Earthing System

Fault currents are established to prevent damage to the system, and for safety to personnel....

https://answers.yahoo.com/question/index?qid=20110331080408AAH6O01&

The earthing design is affected by the voltage of the system you are protecting.

A 400A system is designed to limit iron damage in rotating equipment operating at a high voltage, ie 4000V, where the charging current of the system is relatively high. The 400A rating is chosen based on the sensitivity of the protection equipment, so that selective operation of the protection is possible. The 400 amperes is usually allowed to flow for 10 seconds, enough time to sense & clear the fault.

High resistance grounding can be applied on lower voltage systems, allowing continued operation of the distribution system as long as there is only one phase faulted. Those systems have low charging current, less than 10 amperes, so a low wattage ground resistor can be used. Same reason for your neutral grounding transformer for your high voltage generator, reactance grounding.

Solidly grounded systems are inexpensive, provide plenty of fault current to ensure quick isolation of the fault, and reduce higher voltages associated with other systems during a fault. If you get a ground fault in an expensive large motor, it will allow more damage to occur to the lamination iron, possibly requiring scrapping of the motor, painful for a $20,000 motor, for instance.

All the fault current does is trigger (a) circuit protective device(s) once it is above a predetermined level. A <...400A...> fault requires rather more copper than a <...10A...> fault, and the latter represents a lower total value of equipment installation. It is also potentially [sorry about the pun] less hazardous to personnel who may be in contact with the earth conductor(s) when the fault occurs.

In rotating machines, any stator earthfault would involve the (CRNGO) core in the fault current path. As the core material is highly resistive, the heat due to the fault current flow would be more and thus the core damage. Any winding damage could be rectified at a local re-winder but for core damage rectification, the machine may have to be sent to the manufacturer. This would involve cost and increased downtime. So, core damage has to be avoided, as much as possible. Each rotating machine will have a core damage curve and a typical fault current withstand of CRNGO core is 25A/1sec. So, wherever rotating machines are involved in the network, it is safe to limit the earthfault current to less than 25A. A typical value is 10A.

In case of transformers, even though the winding is capable of handling the rated full load current continuously, to offer safe operator and/or livestock protection, it is safe to limit the earthfault current. At the same time, limiting to a very low current might make detection & isolation difficult. So, to enable detection & isolation, in case of transformers, it is general practice to limit the earthfault current to about 200 to 400A.

In case of low voltage secondaries, with the neutral distributed to cater to single phase loads, non-electrical personnel too handle electric appliances. So, to ensure safety of those personnel, the neutral is solidly grounded, so that, in the event of an earth fault, full three phase short circuit current flows and the transformer's SCPD (Short Circuit Protective Device) would clear the fault in milliseconds, before it could hurt the operator. That is, the fault is cleared without the need for any special earth leakage/earth fault protective device.