What is the difference between a neutral busbar and an earth busbar?

Answer 1

The neutral busbar is the common return path for all of the hot legs. As such, there is current flowing through neutral. If the various hot loads were balanced, effective current in neutral could be zero but, in practice, true balance is hard to maintain.

The grounding busbar is the earth protective ground, used to ensure that any fault within a device is shunted to ground, causing the protective device to trip. This prevents the case of the device from becoming hot and causing an electrocution hazard.

While neutral and ground are connected together at various points starting at the distribution panel and proceeding towards utility power, it is essential that no operational current flow on ground - it must flow on neutral. Any other condition represents a ground fault which must be corrected.

Answer 2

In house wiring any circuit requires two wires, the hot wire and the neutral wire. At one point the neutral wire is connected to earth ground (literally a copper rod driven into the ground).

The neutral wire was grounded as a safety measure. The thinking was, if someone touched an electrical appliance and at the same time touched some metal in the house, a kitchen faucet for example, they would not get a shock. In the past, those two wires were all that was used.

Unfortunately it turned out that in real life, relying on the grounded neutral wasn't really preventing dangerous shocks. As more and more things were turned on in the house (Lights, Radios, TVs, etc.), the neutral wire was really carrying current and so the voltage at the neutral wire was often above the earth ground. People were getting shocks if they touched something that was really grounded (Plumbing is always grounded because the incoming water pipe is buried in the ground) while handling electrical appliances (filling an electric kettle for example).

To alleviate this problem, the code was modified to add a third wire. This wire was also connected to earth ground. But because this wire does not carry current it will remain at true ground potential. This required a third connection at all outlets. This wire is called the "safety ground."

The electrical code specifies the safety ground is colored green for easy identification. The neutral wire must be white. That way anyone working with the wiring will be able to easily identify it. The hot wire is usually black or sometimes red.

So, to answer your question, the neutral busbar is where the neutral (white) wires are terminated and the earth busbar is where the (bare) safety ground wires are terminated. Both busbars are connected to earth ground at this point.