Earth Ground's role in 3 prong AC systems

Question

I've been reading through this explanation where the author goes through the natural progression of why a three-prong AC outlet was made (Why Three Prongs?) , but I have some trouble imagining how earth ground plays a role here.

First off, for just a two prong AC system, which I take it to mean a floating source system, one can't be shocked from touching just one of the two "hot" wires, but only by holding both at the same time and acting as the short circuit between them (does voltage just drop to the value of the wire with the lower potential here, while current spikes?). Just touching one of the hot wires is analogous to only hooking a single wire to one terminal of a battery I imagine--there's no return path for current to start flowing. Even if one were barefoot and standing on a damp floor, with no grounding wire in the system at this point, no danger of shock exists.

So reading further down the link now, how does the fact that the neutral wire (which is grounded) exists allow the hot wire current to go through a person standing on earth ground? The author mentions that by grounding one half of the system, they essentially connected one half of the system under the persons feet. Where specifically is current flowing in the ground to create this loop or return path for current? Is it reaching wiring underground to complete the connection?

To try to drive this idea home, what happens to a power supply when both sides of the circuit are connected to earth ground? How does this make a short circuit between the two sides of the circuit? I thought earth ground was a sort of "reservoir" of sorts that is unaffected by current moving into it.

Any help explaining this concept is greatly appreciated.

Answer 1

AC power distribution systems are two wire circuits for delivering the current to the load. The hot wire conducts the current to the load on the positive AC cycles and from the load on the negative AC cycles. The "neutral wire" is used to conduct the corresponding current from to the load. It conducts current from the load on the positive AC cycles and to the load on the negative AC cycles. The use of the two conductors (Hot and Neutral assures a low resistance circuit path to make the distribution as efficient as possible).

For safety reasons and attempts to stabilize a local reference for the AC power distribution network the "neutral wires" are connected (bonded) to an actual earth ground connection at the entrance to the facility. This means that any given time there is a potential current path from the hot wire through a person's body to anything that is part of the nearby earth ground, be that through wet dirt, damp concrete, water pipes that go into the ground, metal building structures that are buried to foundations in the ground and even water paths to the ground. Any such current that is over some threshold through the human body is lethal. To mitigate this danger the standardized and codified electrical systems in most countries and regions these days require that all AC power distribution systems provide for a low impedance additional conductor path (safety ground) from the usage point (outlet for example) back to the facility entrance point where these additional conductors are tied in common to earth ground and to the "neutral wires". This low impedance path is intended to be connected to any thing that the user may be able to touch that is conductive on an appliance or fixture. The idea is that if there is some type of accidental short from the hot connection inside the appliance to the metal parts of the unit that people can touch that any current that wants to flow would take the preferential path through the safety ground wires back to the service entrance as opposed to going through the person and back through some other path.

These days many circuits in homes and businesses that are located near water, water pipes and other grounded metal systems are required to be equipped with ground fault circuit interrupters (GFCI). These units are designed to constantly monitor the current on the circuit and compare if the current on the hot side of the circuit is within 5mA of the current flowing in the "neutral wires". If they differ by more than this amount then the GFCI trips and open circuits the load point from the power feed. An imbalance could happen in cases where some person touched some metal part of an appliance or fixture where there was a fault or insulation breakdown. Some current could go through the person and into the nearby earth ground network back to the service entrance. Any current over 5mA is considered potentially lethal and so the GFCI trips to protect the user.

Note that not all appliances and devices are equipped with three prong power cords to take advantage of the safety ground feature. Such devices these days, that are manufactured and distributed by honest and reliable businesses, will be designed in such way that it is deemed safe to use the device even if there is an internal short or fault associated with the hot wire connection. These devices often have external plastic cases and are sometimes referred to as being "double insulated".

Answer 2

There are several uses of the third-prong GROUND in domestic wiring. One, is line-voltage shock safety. Another, is static discharge control. A third, is fire safety.

First off, for just a two prong AC system, which I take it to mean a floating source system, one can't be shocked from touching just one of the two "hot" wires, but only by holding both at the same time

There is an unfortunate side effect of the lack of consequence from touching just one wire: it means that an accidental connection to one wire (as, to a piece of grounded metal) has no observable consequence, and will not become known. So, the SECOND connection, some years later and in a different room, comes as a lethal-shock surprise.

For shock safety, a ground conductor capable of opening the circuit breaker (or blowing the fuse) is connected to touchable parts like the metal enclosure of most appliances.
Ground-fault interrupters can also prevent shocks, so are a sensible backup precaution, required in some places.

The use of a ground-floating isolated power circuit is not completely unknown, but is used only in extraordinary circumstances (often combined with sensitive alarm circuitry to inform when a fault occurs), or with harmlessly low voltages.

For static control, all transmission-line equipment has some spark-gap or other 'lightning arrestor' grounding apparatus, but a floating two-wire system does not. Therefore, each building's ground provision is inspected when the electric meter is installed, usually a ground rod connected to one of the conductors. That both protects against voltages that would destroy the insulation, and against lightning-induced currents that come through your home's grounded metal parts (plumbing, for instance). A high voltage between ground and house wiring, just cannot be sustained while the ground rod to wiring bond wire is intact.

Answer 3

Just touching one of the hot wires is analogous to only hooking a single wire to one terminal of a battery I imagine--there's no return path for current to start flowing.

Wrong. The AC distribution system is not floating, each wire has a potential related to the earth. Therefore the return path always exists.

So, why 3rd wire? The PE (Protective Earth) is wired to the metal casing. For example if your boiler heater gets damaged, then you will get hot on its shell case. With PE connected, touching the boiler and the water won't kill you. Further a GFCI device can detect this leak and disconnect the damaged device.