Hello, everyone.
I am reading the material on this website, and have a question regarding grounding, discussed here:

http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

I understand that the purpose of the grounding point is to make any person standing on the ground electrically common with earth ground, and that there is no voltage between electrically common points.

What I fail to understand is the specifics of how this helps.

The book gives scenarios to illustrate the potential danger in not grounding a circuit: if a tree touches the wire, and creates a ground path, then when a person standing on the ground also touches the wire, the circuit is complete, and the person will be shocked.

The second of these scenarios involves a case where there are two people, each standing on the ground. One touches the high wire and one touches the low wire. A tree touching the high wire completes the circuit for the first person, through a ground path. But the second person is also shocked, since the two people are both touching the ground.

What I fail to understand is how adding a ground wire would solve the problem! In all of the danger scenarios that the book depicts, I don't see how adding the grounding wire would prevent the shock.

I thus fail to understand how in any scenario, a person would receive a shock in the absence of a grounding point, but due to the grounding, the person does not receive a shock

Grounding systems are among the most misunderstood. There are many myths about grounding.

Here are the facts: there are two different systems covered by the term "grounding". There is System grounding and equipment grounding.
Equipment grounding is for the purpose of preventing electric shock thru metal parts coming in contact with a live conductor, called a fault.. the equipment is bonded through the ground wire which has continuity in the main panel with the neutral conductor. Upon contact the current travels along the ground wire to the main panel with a very high momentary ampacity which will trip the circuit breaker in the presence of this fault. in the absence of a ground wire if a live conductor comes into contact with a metal part and thus a person, under certain conditions, i.e. barefoot or wet leather soled, the electrons will travel thru the body and through the earth to the grounding electrode up to the neutral bar and on to the neutral wire at the Electric service and back to where it came from at the transformer, thus completing the circuit with the person in series with 120 volts.
This would not occur without a grounding electrode providing path back to neutral unless the service were bonded elsewhere to an underground metal plumbing or gas pipe.
The purpose of the grounding electrode is to prevent overvoltage from nearby lightning strikes which with an EMP can induce electrons in the building to move along the wiring system producing arcs and increasing the potential for fire. Lightning is sky to earth, the electrode provide a more effective path for these lightning induced high voltages to return to.
Not to be confused with direct lightning strikes which will do what damage they do.

 

interesting.

Rule one, To be dangerous to humans a certain current has to flow, say for convenience 10 mA

Rule two, for current to flow, a voltage difference must be present across a resistance

Rule three, resistance on a human is high, say 1 Mohm

Conclusion

,
QED: If human and case are at same voltage, no current flows, so not dangerous.

Also,
a earthed case, if its contacted by a live, the current from live to case / ground is high, by definition its a low impedance,
thus the protection device should blow.

Also,
the impedance from live to case in a short is low, say 0.1 Ohm, the resistance from case ( live ) to ground via the human is high say 1 Mohm, thus we have two resistors in parallel, and only 10 millionth of the current flows through the human to earth as opposed to through the case direct to ground,

 

Welcome to AAC.

When working with line voltage, AC circuits require a ground path so that in circumstances where the case of a machine becomes electrically energized, the ground should safely conduct that current to ground. If the circuit is protected by a fuse or a circuit breaker then they should shut off mains power. If the ground is faulty or non-existent then a human could come in contact with the case and they can become the pathway to ground. It happens from time to time. In electronics class I was operating a signal generator and an oscilloscope. One of the two devices had an electrical issue while at the same time a faulty ground. When I placed one hand on the case of the scope and the other hand on the generator I got a pretty good shocking. Not a full on 110 volts, but it was up there. Strong enough to make my arm muscles contract quickly. The grounded machine became a focal point for when I had my hand on the other machine. Current flowed from the bad machine, through my body, to the grounding of the other machine.

This is the case when working with line (or "mains") voltage. There's commonly a transformer that transforms high voltage down to a useful voltage in the home. In America it's normal to see a split phase transformer (center tapped) to provide 240 VAC from L1 to L2. The center tap (neutral) is also grounded at the pole and the home. From L1 to neutral you get 120VAC and from L2 to neutral you also get 120VAC. Because the system is grounded for safety purposes, if you have a ground circuit in the home that is in proper working order your chances of being shocked are greatly reduced. But if you defeat the ground pin of a plug - like my father used to do all the time - you are no longer protected. My dad built an electric lawnmower from an old washing machine motor. It had three wires but the power cord had the ground pin removed so it could be plugged into the common 2 pin outlets. I remember getting a small shock when cutting the grass when I was barefoot. Somewhere in the motor something was making contact with the case and I became the conductor to ground.

One thing electronics enthusiasts do is use an isolation transformer, whether it's a 1:1 isolation transformer or a 10:1 drop-down transformer. If you have 120VAC fed into the primary of a transformer and you get 12VAC from the secondary - that's a dropping transformer. The mains circuit may be grounded but the 12 volts is isolated. Grounding that circuit is not required for safety - however, it may require grounding depending on what the circuit is doing.

Grounding also reduces noise on an electronic circuit. But proper grounding is required. Don't know if you've read about "Ground Loops", but they can create problems with some electronic circuits. However, on the subject of grounding - there's a whole lot more to it than what I have described. Keep on learning. Especially when it comes to learning about safety. Safety is job #3. Job's 1 & 2 are "Refer to job #3".

 

The resistance of dry skin is high but if you just exercised and are perspiring or washed your hands you will find it much easier to experience a dangerous shock.

 

Another comment:

Neutral and the Ground wire are connected at one point. This point is also Earth usually done with a ground rod.

If there is Fault to ground, the house "sees" the same reference point.

Your metal appliances, copper water lines are also earthed to this same point.

Now for some really wierd stuff, read about grounding swimming pools and sidewalks around swimming pools.

It turns out that lightning storms can make the potential across say a 200' length of Earth different.
This is why "detached" structures also get a ground rod. The detached structure gets a sub-panel where ground and neutral are separated as well. They connect together at the main panel.

The little GFCI breakers measure the difference in current between line and neutral with a transformer. If that exceeds some value, call it 10 mA, the breaker trips. That could be through a person or insulation breakdowm.

What we shoud do and do sometimes when it;s necessary is "home run" every outlet. The outlets are range and known as isolated ground outlets. The center pin goes to the building's ground point. The case ground also goes there, but maybe the outer metal of Bx cable which is common in industry with metal studs.

Not good a drawing pictures, This allows you to have a reference and a ground which you don't have with daisy-chained receptacles.

 

The resistance of dry skin is high but if you just exercised and are perspiring or washed your hands you will find it much easier to experience a dangerous shock.

Here is a pretty basic and understandable good description of humans interact with electricity.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/

 

Because one side of our power mains is grounded, it is safer to have the accessable conductive portions of an appliance also grounded, because that greatly reduces the chance of a person receiving a shock while touching the appliance and ground at the same time. THAT is why grounding of mains powered devices improves safety. The second, and far less common reason is that if some portion of the mains circuit is hit by lightning, having one side of the mains grid grounded will tend to reduce the voltage available to cause a shock.
For an isolated, independent, power source that does not have any portion grounded the situation is different. Unless that isolated source is truck by lightning or some other ground-referenced voltage source, contact with a single point will not result in a shock. This is impossible for may folks to understand, But if some part of an isolated circuit becomes ground connected then no isolated condition exists, and a shock hazard will be present.