have, however, never seen a computer with one.

[1] https://uk.farnell.com/productimages/large/en_US/4578676.jpg

[LiamPowell]

There's simply never a reason for a user to replace a fuse in a properly designed device. If a fuse blows then it means something has gone horribly wrong and replacing the fuse won't fix it.

The exception would be a device that sends mains more-or-less directly to a user device, then a fuse would be protecting against a fault in the user device and should be replaceable. A lamp that takes a regular light bulb would be a good example of this.

[pezezin]

Most computer PSUs have a fuse inside, and it is quite easy to replace them.

I know because many moons ago I blew one, in the era when PSUs had a toggle between 120V and 230V, and I set it to 120V in a country that runs at 230V...

[stephen_g]

I think it's overrated, plenty of equipment has some kind of protection internally anyway.

My country has never had a fuse in the plug and we generally have a very safe electrical system (much stricter earthing rules than the US for example). Adding an extra fuse doesn't really seem to add much, it really doesn't seem to be any kind of significant risk.

[johnwalkr]

The historical reason why is that UK homes were wired early in history for lighting with a ring circuit going throughout the house, and this was also literally set in stone so impractical to phase out for a long time.

So the regulations had to allow one 50A (for example, I don’t know the actual numbers) fuse supplying an unknown number of outlets and devices, rather than requiring one circuit per small area. Such a large fuse will happily let your radio malfunction and start on fire, so local, smaller fuses are necessary.

In other areas a 10A fuse (for example) on a circuit that only goes to one room or one appliance is enough to protect from overloading the circuit as well as most dangerous malfunctions of one device.

[divingdragon]

UK ring circuits are typically protected by 30A or 32A circuit breakers.

[nine_k]

Per-socket circuit breakers are very much a thing here in the US. I assume they are mandatory in wet environments like bathrooms and kitchens. I think they are adequate to protect against a local leak from the live wire to the ground wire, which would likely mean that the connected device is broken or got water inside it, and may be dangerous to touch.

I suppose that a device that suddenly starts to consume far more current than normal under normal voltage is likely broken / fried inside, and it's too late to save it by blowing a fuse. The fuse just prevents a fire, but an automatic circuit breaker in a socket would likely do the same.

There is the case of overvoltage due e.g. to nearby lightning strikes. I suppose a fuse is unlikely to save your computer in such a case, it's too slow. Fast-acting power line protectors exist though, and are cheap and ubiquitous.

[hdgvhicv]

Only socket in my uk bathroom is the shaver socket which is an isolating transformer.

[ssl-3]

The only per-socket "circuit breakers" that I'm familiar with in the US are GFCI outlets. They're the ones that often have a glowy light inside of them, with Test and Reset buttons on the face.

These detect an imbalance of current flow betwixt the two current-carrying conductors and shut off when that imbalance exceeds a threshold, which does reduce the risk of shocks -- particularly in wet environments. The imbalance is evidence of a leak, possibly through a person -- so their intent is to halt that situation when it happens.

But our GFCI outlets have nothing at all to do with what is usually referred to, in the US, as a circuit breaker.

Regular circuit breakers are very different. They only detect overcurrent conditions and switch off -- much a fuse does, but with a reset function. They primarily protect the wiring of the home, and they do not give a fuck if you're being shocked. (Human factors and leakage current are not part of their purvey.)

GFCIs and fuses/circuit breakers are similar in that they both break circuits, but they're different in every other way.

---

Meanwhile, in UK wiring, bathrooms do not have GFCI outlets. Instead, they have a shaver socket. That's a lower-voltage socket that also has a built-in transformer.

The transformer provides galvanic isolation. Galvanic isolation means the current imbalance that a GFCI is meant to detect and shut down can't happen in the first place, so it's safer in that way than a GFCI is.

With a shaver socket: Shaver in one wet hand, other wet hand touching metal water pipe? Perfectly safe: There's no opportunity for current to flow from one hand to the other. It's isolated.

---

Meanwhile: Fuses. British electrical widgets generally use fused plugs. The fuse is to protect the wiring of the device being plugged in.

Why? Because homes are sometimes wired with what is called a ring circuit. This can increase ampacity while using less wire. A ring circuit with 2.5mm wire is typically be fused at 32 amps, which is way spicier than common 2.5mm wire can safely handle, much less a device being plugged in.

But it's OK, because it runs in a ring -- each outlet has 4 current-carrying wires, and they each feed eachother within that ring. The ring (all 4 wires of it) extends all the way to the box where the fuse/MCB [maybe with an RCD], or RCBO live. (In American terms, an MCB is like out central circuit breakers. The RCBO is a combination device that detects and protects against leakage current and overcurrent conditions, like the central GFCI breakers that some homes have for some circuits.)

Rings safe as long as both legs of the ring remain contiguous and are never fucked with improperly.

For the history: The UK does use ring circuits because they had a fuck of a bad time rebuilding after WWII, and they decided that this would save them money and let it gone quicker. They were probably right about this, for them at that time.

But that means their plugs need fuses. So it be.

We don't use ring circuits in the States, because we've never had a post-electrification war here and the opportunity to broadly start over has never forced itself.

We don't usually use fused plugs, either -- our unfused pluggy-inny things are supposed to be able to trip our common 15 or 20a breakers without much drama. (Except when their design doesn't allow that. In those cases, they're supposed to have their own protection devices -- which is why Christmas lights have fused plugs in the US. Their tiny little wires can't carry enough current to trip the branch circuit's breaker in the event of a dead short. We got to choose between using bigger wires for the lights themselves, or fusing the plugs, or having houses burn down. We chose fuses. We were probably right about this, for us.)

---

Two different countries, two different pathways. Both paths work well-enough, but they're not the same.

And it's fine. :)

[quickthrowman]

If you really want to protect a piece of equipment, you need overload or protection relays. These come in various types for various applications, motor overloads, phase loss relays, and ground fault protection relays are a few types.

A three-phase motor circuit (VFD or starter controlled) has either thermal or solid-state overloads after the contactor to protect the motor and wiring in the event of a motor short, any upstream fuses or circuit breakers are just for overcurrent protection.

[inigyou]

Fuses are very imprecise devices. A 10A fuse won't really protect you from an overload of 20A. It could take an hour to blow or not blow at all. Both the 10A and 20A fuse will protect you from 1000A short circuits.

[aix1]

This sounded surprising and so I picked the first fuse I could find on RS and looked at its datasheet [1].

The characteristic curve shows that the 10A fuse is expected to blow after about 4s at 20A. Of course there's sample-to-sample variation and different ambient conditions etc, but how do those four seconds become "an hour to blow or not blow at all"?

[1] https://docs.rs-online.com/bc0e/0900766b81585c97.pdf

[hdgvhicv]

The fuse is there to melt before the wire.

My bedside lamp has a 1mm (or thinner) wire. If it faulted out and drew 10a the RCD won’t blow, but the wire will melt. Not immediately, but after a while.

The 3A fuse will melt long before the wire though.

[UltraSane]

UK plugs have fuses because of the use of 32 amp ring circuits.

[dajonker]

I like the UK sockets because they have a switch.