Can somebody please explain to me the principle of a T-T Earthing system?
The fundamental logic behind this earthing arrangement is purely of safety.  There are 2 scenarios underlying here:

The transformer MEN and consumer board MEN are in physically separate locations, ie pole / house respectively.  In this case all earth potentials around the transformer body and dirt are identical.. meaning zero electrocution risks.  Similarly, the earth potentials around the house, switchboard and dirt are identical again with zero risks.

The transformer and consumer switchboard are in the same location, adjacent. I've used 2 earth rings, one that connects all equipment metalwork including transformer case to earth and to earth electrode, eliminating earth potential electrocution risks. The 2nd only connects the utility earth direct to the earth electrode .


The big issue is when the Utility supply is under fault conditions,  so it's possible to have hundreds (possibly thousands more) of volts presented at the transformer earth, causing step potential electrocution risks.  In scenario 1. above,  the consumer environment remains largly safe.  In scenario 2. above careful engineering is required to avoid a fire, and bonding nearby things like fences, pipes, etc..