Why is the fuse connected to the live wire?

Question

As far as I can understand the concept of AC, it does not matter which wire is live or neutral as this changes like 100 times per second (implying 50Hz) I believe.

I read the fuse must be connected to the live wire, as close as possible to the AC source, so that the user is less likely to interact with the live wire in case of a fault.

Here I understand you are safe to interact with the Neutral wire. Is that right? Why so? Is it because neutral is connected to ground somewhere in an electrical box in my house? What happens if I plug a copper needle in a wall socket of my house, the neutral hole to be more precise, and I touch it?

To me this does not make sense at all as the live wire becomes the neutral wire the millisecond after, and that loops. Proof being when I plug a load in the wall socket, the load will also work if I plug it the other way around. So it does not matter which is live or neutral. Both are live or neutral. To me the distinction only exists I guess for ease of schematics reading.

Please explain why I am wrong as top results on search engines all say fuse must be on the live wire.

[Addendum] Considering your comments, am I right if I declare the following: there is a good and a bad way to plug a load in a system where the live is fused, even though the load would work in both ways.

Answer 1

Indeed the N wire is connected to GROUND and EARTH at the main electrical panel. So this references one leg of your 120VAC to ground. If you touch it nothing happens. So this protects you from 1/2 of the system.

If you fuse the N side and the fuse blows, there is still HOT in the unit. If the unit shorts to ground and the fuse is on the N, nothing happens. If, however, the fuse is on the H, then it blows if there is a short to either N or GROUND and the power is killed to the unit.

Answer 2

Why is the fuse connected to the live wire ?

1. At the transformer:

With the neutral being earthed at the transformer, an earth fault would blow the fuse, disconnect the faulty line and prevent a fire.

2. At the consumer:

With the enclosure being earthed, an earth fault in it would blow the fuse, disconnect the line and render it safe.

Had the fuse been located in the neutral line the enclosure would remain live and continue to present a shock hazard.

Answer 3

The image you have in your mind is like two jump ropes both spinning to make their own sine waves to make two live wires and no neutral.

However, in the US electrical code, the neutral is connected to the ground in the electrical panel. This essentially holds the neutral at "ground" voltage* and you then have to imagine the live has to have peaks and valleys 2x the voltage swings as in the way you were imagining the two spinning jump rope model.

I put the word "ground" in quotes above because neutral at the outlet can be some voltage above ground based on the wire resistance from the outlet back to the panel. Resistance from outlet back to panel x current draw will be the voltage increase over ground.

Answer 4

 the Live wire becomes the Neutral wire the millisecond after, and that loops

No. The neutral wire stays close to 0V w.r.t ground at all times. The voltage on the live wire changes from +170V to -170V (+320 to -320 in Europe) and vice versa every 10 ms or so.

Having a fuse on the live wire switches off the load, and brings the whole circuit close to 0V (he neutral wire remains connected), making it safe to touch.

A fuse in the neutral wire would switch off the load, however, the load would remain at the potential of the live wire, so you'd still get a shock if you touched it.

there is a good and a bad way to plug a load in a system where the Live is fused

No. Loads are either isolated, or use a third terminal in the plug (protective earth) connected to the case. The PE terminal carries essentially the same voltage as the neutral wire, but its contact in the plug is asymmetric, so it's impossible to plug in the load in a way that connects the case to the live wire.

Answer 5

To me this does not make sense at all as the live wire becomes the neutral wire the millisecond after, and that loops.

No, it doesn't. If you imagine wires as pair of strings, the neutral one hangs motionless, while the Live one is swung wildly up and down, ending above or below the neutral string. The load (a rolling pin for example) rests on these and takes the energy from height difference.

2. There are two things to fuses:

   a. House installation fuse - yes, it must be on Live wire, so it gets disconnected the moment something bad happen. In case of house wiring, we know which wire is Live, because they are screwed in and not going to change, unless someone really messes up the service panel outside your house.

   b. Internal appliance fuse.
   Here all bets are off. The plug could be put in either way, or maybe someone swapped Live and Neutral in the socket by accident. What really matters is that fuses breaks the connection between Live and Neutral somewhere inside appliance, making it stop. The fuse is still on what "should" be the Live wire, but it is not externally guaranteed. The appliance is none the wiser which wire is Live, it only cares the voltages swing up and down between the two.
   [caveat: Some grounded appliances do care which wire is Live, because they use ground-referenced sensors.]

Answer 6

Let me briefly start with the idealized description of the wires:

• The neutral wire is nominally at a voltage of 0V (ground).

• The live wire has a voltage which cycles from 230V to -230V and back 50 times per second*.

The term "alternating current" refers to the fact that, if you just connected these wires through a resistor, current would flow from the live wire to the neutral wire when the voltage of the live was positive, then would flow back the other way when the live wire has a negative voltage. The neutral only serves to carry current - the voltages pushing this current around are provided by the live wire. If you switch the wires, the potential difference between the terminals stays the same (changing between 230V and -230V regularly), so plenty of electronic components will work either way.

The massive difference between the wires is how they react when exposed to electrical ground. A neutral wire shouldn't do anything as its voltage is already (nominally) the same as ground. A live wire will start pushing and pulling current to ground. A relatively common risk is that a person might touch a circuit while also being somehow connected to electrical ground - and the question is how to protect such a person.

To justify placing fuses on the live side, consider the following dangerous situation: You stick a metal object into an outlet, connecting the live and neutral wires. A great amount of current flows through it. A fuse blows disconnecting the circuit.

You propose two options:

1. The fuse disconnects the neutral wire. The metal object is now connected only to the live wire and exposes a voltage which regularly shifts between 230 and -230 volts. This is dangerous - if you touched the metal object and something else connected to ground, current will flow through you.

