It is all abput saving money vs provide a sufficient coverage against electric shock

Care to elaborate sir ? 

I think it depends upon the soil conductivity, fault current and voltage of the line. 

No.

Soil resistivity will decide the sizing of the earth strips and quantity of pits. Fault current will be taken care by the size of the strip. The line voltage will play a factor in line current, hence the short circuit values. 

So, why are we using protective earths and seperate earthings from transformer nuetrals in some cases while not in all ?

For instance IT system

1st  Letter: I  (Isolated)- The source of electricity supply, e.g. the star/neutral point of the secondary winding of the supply transformer has no direct connection with earth, although it may be earthed via impedance

2nd Letter: T  (Tera) - The exposed-conductive parts of the electrical installation have their own separate earth as for TT systems.

You save money by removing the needs for earthing cable but. There is common concerned of overvoltage (OV), which mainly affect the IT system as the network is then “unearthed” (not connected to :mass of Earth which bring the potential to Zero Volt). The OV could be caused by

breakdown in the MV/LV transformer,

over voltages due to lightning striking the upstream LV network,

Over voltages due to lightning striking the building in which the installation is housed

For tha reason IT is only used for a special iinstallation ...

Yes I agree Mr. Hartungi. 

IT Is mainly used for Residential buildings, Hospitals and such establishments. 

I read TN, TNC and other systems are mainly used for industrial earthing depending on the equipment -specificity. 

TT- is mainly for consumer power supplies.The owner must install the earthing protection by their own connection to the earth.The advantage is the reductionnoise of high or low frequency,no risk of  failure and  suitable for premises where all AC power circuits are residual current device (RCD) protected.

IT-This system is similar with TT system but different in earthing supply.The distributor system  has only a high impedance connection.This type is not for consumer power supply and used  for power distributor such as substation or generator area.

 TN-S system The consumer’s earthing terminal is usually  connected to the metallic part of the distributor’s cable .This is used for underground power supply to the premise or factory from the distributor substation to customer substation.

 TN-C-S system-This system has the supply neutral conductor of a distribution main connected with earth at source as protective multiple earthing.

TN-C-This system ist a combined PEN conductor fulfills the functions of both a PE(protective conductors) and an N(neutal) conductor

Different types of earthing are needed to suit the particular consumer(domestic/industrial/HT/LT consumer).

https://en.wikipedia.org/wiki/Earthing_system

hello

The main reason for using the card system, conducting ground by creating the shortest and least resistance will be admitted on the basis of telecommunication systems which are of particular sensitivity and To avoid interference and damage to telecommunications facilities separate from the power of the system is intended to improve the level of protection installations.