# Earth Fault Higher Than Three Phase Fault

In a power system, almost always three phase fault current is higher than the earth-fault current. However, earth fault current near the generator is higher than three phase fault. What is the reason for this phenomenon? This article provides an explanation as to the reason of this condition.

### Near Generator Faults

Faults near the generator where the generator transformer is delta-wye connected.

### The Reason Earth faults can be greater than the rated 3-phase short-circuit current if the zero-sequence source impedance behind the circuit breaker is less than the positive-sequence source impedance.

The higher earth fault than three phase fault occurs when the transformer has a delta winding on the generator side and is star (wye) connected on the HV side (refer to Figure 1). In this circuit, the total positive-sequence impedance of the source consists of the generator impedance plus the transformer impedance (refer to Figure 2), whereas, the zero-sequence source impedance is that of the star (wye) connected transformer winding only (refer to Figure 3). This results in the current interrupted in the HV circuit breaker for close-up earth faults, typically 20-30% higher than for a similarly located 3-phase fault.

### Reference

Power Circuit Breaker Theory and Design
Publisher: Institution of Engineering and Technology (IET)
ISBN: 9780906048702