What is the potential of earth relative to mains live and neutral?

Question

I'm confused about the theory of earthing in mains electrics. In particular, how the potential of "earth", the ground or literally the soil beneath my feet relates to the live and neutral wires in household circuits. I am in the UK and would like to discuss UK mains.

So, I've always understood that earthing is an important safety precaution. The earth pins on mains sockets terminate somewhere connected to earth, perhaps via a metal rod stuck into the ground. Metal components of electric appliances are connected to the earth pin on their plug, so that if there is a short anywhere in the appliance, the current would flow straight to earth rather than through a person touching the appliance. This is because the resistance between the appliance and earth is much lower across the earth wire than across the person.

Now, I had compartmentalised the idea that current wants to flow towards earth. I had just taken this for granted, without asking why.

I found out that actually the neutral wire will at some point be "tied" to earth. I think this "some point" can be near the home, maybe even in the home? Or it can be near the transformer? Either way, neutral is actually connected to ground.

So does this mean that the potential of the earth is actually oscillating 180 degrees offset from the live wire in every home, with a potential of 240V between them? This would then explain to me why current wants to flow from live to earth, and how earthing electrical appliances protects a person from shock in-case of a short, but it raises other questions for me ...

1. We seem to be relying on soil being a very good electrical conductor. For example, in the extreme case, where the neutral is connected to earth near the transformer, for the earth to be 240V relative to live wires in all the associated homes and safely source current in the case of a shorting, then the soil/earth/ground would need to very effectively conduct to the surrounding homes, no? This seems very unlikely to me. Is soil in-fact a fantastic electrical conductor or have I missed something?
2. Would it not be safer, in some ways, not to tie neutral to earth, and do away with the earth wire? In this scenario, if a person touches anything that is connected to live, then maybe there wouldn't be any voltage across them because it's now just a "floating" voltage relative to ground?
3. If the PD between earth and neutral is 0, then why even bother with neutral? Why not just connect the negative source of the transformer to earth and replace neutral wire with earth in the home. This would save the cost of cabling and you could still protect from shock by connecting metal appliance bodies to the earth wire as well?

Answer 1

There's a couple of things to separate here.

One is the reason for having a ground ("earth") wire in your house wiring. The other is the reason for having one side of the AC (neutral) connected to the literal earth beneath your feet. They both play together to keep you safe.

The ground wire in your house is connected to neutral to keep the hot and neutral from "floating" to higher voltages. High voltage (static charges and such) can creap across the transformers. If neutral weren't grounded, you could potentially have several thousand volts between your appliances and the ground beneath your feet. Grounding the neutral makes it so that neither the hot nor the neutral can be more than 240VAC from the ground you stand on. It also makes it so that the neutral can't (usually) zap you. The neutral is at the same potential as the ground under your feet - no potential difference, no current flow, no zap.

The ground wire in your house wiring is a fallback to handle things that shouldn't happen, but do. Any metal part that you could touch on any appliance is connected to ground - the earth ground connected to the dirt beneath your feet. That means the metal parts can't zap you. It also means that if a hot wire comes loose inside the appliance and touches metal that it will cause a short circuit through the ground wire back to the neutral - that blows the circuit breaker and indicates that there's a fault in the appliance.

Ground and neutral are run separately because you don't want current flowing through anything you might touch. Neutral carries current all the time - it is part of the normal circuit. If you measure from neutral to ground, you'll usually find several volts difference because the current flowing through the neutral causes a voltage to appear across it. There is normally no current flowing through the ground wire so that anything connected to it is really at zero volts compared to the ground beneath your feet.

Finally, if you only had a neutral and connected the metal parts of the appliances to it, then you'd have a dangerous situation if the neutral were to break. If the neutral wire breaks somewhere and the appliance is connected to the hot, then the metal parts connected to the neutral will have the full line voltage on them.

That third wire - the earth ground - is a safety feature. It is there to keep you from being killed if something goes wrong in your appliances or your house wiring.

Answer 2

You are mixing many different things.

And there are different electrical systems how electricity is distributed to your home, and that is a separate thing how it is distributed inside your home.

Assume earth (the soil, the ground, the planet) is a bad conductor. It still conducts and is at some potential.

Assume a power disitribution transformer has just two output wires. There is 240 VAC between those two wires but they are isolated/insulated from everything so neither are fixed to any potential so the AC output is floating.

The floating system can gather charge compared to earth soil potential and so it would be not very good if say both wires float 1000V above earth potential. When such static charges discharge through some route they can do damage.

So safest thing is to tie one of the transformer outputs to earth potential. As it is now defined to be always at same potential than earth, it is called the mains neutral wire, a blue wire that is always at 0V compared to earth potential.

The other transformer now has 240 VAC compared to 0V and thus earth potential and that is called the mains live wire, coloured brown.