2. The fuse disconnects the live wire. In this case, the outlet is now connected to the neutral only. At least in theory, the metal object is connected to ground through the neutral wire. This is significantly safer - just touching another grounded object shouldn't create a voltage across you**.

Both options will stop current from flowing through the errant object - so at least you won't be setting your house on fire in this thought experiment - but we should prefer option (2) because it merely exposes a neutral wire, which is not as bad as exposing a live wire.

(*Exact voltages and frequencies vary from region to region)

(**For clarity: even if the neutral wire is nominally at ground voltage, you still shouldn't touch it - it's certainly safer than touching a live wire, but it's not something you should gamble with nonetheless)

Answer 7

You are describing what's called an IT system. In an IT system, the two wires are indeed completely equivalent. Fifty or sixty times per second, each wire alternates between being positive and being negative (positive and negative, not live and neutral!) while the other wire does the opposite.

Now imagine you touch one of the two wires. This wire is now connected to ground via you, but nothing happens! Everything keeps working, as you're not in series with the circuit. You might think this is great, and it kind of is, and IT systems are often used in settings where people's life depends on the electricity staying on (e.g. some hospitals).

The disadvantage is that this might lead to a false sense of security. Nobody knows that you're touching one of the wires! So imagine someone else, at the same time, touches the other wire. Unless you're perfectly isolated from each other, you both are now part of an alternative branch in the circuit. The electricity goes from the circuit, through you, into the ground, through the other guy, and back into the circuit. The problem though: the system has no way of knowing whether you're a device being plugged in, or two people being electrocuted! So it will stay on and keep electrocuting you both!

This is the reason IT systems are never used in household installations. Instead, one of the two wires is permanently connected to the ground. This wire is then called the neutral wire. The other wire isn't attached to the ground, and is called the live wire. Now, if you touch the neutral wire, nothing happens, like before. The wire and you (via the ground) are always at the same voltage. (Note that in countries with good safety regulations, there needs to be something called a residual current device that will notice this happening and shut off the electricity anyway, just to make sure.) If you touch the live wire, that's now dangerous. That's why all regulations are designed to avoid people coming into contact with the live wire.

Answer 8

For domestic wiring in the UK:

The short answer is safety.

The reason the live is fused is similar as to why the live is switched and not the neutral. If you switched the neutral, voltage would still be present in the load. By switching the live wire; we ensure that the load is isolated when the circuit is switched off.

The neutral wire is connected to Earth and if isolated will stay at 0V, the live wire will alternate between + and - and effectively pushes or pulls electrons through the neutral wire.

The situation is similar with a fuse. A fuse is designed to protect the equipment and people from a short circuit. There are two cases;

1. The live and neutral get short circuited.
2. The circuit shorts to a conductive material.

In situation 1, it doesn't matter where the fuse is located the fault current will flow through the circuit and blow the fuse wherever it is. In this case the reason for fusing the live is the same as the switching example I gave above. When a fault occurs, we want to isolate the load.

Situation 2 is why we have equipotential bonding (Earth wires on radiators, pipework etc). If we didn't have any equipotential bonding, the equipment may continue to function during a short, but with a serious risk of electrocution. By bonding all the conductive components to earth we ensure that a fault current will flow through the circuit, then through the shorted equipment, and then down to Earth, ensuring that the fuse will blow. In this case, it's possible for very little current to flow through the neutral, and if the fuse was placed on the neutral wire, it may not blow.

Some equipment is said to have a polarity, while other equipment does not. Something like a tungsten light bulb will function either way round because the coil inside looks the same which ever way it is connected. A diode will behave very differently depending on which way round it is connected.

Answer 9

You are confusing the fact that the oscillating voltage is sometimes instantaneously zero with "neutral." The voltage in the neutral wire is constantly zero when the circuit is open, while the live wire has an oscillating voltage that is virtually never zero, even though it is instantaneously zero twice per cycle. (When you connect something and turn it on, both wires have oscillating voltage.)

Imagine a system that provides energy or information through an air-filled tube by using sound waves -- oscillations in the air. This could be a speaking tube in an old ship, for example. Perhaps there could be a membrane in the tube that vibrates with the sound waves, attached to a shaft that does some (presumably microscopic) work. There is a speaker at one end of the tube. The other end of the tube is open; you can hear the sounds coming through the tube at that end.

Suppose the speaker emits a constant tone. Now cut the tube in the middle. You now have two lengths of tube; one has the driving speaker at the far end. You can hear the tone coming out of that tube. The other length of tube is now just an open tube. It does nothing. You hear nothing.

The tube is analogous to a live wire. The open end of the tube is analogous to ground. Cutting the tube is analogous to opening a switch in an electrical circuit, which separates the neutral wire from the live wire. The tube that is connected to the speaker is analogous to the live wire; the other tube to the neutral wire.

In the tube with sound, there is always sound, even though there are moments when the pressure of the air is instantaneously zero relative to the atmosphere. On the other hand, in the other tube, the pressure is always zero relative to the atmosphere.

Answer 10

What really matters is consensus. Everyone agrees that one side should be treated as the "ground." Indeed, as you have noticed, it is tied to an Earth ground.

The situations which are dangerous are those where there is a voltage differential. If everybody agrees that one arbitrary leg is the "high" potential and one is the "low" potential, and everyone agrees to fuse the high OR everybody agrees to fuse the low, malfunctions do not lead to there being an unexpected voltage differential that can shock someone.

Choosing the side which is tied to ground and fusing the "live" side vastly increases the number of surfaces which are complying with the consensus, in addition to the wired devices, making the system safer. That way, if I touch a device with a blown fuse and a metal water pipe, I remain safe.