Earthing

Intro

Fault current is best sent to ground. This includes surge current and lightening current.

An electrode is stuck in a pit in the ground and fault current is directed to it.
The ground should have low resistivity at all times. But ground resistivity varies with space and season. So, electroloytes are added.
With regular earthing, salt and coal is added around the electrode. Salt dissipates gradually and increases conductivity in the surrounding ground.
With “chemical” earthing, special electrolytes and backfills are added, and the electrode itself comes surrounded by a case of electrolytes. As per IEEE 80 14.5 (d), there are certain compounds which has resistivity less than 0.12 ohm meter are called permanent earthing compounds. Which has a life span of more than 10 to 15 years (longer than “salt and charcoal” earthing).

TN-S - most common in older housing stock, with supplier providing a separate earth conductor back to the substation.
TN-C-S - (Protective Multiple Earthing) Supply combines neutral and earth, but they are separated out in the installation. More common in newer houses.
TT - No earth provided by supplier; installation requires its own earth rod etc.
TN-C - No separate earth conductors anywhere - neutral used as earth throughout supply and installation (very rarely used).
IT - Supply is e.g. portable generator with no earth connection, installation supplies own earth rod.

First letter:

T The live parts have one or more direct connections to earth.
I The live parts have no connection to earth, or are connected only through a high impedance.

Second letter:

T All exposed conductive parts are connected via your earth conductors to a local ground connection.
N All exposed conductive parts are connected via your earth conductors to the earth provided by the supplier.

Remaining letters: