Why is the Earth a useful reference of electric potential?

Question

I may be a little confusing, but I honestly need a good help to make me understand certain things. I like very much to learn theory, but I feel that if I learn it without associating to the real life, it would be like 2 isolated boxes with knowledge, with no interaction. In this post I'm trying to stablish a connecting bridge between Electrical Potential, and how it is useful to us, and how this can connects to our daily knowledge.

I get that we use Earth as a reference point, the origin of our referencial, and I get that we only consider the relative values of potential according to Earth (ground).

My question is, why is ground/Earth used as a reference? Why isn't 1 meter above the ground our reference? Is it because its potential relatively to infinite is -500...(something)Volts, so any overcharge in our devices could simply discharge to the ground? But even considering that this is right, why is it useful? What makes it useful to know that in some point, you got X potential relative to our zero referencial?

Per example, If I say my gravitational potential energy referencial is the floor, I can say that a ball 1m above it has $$m\cdot9.8\cdot1 J$$ of energy that can be converted to kinetic energy. So it's useful to know the velocity before it collides! So, how can we make use of our eletric potential referencial assumption?

Also, when we talk about "neutrals", in our homes(per example), I think that they are used because they complete the circuit between Power Source and ground, but are these "neutrals" the same neutrals that we talk in three-phasic configurations (Triangle, Star) ??

Answer 1

You're taking this "earth" or "ground" thing too literally.

If I tell you: "There's 100 V DC on this node." Then then you would assume that I mean "100 V DC relative potential to the ground node of the device I'm measuring at that moment.

You'd call me crazy if I said: "There's 100 V DC on this node relative to the metal hull of an oil-tanker which is in the middle of the Atlantic Ocean".

It would be inconvenient to actually measure like that. There's no guarantee that the oil tanker's hull is properly grounded (also: grounded to what ? The bottom of the sea ?)

This silly example demonstrates that "ground" or "earth" is just a local reference point. We define it as having a zero potential voltage. You always need 2 nodes to measure a voltage, a voltage is actually a potential difference. It is extremely convenient to make one of those potentials zero. And we call that one "ground" or "earth".

Answer 2

In order to have a current you need a circuit... That is, you need a current path in both directions from a voltage supply.

The biggest conductor on the planet is the planet itself. Although dry soil and dirt itself has quite a high resistance per linear meter, when you consider there are 1 trillion cubic kilometers of "earth" that's a lot or resistors in parallel.

As such the ground resistance from a few feet down where I live to wherever in the planet YOU are, is actually quite a small number. The bit of dirt you are standing on is electrically connected to the bit under my chair. Because of that, earth or ground, is quoted as a common potential when talking about electricity in general.

However, it IS only a reference. In any circuit, ground is generally a reference point from which you measure voltages to other points in the circuit. That ground may or may not be connected to the planet. Indeed the ground in your circuit can be kilovolts above true ground potential.

In your house wiring, Neutral and Ground have different functions, though, in most cases, they are actually connected together somewhere. The Neutral is the actual return path to the transformer winding, while ground goes to.. well.. ground.

However, in most countries, and installations, the transformer's neutral is actually "grounded" too. This ensures that everyone in your neighborhood gets the same voltage and they are not all floating at different levels due to static buildup.

^{simulate this circuit – Schematic created using CircuitLab}

When everything is working correctly no current actually flows to ground. The current exiting the transformer coil on the live side should return to it through the neutral line.

Answer 3

Consider AC power coming into your home as an example. If you touch a hot wire (directly or indirectly through a short circuit in a device), what you care about is the potential difference between your hand on the wire and the ground under your feet. That's the potential difference that's doing to determine if a current flows through you or not. That makes it practically a very important reference value in such systems. Likewise for natural phenomena like lightening strikes. For this reason, your home electrical system is literally grounded to the earth.

(This question is tangentially relevant: AC: Why differentiate between Ground and Neutral? Although it's specifically about wiring conventions in houses, the answers address the physical connection to ground of the wires.)

In isolated systems - like, for example, battery powered DC systems - the word "ground" is still used for historical reasons, but the potential between your "ground" in that isolated system and the potential of the literal ground under your feet could be anything, and probably isn't even especially well defined while there's no electrical path connecting them. In this context, the use of the word "ground" is not literal at all.

Answer 4

My question is, why is ground/Earth used as a reference?

Because its a low impedance pathway with a net charge near zero volts. You can dump as much current into the ground as you want and it will still remain near zero. The point is your return pathway for current between two points is pretty much negligible (think milliohms).

Why isn't the height above ours heads the reference?

Because it's a high impedance pathway, you can't dump any current into it (only pA or fA) and it's net charge is usually not zero (electric fields from ESD and the atmosphere, in the day time it swings from ±100V/m, passing storms can bring that potential to 10kV/m)

Is it because its potential relatively to infinite is -500...(something)Volts, so any overcharge in our devices could simply discharge to the ground?

Yes, the ground will 'soak up' any charge (although near power plants the potential of the ground can be a little bit higher due to local currents and the fact that the ground is not a superconductor, it still has small amounts of resistance)

But even considering that this is right, why is it useful?

Consider what would happen if earth potential was a high impedance pathway or insulator (like if the ground were made of teflon). Grounding would be impossible and one couldn't just drive a copper stake into the ground to equal the potential between you and another object, you would have to run an extra wire on every power pole for any return currents for saftey ground, and even then there would be slight difference between point to point because of the resistance of this wire. Communications and power applications would become much much harder.

What makes it useful to know that in some point, you got X potential relative to our zero referencial?

The point is, the earth ground potential doesn't change much and the changes would be negligible and slow and not noticeable for almost all applications.

Answer 5

A reference is nothing more than comparing one point to another. You are seeing how point 'a' looks in comparison to point 'b'. You can use anything as a reference! Technically, B+ can be used as a reference to B+ (looking for an IR drop, perhaps...). In this respect, everything is RELATIVE. What is their RELATIONSHIP to one another? The key question to this equation is, "is there a POTENTIAL DIFFERENCE between these two points of reference? The answer is: there is ALWAYS a difference. The question is how much. Does it really matter (enter differential calculus)? How can I fix this (enter differential calculus)? Yes, ladies and gentlemen, math is very important.