Factor of Earthing

The Factor of Earthing in a three phase system is defined as the ratio of the phase voltage (phase to earth voltage) on a healthy phase during the fault at the rated frequency to the phase voltage of the healthy phase during normal situation.  

When an earth fault occurs in one of the phases of a three phase system, the voltage vectors are distorted.  The neutral shifts in the direction of the fault phase.  This causes the voltage in the other two healthy phases to rise. 

The factor of earthing determines the rise of voltage in the healthy phases when one of the phases has a earth fault.

The factor of safety is 100% for an isolated neutral (floating neutral) system.  It is 80% for an effectively earthed system while it is 57.7% for a solidly earthed system.

The factor of safety is an important parameter while performing an insulation coordination in a system.