Earthing

Earthing has nothing to do with system voltage. When you connect the neutral of a star connection to earth. It is called system earthing. It can be solid earthing, resistance or reactance earthing.

It is provided for various reasons like

a) elimination of arcing grounds.

b) voltage on healthy phase during earth fault on one of the phases will not rise above phase voltage. hence equipment connected to other phases are safe.

c) earth fault protection scheme becomes easy and simple.

Type of earthing depends on

a) fault level of the system.

b) Rotating machines on Secondary side of Transformer.

c) Earth Fault protection scheme.

Why... quite simple really. A MV system can deliver more energy into a fault. So, by using a resistance in the ground path back to the source, we limit the possible current flow to a level that is only high enough to ensure the operation of the overcurrent devices. This limits the energy dumped into a fault.

By limiting the fault current levels we minimize equipment damage caused during a fault event and reduce the cost of installation by allowing for systems less hardened to withstand the effects of high fault currents.

For lower voltage system that can generally deliver far less fault current into a fault, these precautions are seldom needed.