Why Phase‑to‑Phase Faults Hit Harder Than Earth Faults

Electrical faults are never welcome, but understanding how they behave is essential for designing safe, reliable systems.
One of the most common questions in protection engineering is why phase-to-phase faults typically produce
higher fault currents than phase-to-earth faults.

The answer comes down to two simple but powerful ideas: voltage and impedance.

Higher driving voltage

In a standard 400/230 V three-phase system:

• A phase-to-earth fault is driven by the phase voltage (230 V).
• A phase-to-phase fault is driven by the line voltage (400 V).

That alone gives the phase-to-phase fault a 73% higher voltage:

400 / 230 ≈ 1.73

More voltage means more current.

Lower impedance in the fault path

A phase-to-phase fault is almost a direct short between two live conductors. The current path is simply:

• Phase conductor
• Transformer winding
• Other phase conductor

A phase-to-earth fault must travel through a much more resistive route:

• Phase conductor
• Protective earth conductor
• Bonding and MEN link
• Soil/earthing system
• Transformer neutral return

Earth paths always introduce additional impedance, which reduces the fault current.

A numerical example

Let’s use simple, realistic values to illustrate the difference.

Assume:

• Phase conductor impedance: 0.02 Ω
• Earth return path impedance: 0.20 Ω

Phase-to-phase fault

Z_L–L = 0.02 + 0.02 = 0.04 Ω

I_L–L = 400 / 0.04 = 10,000 A

Phase-to-earth fault

Z_L–E = 0.02 + 0.20 = 0.22 Ω

I_L–E = 230 / 0.22 ≈ 1,045 A

Comparison

• Phase-to-phase fault: 10 kA
• Phase-to-earth fault: ~1 kA

The phase-to-phase fault current is roughly ten times larger.

A note on exceptions

In some specialised systems—particularly near generators with delta–wye transformers—the zero-sequence impedance
can be unusually low. In those cases, a phase-to-earth fault may exceed the phase-to-phase fault current.

But in typical distribution networks, phase-to-phase faults are almost always higher.

The Bottom Line

Phase-to-phase fault currents are greater because:

• They use the higher line voltage (400 V).
• They flow through a lower-impedance path.
• Earth faults must return through soil, bonding, and neutral–earth links, all of which add impedance.

The result is a consistently higher current for phase-to-phase faults—critical knowledge for protection settings, breaker selection, and safety compliance.