Earthing

I.E. Rules 61  :   substation and equipments

I.E. Rules 67 : HV and EHV

Maxm Res. :
Large power stations   0.5 ohms
Major Substations      1.0 ohms

Small Substations    2.0 ohms
Others                       8 ohms


I.E. Rules  1956 Chapter I 

Low :          250 V
Medium:    650 V
HV:           11 KV
EHV:         33 KV

Grounding

Advantages of neutral grounding:

• Voltage of phases are limited to phase to ground voltage.
• High voltages due to arcing grounds or transient line-ground faults are eliminated.
• Sensitive protective relays against LG faults can be used.
• Overvoltages due to lightening are discharged to ground.

Advantages of isolated neutral

• It is possible to maintain supply with fault on line.
• Interference with communication lines is reduced because of no Z sequence current.

Effective grounding systems:

X0 / X1 < 3

Least Expensive is = maxm Vphase= 0.8 V line