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Comment by amriksohata | Hacker News Reader

So in the UK we have 3 wires, red, blue and green, the green is ground or earth, what purpose does this serve? Is it the same as ground as you call it in the US

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I'm from the US, so I'll add the caveat that this is answer is based on my foreign understanding and a British electrician may be better suited to answer this.

Your green wire would be what the US calls a ground wire, and what is more accurately called a Equipment Grounding Conductor (EGC). It's purpose is to bond all normally non-current carrying conductive parts of a system together to provide a ground-fault current path back to the source so that the overcurrent protective device can operate properly to open the circuit in the case of a ground fault. It is 100% a safety measure, and will operate as intended to open a breaker whether or not it is connected to the Earth.

Your red and black wires are both ungrounded conductors, and are the conductors which actually create the circuit that allows current to flow to power whatever you connect to the circuit. The circuit will work to provide current without the green wire. However, without the green wire, if either the red or black shorts to something, the breaker will not open and you have an enormous potential to harm someone or burn your house down.

> However, without the green wire, if either the red or black shorts to something, the breaker will not open and you have an enormous potential to harm someone or burn your house down.

This is not true. First of all only the red is hot, if the black shorts to something nothing will happen.

Second if there is a short strong enough to "burn your house down", the breaker most definitely will open.

After researching the UK electrical system, I realize I was mistaken on one regard. I thought they used 2-ungrounded conductors, but I was mistaken. The red wire is the only ungrounded. This would be the same as the black wire in a US house. The red wire is their grounded/neutral. This would be the same as a white wire in a US house.

I am correct on the EGC, though. It works the same as the EGC in the US. If a ground fault causes a house to burn down, then the breaker will absolutely NOT open. That's how the fire starts: current flows through something it shouldn't long enough to heat it up to the point that something ignites. If the breaker opens, the fire wouldn't have started in the first place.

The US NEC now requires arc fault circuit interrupters which are pretty darn expensive circuit breakers with built-in electronics to detect an arcing situation that wouldn't trigger a normal circuit breaker but can cause a fire.

I keep hearing electricians say that they can't detect bad splices which how most residential electrical fires start.

I've seen videos of these on YouTube and they seem useless!

Current British electrical standards require RCDs (GFCIs to you) to be fitted to new installations. So the RCD should trip 30 ms after the current starts flowing where it shouldn't. Plenty of installations won't be brought up to spec for decades of ever, however...

The reason for having ground is simply because if an electrician makes a mistake and reverses the hot and neutral, and you plug something in, the body of the device will now be connected to hot, and you'll get a shock.

By having a separate ground, it's harder to mess that up, and devices can connect the body of the device to that.

Inside the electrical box (mains) the neutral and ground are connected together, and are at the same potential.

The only reason they run separate wires is in case of of mistakes.

> The only reason they run separate wires is in case of of mistakes.

Erm no.

1. In case the neutral gets disconnected, so the frame isn't at the potential of the hot (no voltage drop across the device)

2. So the return voltage drop in the white wire isn't on the body of the device. ~20 volts between adjacent high-current appliances on different phases probably wouldn't be terribly dangerous to most people, but it is sloppy.

3. For GFCI/RCD, it separates bona fide return currents from "accidental" return currents. With only two wires, dropping a toaster into a (PVC-drain) bathtub wouldn't trip. (Erm, I just realized most toasters are actually only two wires. Well uh, don't do that).

If that "toaster circuit" is protected by a GFCI, it will indeed trip with only two wires. The GFCI is sensing the imbalance between hot and neutral (the rest went through the bath water). Both currents pass through the same toroid, but they are in opposite directions and normally exactly cancel, so no flux is induced in the toroid. If there is an imbalance, a current is induced in a secondary winding. This will trip the GFCI internal breaker. Note that this does not involve the green wire.

Except there will be no imbalance if there is no other path for the current to travel by...

The geometry of the electric field in water should probably be similar to that in air (mostly contained to within the case), just don't drop both a toaster and a waffle maker in at separate ends.

The resistance of slightly salty bathwater is much lower than air. While baths are not often plumbed with metal pipes these days, one can still expect some current flow to ground. It only takes milliamps of stray current to trip a GFCI.

Milliamps sounds like a tiny amount in the context of home power, but 5mA at 120V is still only 24kOhm. This is actually pretty low to just assume away. I'd agree there could be some water left over in the drain pipe from the last bath, splashes making a path to the spigot, or something like that. I just wouldn't count on it.

Iirc gfci's are nec code allowed on non grounded lines because they will still trip, but you have to test with the gfci test button because they're only tripping on current difference, not like a breaker.

Yeah, IIRC the "proper" way to upgrade an ungrounded circuit to have a 3-prong outlet (without running new cabling) is to use a GFCI and label it "NO EQUIPMENT GROUND". This is considered safe, but is still ultimately a hack.

It assumes that any dangerous shocking current will be taking a path that isn't just coming back on its own white wire, presumably from taking an unspecified path to ground. This seems a reasonable assumption for safety (and the NFPA surely has looked at the data), but it doesn't really inform the above isolated-bathtub situation.

There is also the matter of unanticipated leakage current between hot and the equipment enclosure. The EGC/green wire keeps the enclosure close to ground potential.

I’m not an electrician but will take a stab at your answer. Please see if you can verify what I said so you don’t fry yourself.

If the green wire is connected between the receptacle and the outlet/switch then it’s “bonding” the two mental items. (Using the terminology from post). Most people refer to this as the ground wire or grounding the receptacle. The purpose, from my understanding, is to have a stable system. In the event the hot wire touches the receptacle.

If the red/blue/green are coming from the same cable (3 wires inside another sheath), then your green is the ground/neutral that connects to the electrical box itself, while the other two are hot/lead.

I posted a similar question above which may clarify things if the OP answers.

In the UK we used to have red, black and green, but now we have brown, blue and green/yellow (I think this is the same as rest of EU). I presume this is because of colour blindness reasons as there are folk who can't distinguish red and black that well.

Hopefully when Brexit is complete, you can return to the true colors as god and the Queen intended.

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