These pages collect the parts of protective earthing that are easy to state
plainly and easy to get wrong in practice: how the earthing arrangement of an
installation is classified, what the letters in TN-S or
TT actually stand for, and which terms are routinely used
interchangeably when they should not be.
Nothing here is a standard. The classification below follows the usual international convention, but the rules that bind you are the national wiring regulations for the installation in front of you, and they differ in details that matter — conductor sizing, disconnection times, and what is permitted to be combined with what.
Bonding is not earthing. Equipotential bonding exists to keep simultaneously accessible conductive parts at the same potential. Earthing exists to give fault current a defined path so that a protective device disconnects the supply. A single conductor may serve both purposes, but the purposes are separate, and sizing rules follow the purpose.
A low resistance to earth is not, by itself, protection. In a TT installation the earth electrode resistance is usually far too high to operate an overcurrent device on a fault. Disconnection is achieved by residual current detection instead. Measuring a good electrode and concluding the installation is safe is a common and serious error.