Earth Fault and Differential Earth Fault

Earth Fault is a fault when there is total or partial insulation failure causing large fault current to earth from the system which can cause total damage to system .also a insulation declivity caused by ageing of wiring or equipment insulations will cause a leakage current called earth leakage current,undetected by MCB or earth Fault relays but detected by ELR-CBCT (Sensitive Earth Fault Protection),ELCB etc .

there is no such thing as "Differential earth fault" but there is a "differential earth fault system" which is used to protect transformers, generators, etc against ground faults.

there are 3 kinds of differential earth fault system

1) restricted earth fault systems

2) partial differential earth fault system

3) modified differential earth fault system (percentage differential protection)

Restricted Earth Fault Protection is a type of zone of protection where Relay is connected in parallel with three phase CTs and one neutral CT. It will look just for faults inside the transformer, and need not to coordinate with relays downstream .the disadvantage is that it is affected by existence of an alternate ground path back to source

partial differential earth fault system is the one in which Differential relays sense the unbalance in the flow of currents in various apparatus or buses. In the absence of a fault in the protected zone, this unbalance tends to be small and the flows into the zone are closely matched to the flows leaving.This type of system is adversely affected by existence of an alternate ground path back to source. The current divides as a function of relative ground impedance parameters . Also It lacks sensitivity when used on a bus system where both buses are served by one source with the tie circuit breaker closed.

modified differential earth fault system (percentage differential protection) is the differential system arranged so that multiple source grounding doesnot affect system performance.