SYSTEM EARTHING
 
The general purpose of earthing is to protect life and property in the event of 50/60 Hz faults (short circuit) and transient phenomena (lightning switching operations). The question of how a e/stern shall be earthed is designed by regulation. The regulation that one point of every system shall be earthed is designed primarily to preserve the security of the system by ensuring that the potential on each conductor is restricted to such a value as is consistent with the level of insulation applied.
 
The choice of the earthing method is based on the following criteria :
 
• Voltage level : the insulation level of material must be in accordance with the induced voltage at the time of fault.

• Insulation co-ordination.

• Limitation of fault current : to reduce the electrodynamics stresses on material, to limit the induced voltage on telecommunications lines and over voltage on LV components.
 
There are 3 methods of neutral earthing:
 
  • Insulated neutral system (fig.1)
  • Direct earthing or solidely earthedx(fig.2)
  • Impedance earthing (fig.3)
    The purpose of this method is to limit the current for greater safety.
    There are 3 type of impedance earthing:
    - Earthing through reactance.
    - Earthing through ARC - Suppression coil.
    - Earthing through resistor.

 

 fig1.gif (2428 octets)
fig2.gif (2777 octets)
fig3.gif (3078 octets)
 

CALCULATION OF NEUTRAL EARTHING RESISTOR
These parameters are needed to specify the earthing resistor:

U : Rated voltage line to line voltage or system voltage (kV)
If : Rated fault current (A)
T s : Rated time (Sec) - Duration of the fault

Resistance value :

When the flow of an electric current through a resistor is relatively short, dissipation is negligible and the heating temperature of that resistor depends on its capacity to store the electric energy (i.e. its heat value itself) in proportion to the mass and specific heat of the material used.

The rise in the resistors temperature will be provided by the relation :

 
When large-scale masses and energies are involved, the real resistance value of the resistor is taken into account, because it varies with the temperature which itself depends on the current flow, time. With that method of calculation we can determine the exact dimensions of the resistor to be built. For resistors heating adiabatically, masses as high as possible are therefore required.
 
As per IEEE-32 for neutral earthing resistor made from stainless; steel resistance material, the allowable temperature rise for 10,30 or 60 sec time rating is 760°K, 610°K for extended time rating and 385°K for continuous rating.