Basic question about circuit diagrams and ground

[Special K]

Originally posted by: silverpig

Originally posted by: Special K
Regarding earth ground - is the entire surface of the earth an equipotential surface?

Also, let's say you take an input AC waveform, and convert it to DC using a diode bridge and a capacitor. Will there be a potential difference between AC "neutral" and DC "ground"?

The surface of the earth is definitely not an equipotential surface (apparently earth ground can vary by hundreds of volts from place to place), but it's generally considered to be 0V.

Is that accurate though? How can it be considered to be 0V if it varies by hundreds of volts?

Also what about my second question?

 

[Born2bwire]

Originally posted by: Special K

Originally posted by: silverpig

Originally posted by: Special K
Regarding earth ground - is the entire surface of the earth an equipotential surface?

Also, let's say you take an input AC waveform, and convert it to DC using a diode bridge and a capacitor. Will there be a potential difference between AC "neutral" and DC "ground"?

The surface of the earth is definitely not an equipotential surface (apparently earth ground can vary by hundreds of volts from place to place), but it's generally considered to be 0V.

Is that accurate though? How can it be considered to be 0V if it varies by hundreds of volts?

Also what about my second question?

Voltage is relative. I could decide to make my folks' overgrown yellow lab as my 0 V ground reference. What is special about the Earth is that it is basically an infinite sink/source of charges. It doesn't matter how many electrons we sink/source from the Earth, the potential does not change in an appreciable amount. Where as the lab will eventually build a measurable charge if I keep dumping electrons on her (think Van De Graaf generator, only cuter). It only becomes a problem when you take multiple ground references tens or hundreds of miles apart. As I recall, what they did with telegraphs is they did not have a return wire, there was only a single signal wire. The telegraph took the negative wire as the Earth ground. This causes a problem because the ground at one telegraph office could be kilovolts in difference from another office. I can't remember how they fixed it though, probably a system of repeaters.

As for your second question, you only get one.














Fine, depends on how you wire your circuit but generally you hook your neutral up to ground and you use the same ground reference on the bridge side so there wouldn't be a voltage difference. Again though, this is arbitrary.

 

[Paperdoc]

Common North American household power systems are AC, usually 60 Hz frequency, and fed to your house from a transformer. Think of the transformer's output (from its secondary winding) as having three wires to your house - two from the two ends of the winding, and one from the center. The transformer is set up so that the two end leads deliver to you an AC supply with an rms voltage of 240 v. So, compared to either end connection, the center lead is 120 v AC. In actual use, the center lead is treated as a common current return line, and the two end leads are separate supply lines, each at 120 v AC from the center. But because the two end leads are exactly 180 degrees out of phase with each other, between those two you get 240 v AC.

Now, for safety, the convention in North America is that, both at the transformer and at your house breaker panel, the common center lead is connected to a true ground lead - that is, a lead really solidly connected to earth ground. Doing this establishes a reference point for all those AC voltages in terms of the earth - one of the leads is at earth potential at all times. Hence that one is usually called the "Neutral" line, and the other two (from the winding ends) are both called "Hot" lines. Just to make it easier to keep track of things, some people use DC concepts and refer to the two "Hot" leads as "+ 120 v" and "- 120 v" lines, but that's not quite right.

Note that the Neutral line is connected to earth ground. In fact, it actually is connected to the SAME earth grounding point as the "Ground" wires - usually, that point is a water pipe entering the house and buried in the ground outside. But here's the important difference. From the breaker panel out to any outlet, the Ground lead is NEVER connected to carry a current in normal use. A circuit will have a "hot" lead to supply power to a device, and a "Neutral" lead to return the current. The wiring cable also will have a bare copper "Ground" wire in it which is connected to the outlet box and to the Ground connector hole in the outlet. But the Ground lead is NEVER involved in carrying current for the devices plugged into the outlet. Its two purposes are to guarantee that there is a true earth ground connection to the device (often used for its exterior surfaces if it has a metal case), AND, in the rare event of a wiring malfunction that connects the Hot lead to the wrong thing, like the outer case, there is a reliable low-resistance return path to Ground at a very high current for all the power available from the Hot lead, sufficient to cause the breaker to trip and shut it off.

Wires are not perfect conductors - they have resistance. So in practice at an outlet connected to a heavy load, the Neutral lead may have a voltage above true Ground, because it is connected to ground farther away at the breaker panel. This is acceptable because the Neutral line is NOT presumed to be safe - it is part of the current-carrying circuit. But as long as we follow the rules and never try to run a current through the Ground lead, it IS safe and at true earth Ground potential at all times, except briefly during an emergency when it does its job to cause the breaker to trip.