Current and Voltage Handling of PCB Ground Planes vs. House/Substation Ground Grids

Hi everyone, new to the forum and I have a question.

In a PCB layout there is a ground plane for the return current and in larger situations such as a house or substation there are ground grids underneath the structure. My question is what determines the amount of current or voltage a specific ground technique, so to speak, can handle and what are the intermediate "techniques" in between a copper layer in a PCB and a ground grid for a house? Thanks for any help!

When used as a return in an item of electronic equipment, the ground part of the circuit will be designed to have sufficient cross-sectional area to take the maximum current that would normally flow.

It may also serve as shielding to prevent radio frequency emissions affecting other equipment, and prevent external interference from affecting the circuit inside.

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Your "intermediate level" example:

The ordinary motor car uses the “ground return” concept for supplying electrical power.

One battery terminal (usually negative) is solidly electrically connected to the metal of the car body.
All the various light bulbs and electrical devices in the car each have one terminal connected to the car body.

Now only one wire from the positive battery terminal (via fuses and switches) needs to be connected to each bulb to light it, as the metal car body serves as the other conductor.

Otherwise, twice as much electrical cable would be required to wire up everything in the car.

The car body has a low enough electrical resistance to provide a current return for most equipment.
Usually, only the starter motor has its own (massive) cable directly connected to the battery.

Thanks for the help!

Now I've heard about high voltage PCB design where they can handle up to 300V or more. Is it unheard of to implement a low-voltage circuit onto a high voltage PCB? I'm sure there are some cost differences, but is there a reason why someone would want a low-voltage potential circuit connect to a high-voltage potential ground?

There are various reasons for using high voltages:
Mains supply - power can be delivered through smaller conductors,
Radio transmission - higher power outputs through aerials,
X-ray machines and devices for emitting various kinds of radiation,
etc.

Control circuits are usually lower voltage both for user safety and economy. The interface between a high voltage equipment and its low voltage control circuit may be electrically isolated for safety, using relays or opto-couplers to keep the two circuits electrically separate.

You will find a great deal of technical information on electrical and electronic devices and their principles of operation on Wikipedia, for example:

http://en.wikipedia.org/wiki/Opto-isolator