Earthing in Three Phase Systems Part II

Resistance Grounding

In Resistance Grounding, the three phase power system is grounded through a series resistance.  This resistance is intended to limit the fault current when there is an earth fault.  Resistance grounded systems are ideal as they enable quick identification of a fault and clearance.  The series resistors used to limit current are designed for thermal stability during fault conditions.  The resistors also have a time rating.  They are designed to be in circuit for a particular period of time till the fault is cleared.  Resistance grounded systems can be classified into

  • High Resistance Grounding and
  • Low Resistance Grounding

High Resistance Grounding restricts the ground fault current to less than 10A.  These systems are advantageous because the system can continue to run when there is a fault between a phase and the earth.  This ensures the system reliability and the system continues to run while the fault can be identified and rectified.  However, care must be taken to ensure that the permitted ground fault current is greater than the charging current of the line capacitances.

This is essential to ensure that there are not transient overvoltages during intermittent earth faults.  The series resistors used in High resistance grounding are designed for longer time rating as they may have to be in circuit as long as the system is running with the fault still present.

High Resistance grounding Systems are not permitted in systems which feed single phase loads.

Modern High Resistance Grounding Systems are equipped with a pulser circuit which is activated when a ground fault is detected.  This pulser circuits generates a pulsating current which can be used to identify the exact location of the ground fault with a handheld device.  This is extremely useful in identifying the fault within a short period and restoring the system.

See Also : 

Types of Earthing