What Is a Good Ground Resistance Value?

The goal in ground resistance is to achieve the lowest ground resistance value possible, that makes sense economically and physically, when contacting the earth, also known as the soil/ground rod interface.
Ideally, a ground should be zero ohms of resistance, but...
Unfortunately, there is not one standard ground resistance threshold recognized by all certifying agencies. 
The NFPA and IEEE recommend a ground resistance value of 5 ohms or less while the NEC has stated to “Make sure that system impedance to ground is less than 5 ohms specified in NEC 50.56. In facilities with sensitive equipment it should be 5ohms or less.”
Copper Earth Rod thermoweld connection at a 33kV Substation in Nakheel, UAE (photo by Muhammad Adnan; Substation Designer via Flickr)
The telecommunications industry has often used 5 ohms or less as their value for grounding and bonding while electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm.
In general, the lower the ground resistance, the safer the system is considered to be. 

why is transformer rated in kVA and not in kW?

1.  Why is transformer rated in kVA and not in kW? 

There are two type of losses in a transformer;

1. Copper Losses
2. Iron Losses or Core Losses or Insulation LossesCopper losses ( I2R)depends on Current which passes through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage.
That’s why the Transformer Rating may be expressed in kVA,Not in kW.