TN-C-S

An earthing system where the supply itself provides a separate earth conductor back to the source. Traditionally that conductor is the lead sheath or armour of the distributor's cable. Neutral and earth stay separate throughout, which keeps the earth path clean but depends entirely on the integrity of that supply conductor.

An earthing system where the installation provides its own earth via an electrode in the ground. Common on rural and overhead supplies, it takes no earth from the distributor at all. The soil path makes the earth fault loop impedance high, so an RCD, not the overcurrent device, is what disconnects earth faults on a TT installation.

Connecting an installation's exposed metalwork to earth and back to the supply source. That gives fault current a deliberate low-impedance path home, turning a live casing into a large, brief fault current that the protective device can detect and disconnect — without it, the casing simply stays live.

Conductors connecting incoming metallic services — water, gas, oil, structural steel — to the main earthing terminal. Bonding does not clear faults; it holds the touchable metalwork of a building at roughly the same potential while a fault lasts, so a person touching a pipe and an appliance at the same moment isn't bridging a dangerous voltage difference.