Grounding wires is important. The green ground wire always brings the question, why do we ground wires? The purpose of a ground wire is to give excess electrical charges a safe place to go. Alternating Current(AC) has two ‘wires’ which are equally dangerous – as the current goes back and forth in each one.
History of AC
When the first AC lines were run from a generator, hundreds of miles to factory, a strange thing happened. People sometimes would get hit with gigantic lighting like bolts just walking within a few meters of the lines. Even though the voltage of the AC lines was no where near enough to arc those distances. There was also issue where if lighting hit a power line, people a hundred miles away could be killed, whoever was unlucky enough to be closest to the power line at the time.
Also if people touched a wire while touching the ground, nothing happened. But if two people touched opposite wires while both touching the ground, they would both get shocked, even if they were a hundred miles away from each other?!
Why Do We Ground Wires?
It was realized that not only does the earth conduct electricity, but that static electricity builds up on the wires, to millions of volts. When air/vapor heats up it leaves the surface of the earth, often while statically charged, and touches the wires, causing them to be statically charged. To fix this they had to earth one of the two wires – ie. attach it to earth every so often on all the power poles. Also where the power enters every house, one of the wires goes down into earth. This means when static builds up on the line, it ends up being drawn to earth and cannot built up. To prevent static on the other line, they connect small loads between the lines – so the static would cause a small DC current and go to earth.
So since one wire is grounded, there is not voltage possible between it and ground. So you can ‘safely’ touch that wire. Even if you are connected to the ground – and so this wire is often called ‘neutral’. While the other wire, being not able to statically charge to millions of volts, will still have the voltage between it and ground for the normal power. So this wire is called ‘hot’ or ‘active’.
A Metal Box
Now consider if you have a metal box. If you have the ‘hot’ wire come loose and touch the metal box – anyone who touches the metal box while touching earth will be zapped. So to prevent this – the metal box is linked to neutral/earth. We ground the wires.
In these ‘suicide’ shower heads. In normal operation the current goes ‘hot’ -> ‘water’ -> ‘neutral’. So if you touch water nothing happens. But what if the neutral wire comes loose and isn’t touching water anymore? The path would be ‘hot’ -> ‘water’ -> ‘person’ -> ‘earth’. Causing them to get shocked and die. With the earth wire in the water it will be ‘hot’ -> ‘water’ -> ‘earth’.
They could also use neutral instead of earth. But earth has the advantage in that. If you run a separate earth wire to the shower head, then you can have a circuit breaker for the ‘hot’ that works by comparing the neutral and active currents. When things are working properly active and neutral have the same current. If something is wrong active and earth will have the current – and the breaker will turn the power off.
Why Do We Ground Wires?
So that is why we use 3 wires, even though 2 of them connect back to each other (Earth and neutral).
Putting the wire in a cup of dirt does nothing. It needs to connect back to the neutral and inturn – the entire earth.