The output now does not float or is isolated, and it won't be able to gather charge and have different reference than earth potential.

The live and neutral are the current carrying wires.

A separate concept is how earth wire is used for safety in your home.

As I mentioned before, at some point in the electrical system (in your house or at transformer) the neutral and earth have connection. It really does not matter where exactly, but at that point you can also start distributing a second wire that is also at earth potential like the neutral, but is not used to carry any current, except in momentary fault conditions for safety. This is the yellow-green earth wire that goes along with neutral and live to your mains sockets.

For example an appliance like an heater or toaster will use live and neutral for getting the energy into heating wires, so whatever current there is on heating element, same current will be on live and neutral wires.

If the equipment breaks and for example live wire touches a metal case, it becomes dangerous to touch as metal case now has 240VAC compared to earth. But if the earth wire is connected to metal case, it safely conducts the live through earth wire to the neutral connection, and as there now is a short circuit inside the device, large enough current flows to blow a fuse or trip a breaker to cut the live supply and de-energize the metal case of faulty equipment.

1. No we don't generally use earth for current. Current to homes is carried in wires. Soil is used as return path only in some special single wire earth return systems.

2. No, not safer to keep neutral disconnected from earth, mentionedd already above why.

3. Depending on your system, you might have only neutral and live coming to your home. The neutral may be earthed at transformer, at your home, or at both places. There may be separate earth wire coming from that point where the earth bond is made. And like mentioned above, to protect from faulty equipment, you need to have the safety earth wire from your mains socket to go to your neutral.

Answer 3

There are two aspects to an earth protection system.

1. In the case of a fault ensure that you do not apply a voltage across a person in contact with a piece of electrical equipment.
2. In the case of a fault blow a fuse or a breaker to disconnect the supply.

In order to achieve the first objective the case of electrical equipment is either non conductive or is connected to "ground" as you said this is the voltage of the earth under your feet. In the UK this protection is extended to metal pipework by demanding that water and gas piping is also bonded to ground. This is a relatively high impedance path and the objective is to make sure that any current runs in the earth wire and not through you.

The second objective is achieved by bonding the neutral to earth at the mains entry into a building. This means that in the case of a live-earth fault a large current will flow from live-protective ground-neutral, this will blow fuses or trip breakers removing the fault circuit.

There is still a risk from high impedance earth faults say from live to the case of electrical appliances. This is taken care of today by the use of Residual current breakers or MCBs. These can detect small current imbalances between live and neutral (i.e. some current is leaking to earth) and trip providing more protection.

Answer 4

We seem to be relying on soil being a very good electrical conductor. For example, in the extreme case, where the neutral is connected to earth near the transformer, for the earth to be 240V relative to live wires in all the associated homes and safely source current in the case of a shorting, then the soil/earth/ground would need to very effectively conduct to the surrounding homes, no? This seems very unlikely to me. Is soil in-fact a fantastic electrical conductor or have I missed something?

It's not in all cases, soil and other conditions can change the resistance to ground. There are codes (depending on region) that specify this, here is an example of one:

What is a ground rod?

A ground rod is usually located very close to your main electrical service panel and is often made of copper or copper coated steel. They’re approximately one-half-inch in diameter and 8 to 10 feet in length. It must be electrically tied to your main service panel to provide an approved ground connection.

If a single ground has a resistance of 25 ohms or less, building codes allow it to be used as the only grounding device. If the resistance of a ground rod is greater than 25 ohms, at least one additional ground rod is required. Source: https://empoweringmichigan.com/ground-rods-what-are-they-and-how-do-they-protect-your-electrical-equipment-and-appliances/

Would it not be safer, in some ways, not to tie neutral to earth, and do away with the earth wire? In this scenario, if a person touches anything that is connected to live, then maybe there wouldn't be any voltage across them because it's now just a "floating" voltage relative to ground?

No, it would not be safer, ground is used as a 'guard' in many instances to redirect dangerous current to ground instead of another pathway (like through a person). Another benefit of this is this can be detected and stopped by saftey circuits like breakers, GFCI's,ect.

In a product (like a power supply) chassis ground is used so if something is miswired or if a fault occurs (like you dip the product in water) the fault would short to ground and blow fuses, switches or breakers.

Ground is also sometimes used in regular wiring (like in a breaker box and sometimes wiring boxes) so if a wire comes loose it will short and create a fault.

If the PD between earth and neutral is 0, then why even bother with neutral? Why not just connect the negative source of the transformer to earth and replace neutral wire with earth in the home. This would save the cost of cabling and you could still protect from shock by connecting metal appliance bodies to the earth wire as well?

This is done in some cases, it's called 'double insulated' and its usually seen with two prong devices. Two sets of insulation can be used instead of ground to ensure that dangerous voltages will not reach someone.