How does equipment ground protect us from electrical shock?

Question

What I have always learned is that current leaves the source and returns back to the source, forming a closed circuit.

Here are the two thought experiments I had and got confused:

Scenario 1:

Here I have a black (hot), white (neutral) and green (ground) wires coming from the wall and into the box where the ground wire is connected to the chassis of the metal test box.

If the black wire touches the metal casing and a person is touching the case, then the ground wire would lower the entire resistance since the person and the ground forms a parallel resistor of the circuit and drop all the voltage upstream so the person would be safe.

Scenario 2:

What if the ground wire isn't connected to the chassis and somehow the hot wire made a connection to the chassis?

My inclination would be the current travels through the person, but here is the dilemma in my head. How does the current return back to the source from the person standing on the concrete ground? Are we really saying the current travels from the concrete, into the building and somehow back to upstream?

If the person is receiving an electric shock from scenario 2, then how does the current get back to the source?

If there is no closed circuit for the fault current to return to source, shouldn't the person be safe since it is an open circuit?

Answer 1

Not quite.

In the case if the black wire touches the metal casing and a person is touching the case, then the ground wire would lower the return circuit resistance, causing a very large current to flow. This will blow the fuse in the plug (UK BS1363 plugs) or the much larger fuse or trip the breaker in the house circuit, keeping the person safe even before they touch the enclosure.

If the ground wire isn't fault current rated or the breaker is too high current for the circuit, a fire ensues instead.

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In the second circuit, only a small current is required to kill the person. If the concrete floor provides a very high resistance, the person feels an unpleasant tingling but probably survives.

If they touch some grounded metal with the other hand, the current increases. Now the RCD (UK) or GFCI (US) can see current flow on Live which isn't balanced by current returning on Neutral, so it trips at 20 mA or so : unpleasant but not usually fatal.

(This may also happen in the first scenario : there's a race between the protection mechanisms. Older houses may not have RCD protection so fuses/overcurrent breakers are still a good idea)

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In either scenario the person is protected IF the protection mechanisms are properly installed, tested and working.

Answer 2

In home wiring, the neutral is connected to ground at the breaker panel.

Then the ground is earth-grounded outside, to stabilize the network's voltage levels. The neutral continues to the distribution network: the current loop is closed by wiring not by conductance of the earth.

Neutral is a circuit conductor that normally completes the circuit back to the source. Neutral is usually connected to ground (earth) at the main electrical panel, street drop, or meter, and also at the final step-down transformer of the supply.

https://en.wikipedia.org/wiki/Ground_and_neutral

Then why not dispense with the ground wire, and connect the chassis to neutral? As currents flow through the neutral to supply the device or other equipment on the same line, the potential of the neutral line is raised above the ground neutral, which could pose a hazard if high enough.

A person is indeed safe if there is no closed circuit, e.g. operating with one hand only, and with isolated feet.

This is also why birds can sit on high-voltage lines without harm.

Then there is live-line working, whereby a worker's both hands can be exposed to the same potential, as long as the rest of the body is not grounded, or connected to a line with a different phase.

live-line working, also known as hotline maintenance, is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. (...) The barehanded approach has a live line worker performing the work in direct electric contact with live parts. Before contact, the worker's body is raised to the same electric potential as the live parts, and then held there by electric connection, while maintaining suitable isolation from the surroundings which are at different potentials, like the ground, other people or trees.

https://en.wikipedia.org/wiki/Live-line_working

Answer 3

In case 1... Before the breaker trips, The person is touching the mid point of 2 resistors, Hot and Ground (of equal value..IF the conductors are of equal size (Ohms/length)). So the voltage between the case and another Earthing point is one half the line voltage! Un-grounded the case is AT the Line voltage. Only saving point is the resistance between the persons and another ground point.