Why Earth?
Many domestic and commercial premises have devices (e.g. washing machines, tumbler dryers, electric heaters and some lighting fittings) which in turn have exposed metalwork or extraneous conductive parts. These “parts” of the installation are potentially dangerous and could become live in the event of a fault. It goes without saying that anyone touching these unearthed parts could then receive an electric shock or even be killed depending on the current flowing their body to earth. To prevent this highly dangerous situation arising, an earthing conductor should be provided to all socket outlets, lighting circuits and any fixed appliances to which their exposed metal parts are then connected.
The earth connection limits the voltage which can appear on the exposed metal parts under fault conditions to a safe value until the fuse blows or the MCB or RCD trips. Note that earthing does not necessarily prevent anyone receiving a shock, but together with the time/current characteristics of the protective device (fuse, MCB or RCD) it should ensure that it is not lethal. It is desirable to make the impedance (resistance) of the earth wiring a low as practicable. (1000A flowing through 0.1 ohm drops 100V!)
It needs to be also noted that exposed metalwork cannot be protected by connection to the neutral because current flowing will cause a voltage drop between the metalwork and true earth. Also, if the neutral connection breaks or the appliance is plugged into a socket with line and neutral reversed (!), the metalwork will be at full mains voltage.
Appliances with an earth connection are called Class I (one): Class II or 'double insulated' appliances incorporate additional insulation to prevent exposed metalwork becoming live, and do not require an earth connection. This means that a 2-core mains lead can be used and internal earth connections are not needed.
A fundamental principle of electrical safety is that no single fault condition should cause a hazardous situation.
This is why some of the regulations that now appear within the 17th edition are robust and without any ambiguity:
it is better to be safe than sorry.
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