Many readers have written asking about grounding and if it is really needed.
Electricity only appears when there is a potential difference between two points. For example, if we take two wires, one with a potential of 12 V, and another with a potential of 0 V, then we'll have a potential difference of 12 V. If both wires have 12 V, there will be no potential difference between them, and electric force will be zero..
Therefore, the mains electrical systems are made up of two wires, one called phase and another called neutral. The neutral wire shows a potential of zero and the phase wire is where the electric energy is transmitted. As there is a potential difference between phase and neutral, there is electric force. In mains distribution network the voltage is alternated, as the electric force of the phase wire varies along the time making a sine wave.
Ground is a signal having a zero volts potential. It is used to level the electric potential between electrical equipments. Usually the ground is connected to the metallic casing of the equipment. In equipments where the cabinet is made of plastic, ground is connected to the internal metallic case existing inside the equipment.
You must be wondering what is the difference between ground and the neutral wire, once both have a zero volts potential.
It happens that the neutral wire potential may vary due to the equipment connected to it and installed in your house or office. For example, it comes out of the public power utilities with a zero volts potential, but due to the connected equipment, the neutral wire may be showing a slightly higher potential, say of 6 volts. The potential difference between neutral and phase will then be 6 volts lower. As electrical equipment usually have a high tolerance this voltage drop will not affect its operation (in this example the voltage lowering will be from 127 volts to 121 volts, and the equipment operation is not affected).
But ground shows a permanent zero volts potential. Ground potential is obtained by means of a steel/copper bar into the ground. As Earth is an perennial source of electrons, its potential is absolutely stable. In case any equipment tries to alter the ground voltage (as it happens to the neutral), the voltage in excess will be diverted to the Earth, thus holding its electrical potential always in zero volts.
The issue is that the ground wire is meaningful only when we are operating with interconnected equipment where there can be no potential difference between them. An electric iron, a mixer or a lamp will operate without that zero volts ground reference, even if the neutral is presenting voltage differences, due to their tolerances. (That is not absolutely true if one considers the user who contacts a metallic part of the equipment while contacting ground simultaneously).