Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics:

Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts between them. Right now, no current flows, because the air is not conductive enough for electrons to flow through it.

Now, we take a light bulb, rated to work at 1.5 volts, and connect it to the battery with a couple of pieces of wire. Now current flows from one battery terminal, through one piece of wire, through the bulb filament, through the other wire, and back to the battery.

Let's now add another piece of wire to one terminal of the battery, and connect the other end of this wire to a rod driven into the earth. No current will flow through this second wire, because there is no connection from the earth to the other battery terminal.

Now, we have two conductors; one of them happens to be grounded. We can call one conductor 'grounded' and the other 'ungrounded'. The only difference is that a voltmeter with one lead grounded will read zero volts on one wire and 1.5 volts on the other.

We can say that one wire, the one not grounded, is 'hot', and we could (improperly) call the grounded one the 'neutral'. Improperly because it doesn't qualify under the definition of 'neutral', so we'll call it the 'grounded' condcutor instead. With me so far?

Okay, now let's take two batteries, stacked like they would be in a flashlight, with the tip of one contacting the base of the other. They are said to be 'in series'. Let's take a piece of metal and insert it between the batteries; we'll call this the 'center tap'.

If we test with a voltmeter from the top of the upper battery to the bottom of the lower battery, we would measure 3 volts. If we test from the center tap to either end, we would measure 1.5 volts. Is this starting to sound familiar? Stay with me.

Now, let's take a piece of wire, and connect it to the center tap, and connect the other end of it to our ground rod. Again, no current will flow, because there is no conductive pathway from any other part of our circuit to the earth; only one point is grounded.

Let's take two 1.5-volt bulbs, and connect one from the top of the upper battery to the center tap, and the other from the bottom of the lower battery to the center tap. Each bulb receives 1.5 volts from its battery, and each can be turned on and off independently.

Okay, now we have a conductor, connected to the metallic center tap, that is shared by both of the bulbs. This conductor does indeed qualify as being called a 'neutral'. It also happens to be grounded, but that has no bearing or effect on our circuit.

We could also add a 3-volt bulb, connecting it from the top of the upper battery to the bottom of the lower battery. It can operate independently from the other two bulbs. In fact, we could add a plethora of both 1.5-volt and 3-volt bulbs to our batteries.

If we happen to add matching wattages of 1.5-volt bulbs to the two 1.5-volt halves of our 3-volt system, no current will flow through the wire to the center tap. However, if we have an imbalanced set of loads, the difference current will flow on the neutral.

For example, let's say we have 2 amps flowing through the upper wire, and three amps flowing on the lower wire. That's a difference of one amp, and that will flow on the neutral conductor to the center tap. However, nothing will flow into the earth.

Now, let's see if we can induce some current to flow into the earth. We already have one wire, the neutral, grounded via the ground rod. The only way we can get current to flow through the rod's wire is to ground one of the other battery terminals.

In other words, we would have to connect two different points of the circuit, with a voltage difference (aka potential) between them, to two different points of the earth. We call this a 'ground fault', meaning an accidental grounding of a conductor.

The current from the accidental grounding will only attempt to flow toward the intentionally-grounded point of the circuit, where the neutral is grounded, and no farther. The current will flow through any availabe pathway between potential differences.

Okay, now let's convert this 3-volt battery supply into a utility transformer that delivers 240 volts between the two ungrounded conductor, and 120 volts between either ungrounded conductor and the neutral, which also happens to be grounded.

There is one main difference: unlike the direct current (DC) from one or more batteries, the utility delivers alternating current (AC), which simply means that the polarity swaps back and forth. The main reason for this is that transformers require AC.

As I said earlier, the differences between AC and DC are not relevant to this discussion, except that the transformer's secondary winding receives the energy to produce the 240 volts from the electricity delivered to the primary.

Any current from a ground fault on the secondary side will only attempt to flow toward a point with a potential difference, which would be the grounded neutral conductor, and no farther. The transformer isolates the secondary from the rest of the world.

The only time current in the secondary system might attempt to flow farher upstream, towards the sub-station supplying the primary system, is in the case of a primary-to-secondary fault within or outside the transformer enclosure.

So, that one conductor of any system is grounded has no effect on the normal flow of current. In a manner of speaking, it makes an accidental contact with a hot wire more dangerous, but it also limits the secondary voltage in case of a primary-to-secondary fault.